SYNC™

SynC · SynC Standards

Grounding and Bonding for Electrical Systems

Written and maintained by SynC.

Rev9
IssuedOct 19, 2026
LicenseCC BY 4.0
Contents

Revision history

Download
Build a datasheet from this standard Start a project with this standard already attached — one click, no setup.
Use in a project

1 Scope

NOTE This standard governs the grounding and bonding of the electrical system as one system: the electrodes in the earth, the conductor that ties the system to them, the single bond between the grounded conductor and the equipment ground at the service and at each separately derived system, the equipment grounding conductor that carries fault current from every enclosure back to that bond, the bonding of the metal piping and structure that could become energized, the common connection to the other grounded systems in the building, the connections that join all of it, and the tests that prove it. (1.1)
NOTE Grounding and bonding do two different jobs. Grounding connects the system to earth so that the voltage to earth is held steady in normal operation and so that lightning and line surges have a path to dissipate. Bonding connects the metal parts to one another and back to the source so that a fault raises enough current to open the overcurrent device. Earth is a poor conductor and never the fault-clearing path; a ground rod at a motor with no bonding conductor back to the panel leaves the frame energized at line voltage while the breaker holds. Most of the requirements in this standard exist because the two jobs are confused in the field. (1.2)
NOTE The equipment standards and this standard divide the work in one way throughout the corpus. The standard for a transformer, a switchboard, a motor, a pump station or a luminaire names what on that equipment is bonded and defers the conductor, its size, its termination and its test to this standard. This standard states the rule once; the equipment standard does not restate it. (1.3)
NOTE The following are governed elsewhere and are outside this standard: (1.4)
  • the ground grid of a substation designed under IEEE 80 and the system grounding of circuits above 1000 V, including neutral grounding resistors and reactors, which no SynC standard yet owns
  • the shield grounding of a medium-voltage cable, whether single-point, multi-point or cross-bonded, under Medium Voltage CablesMedium-Voltage CablesResolves to the current adopted revision.sync/medium-voltage-cables
  • the strike termination network, its down conductors and its own earth terminations, under Lightning ProtectionLightning Protection SystemsResolves to the current adopted revision.sync/lightning-protection
  • the primary bonding busbar and everything the telecommunications backbone serves downstream of it, under Telecommunications BondingTelecommunications Bonding and GroundingResolves to the current adopted revision.sync/telecommunications-bonding
  • impressed-current and galvanic corrosion control of buried and submerged steel and iron, under Cathodic ProtectionCathodic ProtectionResolves to the current adopted revision.sync/cathodic-protection
  • the protection technique, wiring method and sealing inside an explosive atmosphere, under Electrical Hazardous LocationsElectrical Installations in Hazardous (Classified) LocationsResolves to the current adopted revision.sync/electrical-hazardous-locations
  • the utility point of delivery, the service drop or lateral, the meter and the service disconnecting means, under Electrical Service EntranceElectrical Service EntranceResolves to the current adopted revision.sync/electrical-service-entrance
  • the building wire itself, its stranding class and its jacket, under Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables
  • conduit, tubing and their fittings, under Raceways And ConduitRaceways and ConduitResolves to the current adopted revision.sync/raceways-and-conduit, and the tray and its splice plates, under Cable TrayCable Tray SystemsResolves to the current adopted revision.sync/cable-tray
  • the terminal bar inside a panelboard, switchboard, switchgear or motor control center, under PanelboardsPanelboardsResolves to the current adopted revision.sync/panelboards, Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current adopted revision.sync/low-voltage-switchboards, Low Voltage SwitchgearLow Voltage SwitchgearResolves to the current adopted revision.sync/low-voltage-switchgear and Motor Control CentersMotor Control CentersResolves to the current adopted revision.sync/motor-control-centers
  • the testing organization, its instruments, its safety practice and the compiled report, under Electrical Acceptance TestingElectrical Acceptance TestingResolves to the current adopted revision.sync/electrical-acceptance-testing
  • label materials and lettering, under Electrical IdentificationWiring System IdentificationResolves to the current adopted revision.sync/electrical-identification
  • the piping, ductwork, tanks and structural steel that are bonded, each under its own standard
1.5 The grounding and bonding of every electrical system rated 1000 V and below on the project shall comply with this standard.
1.6 The requirements of this standard shall apply to the service equipment, to every separately derived system, to every feeder and branch circuit, to every enclosure, raceway, cable tray, busway and wireway, and to every item of utilization equipment, whether or not the standard governing that item restates them.
1.7 Where the Contractor and the Engineer of Record disagree whether a metal part is likely to become energized, or whether a connection is accessible, the Engineer of Record shall make the initial determination.

2 Referenced Standards

2.1 Materials and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
2.2 Where the contract documents, the Authority Having Jurisdiction, or two referenced standards impose conflicting requirements, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
Standard Title
NFPA 70 National Electrical Code (Article 250)
NFPA 70E Standard for Electrical Safety in the Workplace
NFPA 780 Standard for the Installation of Lightning Protection Systems
UL 467 Grounding and Bonding Equipment
IEEE 81 Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System
IEEE 142 Recommended Practice for Grounding of Industrial and Commercial Power Systems
IEEE 3003.2 Recommended Practice for Equipment Grounding and Bonding in Industrial and Commercial Power Systems
IEEE 837 Standard for Qualifying Permanent Connections Used in Substation Grounding
IEEE 1100 Recommended Practice for Powering and Grounding Electronic Equipment
ANSI/NETA ATS Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems
ANSI/TIA-607 Generic Telecommunications Bonding and Grounding (Earthing) for Customer Premises
ASTM B3 Standard Specification for Soft or Annealed Copper Wire
ASTM B8 Standard Specification for Concentric-Lay-Stranded Copper Conductors, Hard, Medium-Hard, or Soft
ASTM B187 Standard Specification for Copper, Bus Bar, Rod, and Shapes and General Purpose Rod, Bar, and Shapes
NEMA GR 1 Grounding Rod Electrodes and Grounding Rod Electrode Couplings

3 Submittals

3.1 Action Submittals

3.1.1 The Contractor shall submit the following for review before any grounding electrode is installed or any grounding connection is made:
  • product data for every grounding and bonding component, including rod electrodes, plate and electrolytic electrodes, ground bars and their insulators, connectors, clamps, bonding bushings and locknuts, exothermic molds and weld metal, compression connectors and the dies that set them, and ground enhancement material
  • a grounding riser diagram showing every electrode, the grounding electrode conductor and its route, the main bonding jumper, every separately derived system with its system bonding jumper and its grounding electrode connection, every ground bar, the intersystem bonding termination, and the connection point of each other grounded system
  • an electrode layout showing the location of each rod, ring, plate and concrete-encased electrode and of each test well
  • a sizing schedule listing each grounding electrode conductor, bonding jumper, system bonding jumper and ground bar bonding conductor with the NFPA 70 table and the conductor size it was sized from
  • the soil resistivity survey, where the datasheet selects the measured basis
Action Submittalscheckbox
☑ Product data for grounding and bonding components
☑ Grounding riser diagram
☑ Electrode layout
☑ Conductor and jumper sizing schedule
☐ Soil resistivity survey
3.1.2 No grounding electrode shall be concealed before the electrode layout covering it has been reviewed and returned.
NOTE The riser diagram is the document that catches the second neutral-to-ground bond, the transformer with no grounding electrode conductor, and the telecommunications backbone landed on a panel ground bar instead of the electrode system. It is reviewed once, on paper, when each of those costs nothing to move. (3.1.3)

3.2 Closeout Submittals

3.2.1 The Contractor shall submit the following before the grounding system is accepted:
  • the field test reports for the electrode resistance, bonding continuity and objectionable current tests, with the method, the instrument, the measured values and the acceptance value for each
  • the record grounding riser diagram and electrode layout, showing the electrodes, conductors, connections and test wells as installed
  • a photographic record of each connection made inaccessible by backfill, concrete or finishes, taken before it was concealed
  • the record of the Authority Having Jurisdiction's inspection of the electrode system before concealment
Closeout Submittalscheckbox
☑ Field test reports
☑ Record grounding riser diagram and electrode layout
☑ Photographic record of concealed connections
☑ Inspection record before concealment
NOTE The record test values are the baseline every later measurement of the electrode system is compared against; a resistance that has doubled in five years is a corroded electrode or a cut conductor, and that is visible only against the first reading. (3.2.2)

4 Quality Assurance

4.1 Installer and Connection Qualifications

4.1.1 Grounding and bonding work shall be performed under the supervision of a licensed electrical contractor by electricians who have installed grounding electrode systems on not fewer than three completed projects.
4.1.2 Every person who makes an exothermic welded connection shall have been trained by the connection system's manufacturer in the molds and weld metal used on the project and shall make one sample connection of each type used, which shall be sectioned and shown to be free of porosity and slag before that person makes a production connection.
4.1.3 Every person who makes a compression connection shall use the die and the tool the connector's listing names for that connector and conductor, and the tool shall be of the type that completes the full crimp before it releases.
4.1.4 Connections that are buried, encased in concrete or otherwise inaccessible after installation shall carry the qualification indicated in the datasheet.
Qualification of Inaccessible Connectionsradio
● Listing to UL 467
○ Listing to UL 467 with qualification to IEEE 837
NOTE UL 467 lists a connector for its intended conductors and surfaces and for direct burial or concrete encasement, which is the listing NFPA 70 250.70 requires and the one every building project relies on. IEEE 837 subjects a connection to repeated fault-current, thermal cycling and corrosion exposure that an ordinary mechanical lug does not survive, and is the qualification a substation, a generating station or an Owner's corporate standard asks for where the connection carries fault current from a stiff source and will never be inspected again. (4.1.5)

4.2 Listing of Grounding Components

4.2.1 Every connector, clamp, bonding bushing, bonding locknut, ground bar and rod electrode shall be listed and labeled to UL 467 by a Nationally Recognized Testing Laboratory.
4.2.2 Every connector shall be listed for the conductor material and size range and for the surface it is applied to, and a connector that joins aluminum or copper-clad aluminum to copper shall be listed for that combination.
4.2.3 Every connector in contact with earth or embedded in concrete shall be listed for direct burial or for concrete encasement.

4.3 Inspection Before Concealment

4.3.1 The grounding electrode system, every connection to an electrode, and every conductor to be buried or encased shall be inspected by the Authority Having Jurisdiction and by the Engineer of Record's representative before it is concealed by backfill, concrete or finishes.
4.3.2 The Contractor shall give not less than two working days' notice of each inspection and shall not conceal any part of the electrode system before it has been released.
4.3.3 A connection concealed before inspection shall be exposed at the Contractor's cost for inspection and shall be remade where the exposure damaged it.

5 Soil and Site Conditions

5.1 Soil Resistivity Design Basis

5.1.1 The electrode system shall be designed to the soil resistivity basis indicated in the datasheet.
Soil Resistivity Design Basisselect
Measured on site by the four-point method of IEEE 81
Taken from the geotechnical report
Assumed from regional soil data and confirmed by the electrode resistance test
NOTE A four-point survey on the site measures the resistivity at several depths and spacings and is the only basis that predicts the resistance of a ring or a rod field before it is built, at the cost of a survey crew and a day on an undisturbed site; a resistivity taken from the geotechnical report uses a value the geotechnical engineer measured or inferred for corrosion purposes, at one or two locations and often at one depth, which is enough for a building whose electrodes are the footing steel and a pair of rods; an assumed regional value costs nothing and leaves the resistance to be discovered at the test, which suits a project where the code electrodes are expected to meet the target with margin and remedial rods are cheap to add. (5.1.2)
5.1.3 The design soil resistivity shall be the value indicated in the datasheet.
Design Soil Resistivityrange
Ω·m
110000
Per drawings — the geotechnical report (deferred by default)
NOTE Soil resistivity spans four orders of magnitude, from wet clay near 10 Ω·m to dry sand and rock above 1000 Ω·m, and the resistance of a rod is nearly proportional to it. The value fixes how many rods, how long a ring, and whether ground enhancement material is needed, which is why it is settled before the electrodes are priced rather than after the first test fails. (5.1.4)
5.1.5 Where the measured or reported resistivity exceeds the design value by more than 50%, the Contractor shall report the difference to the Engineer of Record before any electrode is installed, and the Engineer of Record shall direct any change to the electrode system in writing.

5.2 Corrosive Soil and Dissimilar Metals

5.2.1 Every buried electrode shall be of the material indicated in the datasheet.
Electrode Materialradio
● Copper-bonded steel
○ Solid copper
○ Stainless steel
○ Galvanized steel
NOTE A copper-bonded steel rod drives like steel and corrodes like copper, lasts thirty years or more in ordinary soil, and is the rod nearly every building project installs; a solid copper rod bends in driving and is chosen for a plate or a short rod in soil that attacks the steel core at a nick in the bonding; a stainless steel rod resists soil with high chloride or sulfate content, low pH or a cinder fill, and resists the galvanic attack a copper system causes on nearby buried steel, at several times the cost; a galvanized steel rod is the cheapest and is consumed in a few years once the zinc is gone, which is accepted where the Owner's standard is galvanized throughout or the electrode is easy to replace. (5.2.2)
5.2.3 Electrodes of dissimilar metal shall not be installed in one electrode system where they are interconnected through earth by buried bare conductor, unless the Engineer of Record accepts the galvanic exposure in writing.
5.2.4 Where the soil has a pH below 5, a chloride or sulfate content the geotechnical report classes as aggressive, or a cinder, slag or ash fill, every buried connection shall be exothermic welded or a compression connection of the same metal as the conductor, and every buried conductor shall be solid copper or stainless steel.
NOTE Copper buried in earth and bonded to the building steel, the water service and the reinforcing steel makes every one of them the anode of a galvanic cell with the copper as its cathode. In ordinary soil the current is small and the steel lasts the life of the building; in wet, low-resistivity, low-pH soil it consumes a bare steel pipe or a galvanized rod in a few years, which is the reason a cathodic protection system under Cathodic ProtectionCathodic ProtectionResolves to the current adopted revision.sync/cathodic-protection puts a decoupling device in the bond rather than cutting it. (5.2.5)

5.3 Frost and Seasonal Moisture

5.3.1 Where Frost DepthFrost DepthParameterEach project supplies its own value.frost-depth exceeds the ground ring burial depth indicated in the datasheet, the resistance target shall be met by rod electrodes extending below the frost depth and not by the ring alone.
5.3.2 Rod electrodes shall be driven so that the full length of the rod below its top is in contact with soil and the lower end of each rod is not less than 2 ft below Frost DepthFrost DepthParameterEach project supplies its own value.frost-depth.
NOTE Frozen soil has a resistivity ten to a hundred times that of the same soil thawed, and an electrode system that meets its target in October and fails in February is one whose electrodes all lie in the frost zone. The rod that reaches permanently moist soil below the frost line carries the system through the winter. (5.3.3)

6 Grounding Electrode System

6.1 Electrodes Relied On

6.1.1 Every grounding electrode present at the building or structure that NFPA 70 250.52(A) recognizes shall be bonded together to form one grounding electrode system, in accordance with NFPA 70 250.50, whether or not it is selected in the datasheet.
6.1.2 The electrodes the design relies on to meet the resistance target shall be as indicated in the datasheet.
Grounding Electrodes Relied Oncheckbox
☑ Concrete-encased electrode
☐ Ground ring
☑ Rod electrodes
☐ Pipe electrodes
☐ Plate electrodes
☐ Structural metal in direct contact with earth
☐ Metal underground water pipe
☐ Electrolytic electrodes
NOTE The selection records which electrodes the design counts on; it never relieves the obligation to bond every qualifying electrode that exists. The common field error is to drive two rods and ignore the footing steel, the building columns and the metal water service, each of which NFPA 70 250.50 requires to be bonded and each of which lowers the resistance more than the rods do. (6.1.3)
6.1.4 Where no electrode of the types NFPA 70 250.52(A)(1) through (A)(7) exists at a building or structure, one or more of the electrodes of 250.52(A)(4) through (A)(8) shall be installed, in accordance with NFPA 70 250.50.
6.1.5 A metal underground gas piping system, an aluminum electrode, and the structures and reinforcing steel of a swimming pool shall not be used as grounding electrodes, in accordance with NFPA 70 250.52(B).

6.2 Concrete-Encased Electrode

6.2.1 Where a footing or foundation in direct contact with earth is poured on the project, a concrete-encased electrode shall be provided in it, in accordance with NFPA 70 250.52(A)(3), and shall be of the form indicated in the datasheet.
Concrete-Encased Electrode Conductorradio
○ Bare copper conductor in the footing
○ Reinforcing steel of the footing
NOTE A bare copper conductor laid in the footing is a dedicated electrode the electrical trade installs and inspects itself, with a tail that leaves the concrete wherever the design wants it, at the cost of 20 ft or more of 4 AWG copper and a trip to the site before the pour; the reinforcing steel is already in the footing, forms a far longer electrode than the copper conductor, and costs only the listed connector and the stub, at the cost of depending on the reinforcing trade to tie the bars electrically continuous and to leave the stub where the electrical trade needs it. (6.2.2)
6.2.3 Where the bare copper conductor form is selected, the electrode shall consist of not less than 20 ft of bare copper conductor not smaller than 4 AWG, encased by not less than 2 in. of concrete and located near the bottom of a footing or foundation that is in direct contact with earth.
6.2.4 Where the reinforcing steel form is selected, the electrode shall consist of not less than 20 ft of bare, zinc-coated or electrically conductive coated reinforcing bar not smaller than 1/2 in. in diameter, made electrically continuous by the usual steel tie wires or by exothermic welding or listed connectors, located as the copper form is located, and connected to the grounding electrode conductor through a listed connector or a bar that extends out of the concrete.
6.2.5 The connection to a concrete-encased electrode shall be made to a conductor or bar that extends out of the concrete, or by a listed connector installed before the pour, and no connection shall be made by chipping into cured concrete.
6.2.6 The Contractor shall coordinate the location of the electrode and the point where its tail or stub leaves the concrete with the concrete and reinforcing trades before reinforcement is placed, and shall protect the tail from the pour until it is terminated.
NOTE Concrete below grade draws moisture from the soil and holds it, so a footing electrode has a lower and more stable resistance than any rod field a building project would install, and it costs almost nothing when it is placed before the pour. It cannot be added afterward, which is why its installation is a hold point and why NFPA 70 250.50 requires it wherever a footing is poured. (6.2.7)

6.3 Ground Ring

6.3.1 Where a ground ring is selected, it shall encircle the building or structure, shall be in direct contact with earth, shall consist of not less than 20 ft of bare copper conductor of the size indicated in the datasheet, and shall be buried at not less than the depth indicated in the datasheet, in accordance with NFPA 70 250.52(A)(4) and 250.53(F).
Ground Ring Conductor Sizeselect
2 AWG
1/0 AWG
2/0 AWG
4/0 AWG
250 kcmil
500 kcmil
Ground Ring Burial Depthrange
Not less than
in
3036424860
NOTE A ring of 2 AWG is the smallest NFPA 70 permits and is enough where the ring exists to tie rods together and give the service a low-resistance electrode; a ring of 1/0 through 4/0 survives mechanical damage during later excavation and corrosion over the life of the building and is the size a data center, a communications site or a lightning-exposed structure specifies; 250 kcmil and 500 kcmil rings are installed where the ring carries the fault current of a large generator plant or a substation on the same site and the ring is sized for that current. A ring at 30 in. meets the code minimum; a deeper ring lies below the frost line and the dry surface layer and reads a lower and more constant resistance, at the cost of a deeper trench and a hazard to other utilities in the same corridor. (6.3.2)
6.3.3 The ring shall be installed not less than 2 ft from the foundation wall or footing, and rod electrodes bonded to the ring shall be connected to it by exothermic welding or by listed irreversible compression connectors.
6.3.4 The ring shall be installed in one continuous length wherever practicable, and a splice in the ring shall be exothermic welded or a listed irreversible compression connection.

6.4 Rod, Pipe, and Plate Electrodes

6.4.1 Rod electrodes shall conform to NEMA GR 1, shall be listed to UL 467, and shall be of the length and diameter indicated in the datasheet.
Rod Electrode Lengthrange
Not less than
ft
8101216203040
Rod Electrode Diameterrange
Not less than
in
0.50.6250.751
NOTE An 8 ft rod is the shortest NFPA 70 250.52(A)(5) permits and the rod a residential service drives; a 10 ft rod of 3/4 in. diameter is the rod commercial projects specify, because the extra length reaches past the dry surface layer and the larger diameter resists bending in driving and couples reliably when sections are added; rods of 16 ft and longer are sectional rods coupled as they are driven, used where the resistivity survey shows a conductive layer at depth and a long rod reaches it for less than a field of short ones. A 1/2 in. rod is permitted only where it is listed; a 1 in. rod is specified for driving into dense soil or for a design life beyond thirty years in corrosive soil. (6.4.2)
6.4.3 Rod and pipe electrodes shall be installed so that not less than 8 ft of length is in contact with the soil and shall be driven to a depth of not less than 8 ft, except that where rock is encountered the electrode shall be driven at an angle not exceeding 45° from vertical or buried in a trench not less than 30 in. deep, in accordance with NFPA 70 250.53(G).
6.4.4 The number of rod electrodes installed shall follow the basis indicated in the datasheet.
Rod Electrode Count Basisradio
● Two rods installed without measuring the first
○ Single rod, supplemented only where its measured resistance exceeds 25 Ω
NOTE NFPA 70 250.53(A)(2) permits a single rod whose resistance to earth is 25 Ω or less and requires a second where it is not. Installing two rods without the measurement costs one rod and saves the test visit, and is what nearly every commercial project does; measuring the first rod and stopping there is the arrangement of a small structure in conductive soil where the rod is the only electrode and the single test is cheaper than the second rod. (6.4.5)
6.4.6 Rod electrodes shall be spaced not less than 6 ft apart and not less than the length of the longer rod, and where more than two rods are installed they shall be arranged in a line or around the structure so that no rod lies within the length of another.
NOTE Two rods driven a foot apart share the same shell of soil and read little better than one; at a spacing equal to the rod length each works in its own soil and the pair reads close to half of one rod's resistance, which is where the spacing rule comes from. (6.4.7)
6.4.8 Pipe electrodes shall be not smaller than metric designator 21 (trade size 3/4), galvanized or otherwise coated against corrosion, and installed as rod electrodes are installed.
6.4.9 Plate electrodes shall expose not less than 2 ft² of surface to the soil, shall be not less than 0.06 in. thick where of bare or electrically conductive coated iron or steel and not less than 0.06 in. thick where of solid uncoated nonferrous metal, and shall be buried not less than 30 in. below grade, in accordance with NFPA 70 250.52(A)(7) and 250.53(H).

6.5 Structural Metal and Metal Water Pipe Electrodes

6.5.1 A metal in-ground support structure in direct contact with earth for 10 ft or more, with or without concrete encasement, shall be used as a grounding electrode, in accordance with NFPA 70 250.52(A)(2).
6.5.2 A metal underground water pipe in direct contact with earth for 10 ft or more shall be used as a grounding electrode and shall be supplemented by at least one other electrode of a type NFPA 70 250.52(A)(2) through (A)(8) recognizes, in accordance with NFPA 70 250.52(A)(1) and 250.53(D)(2).
6.5.3 The connection to a metal water pipe electrode shall be made within the first 5 ft of the pipe's entrance into the building, and every water meter, filter, dielectric fitting or other removable device in that length shall be bypassed by a bonding jumper sized as the grounding electrode conductor is sized, in accordance with NFPA 70 250.53(D)(1) and 250.68(B).
NOTE A water utility replaces a metal service with plastic without telling the building, and a plumber installs a dielectric union at the meter. Either one removes the water-pipe electrode, which is why NFPA 70 never lets it stand alone and why the connection and the jumper around the meter are made where they can be seen. (6.5.4)

6.6 Ground Enhancement and Electrolytic Electrodes

6.6.1 Ground enhancement material shall be used as indicated in the datasheet.
Ground Enhancement Materialradio
● Not used
○ Applied at every rod and plate electrode
○ Applied as a remedial measure where the measured resistance exceeds the target
NOTE Ground enhancement material is a conductive, hygroscopic backfill placed around an electrode to lower the contact resistance between the metal and a poor soil. It is unnecessary where the soil resistivity is low and the footing steel and rods meet the target on their own, which is most buildings; applying it at every rod is the design of a site on sand, gravel or rock where the survey shows the rods alone cannot reach the target; applying it as a remedial measure lets the project install the code electrodes first and spend on enhancement only at the electrodes that need it, at the cost of a second mobilization after the test. (6.6.2)
6.6.3 Where ground enhancement material is used, it shall be a listed, permanent, non-corrosive material with a resistivity not exceeding 0.2 Ω·m, shall be placed around the electrode in an augered hole or a trench in accordance with the material manufacturer's instructions, and shall not be a salt, a chemical that leaches into the soil, or bentonite alone.
6.6.4 Where electrolytic electrodes are selected, each shall be a listed copper tube electrode charged with a hygroscopic salt that leaches through breather holes into the surrounding soil, installed vertically or horizontally in ground enhancement material in accordance with its listing, with its cap accessible for recharging in a test well.
NOTE Bentonite holds water while the ground stays wet and shrinks away from the electrode when the soil dries, and a salt backfill lowers the resistance for a season and then corrodes the electrode it surrounds. The listed enhancement materials set permanently around the electrode and do neither, which is why the clause names what the material is not. (6.6.5)

6.7 Auxiliary Electrodes at Outdoor Equipment

6.7.1 An electrode installed at a light pole, a photovoltaic array, a pedestal, a pad-mounted enclosure or other outdoor equipment shall be an auxiliary electrode in accordance with NFPA 70 250.54, shall be bonded to the equipment grounding conductor of the circuit serving the equipment, and shall not be used in place of that conductor.
6.7.2 An auxiliary electrode is exempt from the resistance target and the bonding rules of this standard, and the equipment grounding conductor to the equipment shall be sized and installed as though the electrode did not exist.
6.7.3 Where a separate building or structure is supplied by a feeder or branch circuit, a grounding electrode system shall be installed at that building or structure and connected to the equipment grounding conductor of the feeder, in accordance with NFPA 70 250.32, and the grounded conductor shall not be connected to the electrode system or to the equipment ground at the separate building.
NOTE A rod at a light pole gives lightning on the pole a path to earth that does not run through the building; it does nothing for a fault on the pole circuit, whose current must still flow back to the source on the equipment grounding conductor to open the breaker. The two are confused often enough that NFPA 70 250.54 says so in words. (6.7.4)

6.8 Resistance to Earth Target

6.8.1 The resistance to earth of the grounding electrode system, measured as the Testing article requires, shall not exceed the value indicated in the datasheet.
Resistance to Earth of the Grounding Electrode Systemrange
Not more than
Ω
1235101525
NOTE NFPA 70 names 25 Ω only as the threshold at which a single rod must be supplemented, and places no resistance requirement on the electrode system as a whole. The value a project selects is a design decision, not a code reading: 25 Ω accepts the code electrodes as installed and suits a structure in conductive soil with no sensitive loads; 5 Ω is the value most commercial and institutional projects write, gives margin against seasonal drying, and is reached by the footing steel and a few rods in ordinary soil; 1 to 3 Ω is the value a data center, a communications site, a hospital or a facility with a lightning protection system specifies under IEEE 1100 and the equipment makers' installation requirements, and in high-resistivity soil it is reached only with a ring, long rods or enhancement material designed from a resistivity survey. The lower the value, the more the electrode system costs, and below the point where the system meets NFPA 70 the lower value buys surge performance and a stable reference for electronics, not fault clearing. (6.8.2)
6.8.3 Where the datasheet states no value, the supplementation rule of NFPA 70 250.53(A)(2) shall be the only resistance requirement on the electrode system.
6.8.4 Where the measured resistance exceeds the value in the datasheet, the Contractor shall add rod electrodes, extend the ring or apply ground enhancement material, as the Engineer of Record directs, and shall retest, and the cost of the first two additional electrodes shall be borne by the Contractor.
6.8.5 Where the value in the datasheet is not reached after the additional electrodes the Engineer of Record directs have been installed, the Engineer of Record shall set the accepted value in writing, and the additional electrodes beyond the first two shall be paid for as a change to the Work.

7 Grounding Electrode Conductor

7.1 Conductor Material

7.1.1 The grounding electrode conductor shall be of the material indicated in the datasheet.
Grounding Electrode Conductor Materialradio
● Copper
○ Aluminum
○ Copper-clad aluminum
NOTE A copper grounding electrode conductor runs in earth, in concrete and outdoors to the electrode without restriction and is the conductor nearly every project installs, because the conductor nearly always ends at an electrode in or near the earth; aluminum and copper-clad aluminum cost less and are chosen for a long run inside a building to a structural steel electrode, and each is barred by NFPA 70 250.64(A) from contact with masonry or earth, from corrosive conditions and from outdoor runs within 18 in. of earth, so that the last length to a buried electrode is copper in any case. (7.1.2)
7.1.3 Where aluminum or copper-clad aluminum is selected, the conductor shall not be installed in direct contact with masonry or earth, in a corrosive location, or outdoors within 18 in. of earth, and the final length to any electrode in earth or concrete shall be copper joined to the aluminum through a listed connector in accordance with NFPA 70 250.64(A).
7.1.4 Copper grounding electrode conductors shall be soft-drawn copper conforming to ASTM B3, stranded in accordance with ASTM B8 where larger than 8 AWG, and bare or insulated as the Continuity, Routing, and Protection article requires.

7.2 Conductor Size

7.2.1 The grounding electrode conductor at the service shall be sized in accordance with NFPA 70 Table 250.66 from the largest ungrounded service-entrance conductor or the equivalent area of parallel conductors, and shall be the size indicated in the datasheet.
Grounding Electrode Conductor Sizeselect
8 AWG
6 AWG
4 AWG
2 AWG
1/0 AWG
2/0 AWG
3/0 AWG
250 kcmil
Derived — the largest ungrounded service-entrance conductor, per NFPA 70 Table 250.66 (by default)
7.2.2 The portion of the grounding electrode conductor that is the sole connection to a rod, pipe or plate electrode is not required to be larger than 6 AWG copper; the portion that is the sole connection to a concrete-encased electrode is not required to be larger than 4 AWG copper; and the portion that is the sole connection to a ground ring is not required to be larger than the ring conductor, in accordance with NFPA 70 250.66(A) through (C).
7.2.3 Where one conductor serves more than one electrode, the full size from NFPA 70 Table 250.66 shall apply to every portion of the conductor that serves more than one of them.
7.2.4 The grounding electrode conductor shall not be sized from NFPA 70 Table 250.122.
NOTE Table 250.66 sizes the conductor from the service conductors, because the service has no overcurrent device ahead of it and the conductor must survive whatever the utility can deliver; Table 250.122 sizes an equipment grounding conductor from the breaker that protects the circuit. Reading one table for the other is the most common sizing error in a grounding submittal and produces a conductor a size or two wrong in either direction. (7.2.5)

7.3 Continuity, Routing, and Protection

7.3.1 The grounding electrode conductor shall be installed in one continuous length without splice or joint from the point of connection at the service or derived system to the electrode, except that it may be spliced by exothermic welding, by a listed irreversible compression connector, or at a ground bar, in accordance with NFPA 70 250.64(C).
7.3.2 The grounding electrode conductor shall be routed as directly as the structure permits, with no bend sharper than the conductor manufacturer permits, and shall be secured to the surface it follows at intervals not exceeding 5 ft.
7.3.3 The grounding electrode conductor shall be installed in the raceway indicated in the datasheet, and a conductor smaller than 6 AWG, and any conductor exposed to physical damage, shall be installed in rigid metal conduit, intermediate metal conduit, rigid PVC conduit, electrical metallic tubing or cable armor in accordance with NFPA 70 250.64(B) whatever the datasheet selects.
Grounding Electrode Conductor Racewayradio
○ None, conductor secured to the surface where 6 AWG or larger
○ Rigid PVC conduit
○ Ferrous metal raceway bonded to the conductor at each end
NOTE A bare conductor secured to the wall is the simplest installation, is permitted for 6 AWG and larger where it is not exposed to damage, and is what an electrical room with no traffic gets; rigid PVC conduit protects the conductor without the magnetic effect of a steel raceway and is the raceway most projects use where the conductor crosses a floor, a loading area or a mechanical room; a ferrous raceway gives the strongest protection and is chosen where the conductor runs in a space with vehicle or cart traffic, at the cost of bonding the raceway to the conductor at both ends, which NFPA 70 250.64(E) requires and which the field misses more often than any other grounding detail. (7.3.4)
7.3.5 Where a ferrous metal raceway or enclosure encloses the grounding electrode conductor, the raceway shall be electrically continuous from the enclosure to the electrode and shall be bonded to the conductor at each end and at every intermediate enclosure by a bonding jumper of the same size as the conductor, in accordance with NFPA 70 250.64(E).
NOTE A steel conduit around a conductor carrying surge or fault current acts as a choke that raises the impedance of the path many times over, because the current is alternating and the steel is magnetic. Bonding the conduit to the conductor at both ends puts the conduit in parallel with the conductor and cancels the effect; leaving it unbonded leaves the electrode system connected through a choke. (7.3.6)
7.3.7 An insulated grounding electrode conductor shall be green, green with a yellow stripe, or bare, and where a bare conductor is installed in a raceway with other conductors it shall be identified at each termination as the grounding electrode conductor.

8 Service Grounding and Main Bonding Jumper

8.1 Main Bonding Jumper

8.1.1 The grounded service conductor, the equipment grounding conductors and the service equipment enclosure shall be connected together by one main bonding jumper at the service disconnecting means, in accordance with NFPA 70 250.24(B) and 250.28, and the grounding electrode conductor shall be connected to the grounded conductor at the same point.
8.1.2 Where the service has more than one disconnecting means in separate enclosures, a main bonding jumper shall be installed in each, and the grounded conductor shall be run to each in accordance with NFPA 70 250.24(C).
8.1.3 The main bonding jumper shall be the bus, strap or screw furnished with the listed service equipment and identified by its manufacturer for that purpose, or, where the equipment furnishes none, a wire-type jumper of copper or aluminum sized in accordance with NFPA 70 250.28(D).
8.1.4 A wire-type main bonding jumper shall be not smaller than the size NFPA 70 Table 250.102(C)(1) gives for the service-entrance conductors, and where the service-entrance conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, not smaller than 12.5% of the area of the largest ungrounded conductor.
8.1.5 A bonding screw furnished as the main bonding jumper shall be green, shall be visible after installation, and shall be installed only in the service equipment, and the screw or strap furnished in every panelboard downstream of the service shall be removed and discarded.
NOTE The main bonding jumper is the single point at which the neutral and the equipment ground are joined for a service, and every fault anywhere downstream returns to the source through it. It is one of the few connections in the building whose absence is invisible until a fault, when the breaker does not trip and every enclosure on the system rises to line voltage. (8.1.6)

8.2 Bonding of Service Raceways and Enclosures

8.2.1 Every metal raceway, cable armor, auxiliary gutter, meter enclosure and other enclosure that contains service conductors shall be bonded together and to the service equipment by one of the means NFPA 70 250.92(B) recognizes: a threaded coupling or hub made up wrench-tight, a bonding-type locknut or bushing, or a bonding jumper installed around a knockout.
8.2.2 Standard locknuts and bushings shall not be relied on as the bonding means for any enclosure containing service conductors, and a bonding jumper shall be installed around every concentric, eccentric or oversized knockout in the service path.
8.2.3 Supply-side bonding jumpers shall be sized in accordance with NFPA 70 250.102(C) from the largest ungrounded supply conductor in the raceway or enclosure they bond.
NOTE A service raceway carries current from a fault on the service conductors with no overcurrent device ahead of it except the utility's, which can take seconds to clear. A single locknut cannot be assured to carry that current, which is why the bonding of the service path is a separate and stricter requirement than the bonding of the raceways downstream. (8.2.4)

8.3 Neutral Isolation Downstream of the Service

8.3.1 The grounded conductor shall not be connected to the equipment grounding conductors, to an enclosure or to a ground bar at any point on the load side of the service disconnecting means, except as NFPA 70 250.142(B) permits, and the neutral bus of every panelboard, switchboard and transfer switch downstream of the service shall be insulated from its enclosure.
8.3.2 Before energization, the Contractor shall verify at every downstream panelboard and distribution assembly that the neutral bus is isolated from the enclosure and that no bonding screw or strap remains in place, and shall record the verification on the continuity test report.
8.3.3 The grounding and bonding system shall be arranged so that no current flows on an equipment grounding conductor, a bonding jumper, a raceway or the structure under normal operation, in accordance with NFPA 70 250.6, and where such current is found it shall be removed by correcting the wiring error and never by interrupting a grounding or bonding connection.
NOTE A second neutral-to-ground bond at a downstream panel puts the building's neutral current onto every parallel path between that panel and the service: the conduit, the building steel, the water pipe and the equipment grounding conductors. The result is a current on the ground system that heats connections, corrodes the pipe at every joint, and couples noise into every signal cable in the building. The cure is to find the bond and remove it; opening a ground to stop the current leaves the fault path open too. (8.3.4)

8.4 Intersystem Bonding Termination

8.4.1 An intersystem bonding termination shall be provided at the service equipment of each building and at the disconnecting means of each separate building or structure supplied by a feeder, in accordance with NFPA 70 250.94, and shall be of the form indicated in the datasheet.
Intersystem Bonding Terminationradio
● Listed external termination device at the service equipment
○ Terminals integral to the meter or service enclosure
○ Separate bus bar near the service equipment
NOTE The listed external device mounts on the outside of the service equipment or on the grounding electrode conductor, offers its terminals without opening a cover, and is the form nearly every project installs; terminals integral to the meter or service enclosure are furnished by some equipment makers and save the device where they exist, at the cost of a termination the communications installer needs the enclosure opened to reach; a separate bus bar near the service equipment is the form a building with many communications entrances uses, where a dozen bonding conductors land and the three terminals of a listed device would not do. (8.4.2)
8.4.3 The intersystem bonding termination shall provide not fewer than three terminals, shall be accessible without removing a cover or a finish, shall be connected to the service equipment enclosure, to the grounded service conductor or to the grounding electrode conductor by a conductor not smaller than 6 AWG copper, and shall not interfere with the opening of any enclosure.
8.4.4 The protector, surge arrester or cable shield of every telecommunications, television, antenna, radio and coaxial system entering the building shall be bonded to the intersystem bonding termination or to the grounding electrode system by a conductor the standard governing that system sizes, in accordance with NFPA 70 Chapter 8 and Article 810.
NOTE The intersystem bonding termination exists so that the telephone, cable television, antenna and satellite installers who come after the building is finished have one labeled place to bond, instead of a clamp on the nearest water pipe. A protector grounded to a different electrode than the power system puts the difference in potential between the two electrodes across the equipment during a surge, which is how a lightning strike a mile away destroys a modem. (8.4.5)

9 Separately Derived Systems

9.1 System Bonding Jumper

9.1.1 Each separately derived system, including the secondary of every transformer, the output of every generator whose neutral is switched in the transfer switch, and the output of every uninterruptible power supply whose output neutral is not solidly connected to the input neutral, shall be grounded and bonded in accordance with NFPA 70 250.30 and this article.
9.1.2 A system bonding jumper shall be installed for each separately derived system, connecting the grounded conductor of the derived system to the equipment grounding conductor and the enclosure, at the location indicated in the datasheet and at no other point.
System Bonding Jumper Locationradio
● At the source
○ At the first disconnecting means
NOTE A system bonding jumper at the source, in the transformer or at the generator terminals, puts the bond where the derived neutral originates, makes the feeder from the source to the first disconnect a four-wire feeder with a separate equipment grounding conductor, and is where nearly every project puts it; a system bonding jumper at the first disconnecting means is chosen where the source is outdoors or inaccessible and the project wants the bond and the grounding electrode conductor in the switchboard, and it requires the supply-side bonding jumper between the source and the disconnect to be sized for the derived conductors under NFPA 70 250.30(A)(2). (9.1.3)
9.1.4 The system bonding jumper shall be sized in accordance with NFPA 70 250.28(D) from the largest ungrounded derived conductor, and shall be a bus, strap, screw or wire-type jumper as the Main Bonding Jumper article requires for a main bonding jumper.
9.1.5 The grounded conductor of a separately derived system shall be insulated from the enclosure at every point other than the system bonding jumper, and the Contractor shall remove any bonding strap or screw furnished at the point not selected.
NOTE Two system bonding jumpers on one derived system, one at the transformer and one at the panel it feeds, put the derived neutral current on the equipment grounding conductor between them, which is the downstream-bond defect of the Neutral Isolation Downstream of the Service article, moved one transformer down. NFPA 70 250.30(A)(1) says one point, and the datasheet is where that point is written down. (9.1.6)

9.2 Grounding Electrode Conductor of a Derived System

9.2.1 Each separately derived system shall be connected to the grounding electrode system by a grounding electrode conductor installed at the same point as the system bonding jumper, sized in accordance with NFPA 70 Table 250.66 from the largest ungrounded derived conductor, and arranged as indicated in the datasheet.
Derived System Grounding Electrode Conductor Arrangementradio
○ Individual conductor from each derived system to the nearest qualifying electrode
○ Taps from a common grounding electrode conductor
○ Connection to a ground bar bonded to the grounding electrode system
NOTE An individual conductor from each transformer to the nearest structural steel column or metal water pipe that is bonded to the electrode system is the arrangement NFPA 70 250.30(A)(5) describes first and the one a building with a few transformers uses; a common grounding electrode conductor of 3/0 copper or larger run through the building with a tap to each derived system under NFPA 70 250.30(A)(6) suits a high-rise or a plant with many transformers on one riser, where one conductor replaces a dozen; a ground bar in each electrical room, bonded to the electrode system and serving as the tap point for every derived system in the room, gives the same result with a landing the maintenance staff can see and test, and is the arrangement the electrical room standard provides for. (9.2.2)
9.2.3 The electrode a derived system is connected to shall be the nearest of a metal in-ground support structure that is bonded to the electrode system or a metal water pipe electrode within 5 ft of its entrance, and where neither is available, another electrode of the grounding electrode system, in accordance with NFPA 70 250.30(A)(5).
9.2.4 Where the common conductor arrangement is selected, the common grounding electrode conductor shall be not smaller than 3/0 AWG copper or 250 kcmil aluminum, each tap shall be sized from the derived conductors of the system it serves, and the taps shall be connected to the common conductor by exothermic welding, by listed irreversible compression connectors or at a ground bar, so that the common conductor remains without splice.
9.2.5 Where the ground bar arrangement is selected, the bar shall be bonded to the grounding electrode system by a conductor sized in accordance with NFPA 70 Table 250.66 from the largest derived system or service it serves, and the conductor from each derived system to the bar shall be sized from that system's derived conductors.
9.2.6 Where a structural steel column serves as the grounding electrode connection for a derived system, the structural metal shall itself be bonded to the grounding electrode system in accordance with the Bonding of Piping, Structure, and Other Metal Systems article, and the connection to the column shall be exothermic welded or a listed clamp applied to cleaned bare steel.

9.3 Generator, Transfer Switch, and UPS Sources

9.3.1 Whether a generator or uninterruptible power supply is a separately derived system follows from whether its neutral is switched in the transfer equipment or isolated from the normal source, a selection made under Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches, Generator Paralleling SwitchgearGenerator Paralleling SwitchgearResolves to the current adopted revision.sync/generator-paralleling-switchgear and Uninterruptible Power Supply SystemsUninterruptible Power Supply SystemsResolves to the current adopted revision.sync/uninterruptible-power-supply-systems, and the Contractor shall confirm that selection in writing before bonding the source.
9.3.2 Where the transfer equipment switches the neutral, the generator or UPS output shall be grounded and bonded as a separately derived system under this article, with its system bonding jumper at the source or at the first disconnecting means as the datasheet selects.
9.3.3 Where the transfer equipment does not switch the neutral, the generator or UPS output shall not be bonded to ground at the source, the neutral shall be carried unbroken to the service main bonding jumper, and the source frame, enclosure and sub-base tank shall be bonded to the equipment grounding conductor of the feeder only.
9.3.4 Where a generator set is shipped with its neutral bonded to its frame and the transfer equipment does not switch the neutral, the Contractor shall remove the factory bond and record the removal on the continuity test report.
NOTE A generator bonded at the set while the transfer switch carries a solid neutral has two neutral-to-ground bonds on the same system, one at the service and one at the generator, and the neutral current of every emergency load splits between the neutral and the ground path during a utility outage. A generator not bonded while the switch breaks the neutral has no ground reference at all when it is the only source. Either error passes every test performed on utility power and appears only in the first outage, which is why the neutral arrangement is confirmed in writing before the set is bonded. (9.3.5)

10 Equipment Grounding Conductors

10.1 Feeder and Branch Circuit Equipment Grounding

10.1.1 Every feeder shall carry the equipment grounding conductor indicated in the datasheet.
Feeder Equipment Grounding Conductorradio
● Wire-type conductor in every raceway and cable
○ Metallic raceway or cable armor permitted as the conductor
10.1.2 Every branch circuit shall carry the equipment grounding conductor indicated in the datasheet.
Branch Circuit Equipment Grounding Conductorradio
● Wire-type conductor in every raceway and cable
○ Metallic raceway or cable armor permitted as the conductor
NOTE NFPA 70 250.118 recognizes rigid metal conduit, intermediate metal conduit, electrical metallic tubing, listed cable armor and several other wiring methods as equipment grounding conductors in their own right, and a project that relies on them saves a conductor in every raceway. A wire-type conductor in every raceway gives a fault path that does not depend on every coupling, connector and locknut staying tight for the life of the building, survives a corroded or separated raceway, and is what nearly every commercial, institutional and industrial project specifies for feeders and branch circuits alike; relying on the raceway is the arrangement of a project under cost pressure with a well-supervised raceway installation, and it is more often accepted for branch circuits in threaded conduit than for feeders, which is why the two circuit classes are selected separately. (10.1.3)
10.1.4 Where a metallic raceway or cable armor is relied on as the equipment grounding conductor, every joint shall be made up wrench-tight, every connector and coupling shall be listed for grounding, no raceway run shall include a nonmetallic section, expansion fitting or flexible section without a bonding jumper across it, and the raceway shall be bonded at each enclosure by a means the Bonding of Service Raceways and Enclosures article recognizes wherever a concentric or eccentric knockout or a reducing washer interrupts the metal-to-metal contact.
10.1.5 A wire-type equipment grounding conductor shall be installed in every raceway and cable within a hazardous (classified) location and in the raceway path from that location back to the service or the derived system, and in every raceway and cable serving a patient care space, an isolated-ground circuit, a swimming pool or fountain, or an agricultural building, whatever the datasheet selects.
10.1.6 The equipment grounding conductor of every circuit shall be run in the same raceway, cable or trench as the circuit conductors it serves, in accordance with NFPA 70 300.3(B) and 250.134.

10.2 Flexible Conduit and Cord

10.2.1 The equipment grounding of every length of flexible metal conduit, liquidtight flexible metal conduit and liquidtight flexible nonmetallic conduit shall be as indicated in the datasheet.
Flexible Conduit Equipment Groundingradio
● Wire-type conductor in every length of flexible conduit
○ Flexible metal conduit relied on as the conductor within the limits of NFPA 70 250.118
NOTE NFPA 70 250.118 permits flexible metal conduit and liquidtight flexible metal conduit to serve as the equipment grounding conductor only in short lengths, on small circuits, with listed fittings and where flexibility is not needed after installation. A wire-type conductor in every length of flex removes the question for the inspector and the maintenance electrician and is what nearly every project specifies, because a flex whip to a motor or a transformer is exactly the place where vibration loosens a fitting; relying on the flex within the code limits is the arrangement of a lighting whip or a short connection to a fixed appliance on a project that accepts the limits and polices them. (10.2.2)
10.2.3 Liquidtight flexible nonmetallic conduit and every flexible connection to equipment that vibrates or is moved for service shall contain a wire-type equipment grounding conductor sized for the circuit, whatever the datasheet selects.
10.2.4 Cord-and-plug-connected equipment shall be grounded through the equipment grounding conductor of the cord and the grounding contact of the plug, and the box or enclosure the receptacle is mounted in shall be bonded to the circuit's equipment grounding conductor in accordance with NFPA 70 250.146 and 250.148.

10.3 Wire-Type Conductor Material and Covering

10.3.1 Wire-type equipment grounding conductors shall be of the material indicated in the datasheet.
Wire-Type Equipment Grounding Conductor Materialradio
● Copper
○ Aluminum
○ Copper-clad aluminum
NOTE A copper equipment grounding conductor terminates on any listed lug, runs with copper or aluminum circuit conductors, and is what nearly every project installs; an aluminum or copper-clad aluminum conductor costs less on a large feeder and is chosen where the feeder conductors are aluminum and the terminations are already rated for it, and NFPA 70 250.120(B) bars it from direct contact with masonry, from earth, from corrosive conditions and from termination within 18 in. of earth. (10.3.2)
10.3.3 Wire-type equipment grounding conductors shall be covered as indicated in the datasheet, and an insulated or covered conductor shall be continuous green or green with one or more yellow stripes along its entire length, in accordance with NFPA 70 250.119, except that a conductor larger than 6 AWG may be identified green at each end and at every point where it is accessible.
Wire-Type Equipment Grounding Conductor Coveringradio
● Insulated, green
○ Bare
NOTE An insulated green conductor is identifiable in a crowded box or gutter and cannot touch a live terminal in a panel, and it is what nearly every project specifies in raceway; a bare conductor is permitted by NFPA 70, saves the insulation cost on a large feeder, is the conductor inside a listed cable assembly, and is chosen for a feeder in a dedicated raceway or a tray where nothing else can contact it. (10.3.4)
10.3.5 Green or green-with-yellow-stripe insulation shall not be used for any conductor other than an equipment grounding conductor or a grounding electrode conductor.

10.4 Equipment Grounding Conductor Size

10.4.1 Every wire-type equipment grounding conductor shall be sized in accordance with NFPA 70 Table 250.122 from the rating of the overcurrent device ahead of the circuit, and shall not be required to be larger than the circuit conductors it accompanies.
10.4.2 Where the ungrounded conductors of a circuit are increased in size above the minimum their ampacity requires, for voltage drop or any other reason, the wire-type equipment grounding conductor shall be increased in size in proportion to the increase in circular mil area of the ungrounded conductors, in accordance with NFPA 70 250.122(B), and the Contractor shall verify the increase on every feeder shown upsized on the one-line diagram and feeder schedule.
10.4.3 Where one equipment grounding conductor serves several circuits in one raceway, cable or tray, it shall be sized from the largest overcurrent device protecting any of those circuits, in accordance with NFPA 70 250.122(C).
10.4.4 Where circuit conductors are installed in parallel in more than one raceway or cable, a wire-type equipment grounding conductor shall be installed in each raceway or cable and each shall be sized for the full rating of the overcurrent device, in accordance with NFPA 70 250.122(F).
10.4.5 The equipment grounding conductor to a motor shall be sized from the rating of the branch-circuit short-circuit and ground-fault protective device, in accordance with NFPA 70 250.122(D).
NOTE A feeder upsized two sizes for voltage drop with its equipment grounding conductor left at the table value is the defect a reviewer finds most often in a feeder schedule, and it leaves the fault path with a higher impedance than the circuit it protects. The proportional rule exists so that the fault current on a long feeder still rises high enough, fast enough, to open the breaker. (10.4.6)

10.5 Bonding of Enclosures, Boxes, and Equipment Frames

10.5.1 Every enclosure, cabinet, box, frame, rack, pedestal, pole, tray, wireway, busway housing, motor frame, transformer case, generator frame and other normally non-current-carrying metal part of electrical equipment shall be bonded to the equipment grounding conductor of the circuit that serves it, by a listed terminal, lug, bonding jumper or grounding screw, in accordance with NFPA 70 250.110 through 250.136.
10.5.2 Where an equipment grounding conductor enters a metal box, it shall be bonded to the box by a listed grounding screw used for no other purpose, a listed grounding clip or a listed device, and every equipment grounding conductor in the box shall be spliced or joined so that the removal of a device does not interrupt the grounding continuity of the other conductors, in accordance with NFPA 70 250.148.
10.5.3 Every metal wireway, auxiliary gutter and cable tray shall be made electrically continuous across each joint, shall be bonded to the equipment grounding conductor of the largest circuit it contains, and where it contains more than one circuit shall carry or be bonded to an equipment grounding conductor sized in accordance with the Equipment Grounding Conductor Size article for that circuit.
10.5.4 The ground bus of every switchgear, switchboard, motor control center, drive lineup and other assembly rated above or below 1000 V shall be bonded to the grounding electrode system by a conductor sized in accordance with NFPA 70 Table 250.66 from the service or derived conductors supplying the assembly, or in accordance with Table 250.122 from the feeder overcurrent device where the assembly is fed by a feeder.
10.5.5 The grounding terminal of every industrial control panel, instrument enclosure, network cabinet and equipment rack shall be bonded to the equipment grounding conductor of the circuit serving it, and an isolated instrument or signal reference ground bus furnished inside such an enclosure under Industrial Control PanelsIndustrial Control PanelsResolves to the current adopted revision.sync/industrial-control-panels shall be connected to the grounding electrode system by one insulated conductor not smaller than 6 AWG copper at one point only.
10.5.6 Every metal pole shall be bonded to the equipment grounding conductor of its circuit at a grounding terminal inside the handhole, and the grounding terminal of every meter, transmitter, valve actuator and similar field instrument shall be bonded to the equipment grounding conductor of the raceway or cable serving it.
10.5.7 Where a raised access floor understructure, an equipment room grid or a signal reference grid is used as part of the grounding system, the understructure or grid shall be bonded to the room ground bar and to the equipment grounding conductors of the circuits in the room, and shall supplement and never replace the equipment grounding conductor of any circuit.

11 Isolated-Ground Circuits

11.1 Isolated-ground branch circuits shall be provided as indicated in the datasheet, and where provided shall serve the outlets indicated on the electrical plans.
Isolated-Ground Branch Circuitsradio
● Not used
○ Provided
NOTE An isolated-ground circuit carries an insulated equipment grounding conductor that passes through the boxes and panelboards on its route without bonding to them, so that the grounding terminal of the receptacle sees only the grounding point of its source and not the noise current circulating on the raceway system. It is specified for recording, laboratory and some medical instrumentation whose maker requires it, and it is specified far more often than that for ordinary office and data equipment whose power supplies do not need it, where it adds a conductor, an insulated bus and a receptacle to every outlet and buys nothing measurable. (11.2)
11.3 The insulated equipment grounding conductor of each isolated-ground circuit shall be routed with the circuit conductors and shall follow the path indicated in the datasheet to the grounding point of the service or the separately derived system that supplies the circuit, in accordance with NFPA 70 250.146(D).
Isolated Grounding Conductor Pathradio
● Collected on an isolated ground bus in the serving panelboard, with one insulated conductor from that bus to the grounding point of the source
○ Run unbroken from each outlet to the grounding point of the source
NOTE Collecting the isolated conductors on an insulated bus in the panelboard, furnished under PanelboardsPanelboardsResolves to the current adopted revision.sync/panelboards, and running one insulated conductor from that bus to the transformer or service grounding point is the arrangement nearly every isolated-ground design uses, because it keeps the home runs short and gives the maintenance electrician one conductor to test; running each conductor unbroken to the source is the arrangement an instrument maker's grounding specification sometimes requires, and it multiplies the conductors in the feeder raceway by the number of outlets. (11.4)
11.5 The isolated equipment grounding conductor shall be insulated along its entire length, shall not be bonded to any box, raceway, enclosure or panelboard cabinet it passes through, and shall terminate only at the isolated ground bus or at the grounding point of the source.
11.6 The raceway, the boxes and the ordinary equipment grounding conductor of an isolated-ground circuit shall be installed and bonded as for any other circuit, and the isolated conductor shall be in addition to, and never in place of, that grounding path.
11.7 The isolated equipment grounding conductor shall not be connected to a separate electrode, to the structure or to any point other than the grounding point of the source, and an isolated-ground circuit that is grounded only to an electrode is a code violation that leaves the receptacle with no fault-clearing path.
11.8 Every isolated-ground receptacle, furnished under Wiring DevicesWiring DevicesResolves to the current adopted revision.sync/wiring-devices, shall be identified by an orange triangle on its face, and every isolated-ground circuit shall be identified as such in the panelboard directory.

12 Bonding of Piping, Structure, and Other Metal Systems

12.1 Code-Required Bonding of Piping and Structural Metal

12.1.1 The metal water piping system of each building shall be bonded to the service equipment enclosure, to the grounded service conductor, to the grounding electrode conductor or to a grounding electrode, in accordance with NFPA 70 250.104(A), by a bonding jumper sized in accordance with NFPA 70 Table 250.102(C)(1) from the service-entrance conductors and of the size indicated in the datasheet.
Water Piping Bonding Jumper Sizeselect
8 AWG
6 AWG
4 AWG
2 AWG
1/0 AWG
2/0 AWG
3/0 AWG
250 kcmil
350 kcmil
500 kcmil
Derived — the service-entrance conductors, per NFPA 70 Table 250.102(C)(1) (by default)
12.1.2 The metal water piping in the area served by each separately derived system shall be bonded to the grounded conductor at the system bonding jumper, to the grounding electrode conductor of that system or to its electrode, by a jumper sized in accordance with NFPA 70 Table 250.102(C)(1) from the derived conductors, in accordance with NFPA 70 250.104(D).
12.1.3 Every metal gas piping system, including corrugated stainless steel tubing, shall be bonded to the service equipment enclosure, the grounded service conductor, the grounding electrode conductor or a grounding electrode by a jumper sized in accordance with NFPA 70 Table 250.122 from the largest circuit likely to energize the piping and not smaller than 6 AWG copper, in accordance with NFPA 70 250.104(B), and the bonding of the gas piping shall be coordinated with the connection point and the fittings Natural Gas PipingNatural Gas PipingResolves to the current adopted revision.sync/natural-gas-piping and Fuel Gas PipingFacility Liquefied-Petroleum and Fuel Gas PipingResolves to the current adopted revision.sync/fuel-gas-piping provide for it.
12.1.4 Exposed structural metal that is interconnected to form a building frame and is not intentionally grounded or bonded shall be bonded to the service equipment enclosure, the grounded service conductor, the grounding electrode conductor or a grounding electrode by a jumper sized in accordance with NFPA 70 Table 250.102(C)(1) from the service-entrance conductors, in accordance with NFPA 70 250.104(C), and the connection shall be made to a column at a point cleaned to bare steel.
12.1.5 Other metal piping that is likely to become energized, including sprinkler, process, compressed air, fuel oil and hydronic piping with an electric pump, heater or valve on it, shall be bonded in accordance with NFPA 70 250.104(B) by the equipment grounding conductor of the circuit likely to energize it, and the equipment grounding conductor of that circuit shall be the bonding means.
12.1.6 Every bonding jumper to piping or structure shall be connected by a listed pipe clamp, a listed clamp for structural steel, an exothermic weld or a listed lug on a tapped hole, applied to a surface cleaned to bare metal, and shall be accessible except where it is exothermic welded or is a listed connector embedded in concrete.
NOTE The water pipe, the gas pipe and the building steel are bonded so that a fault that energizes them, from a water heater element, a furnace igniter or a conduit chafed against a beam, trips the breaker instead of leaving the pipe at line voltage for the next person to touch it. The bond is sized from the service, not from the circuit, because the pipe can be energized by any circuit in the building. (12.1.7)

12.2 Supplementary Bonding of Metal Systems

12.2.1 The metal systems indicated in the datasheet shall be bonded to the grounding electrode system or to the equipment grounding conductor of the nearest electrical equipment, in addition to the systems NFPA 70 250.104 requires, by a conductor not smaller than 6 AWG copper connected by a listed clamp, lug or exothermic weld at a point cleaned to bare metal.
Supplementary Bonding of Metal Systemscheckbox
☐ Metal duct systems
☐ Fire sprinkler and standpipe piping
☐ Process and compressed air piping
☐ Metal roof and wall panels
☐ Metal doors, frames, ladders and racks in electrical rooms
☐ Water storage tanks
☐ Fuel storage tanks and fuel piping
☐ Static-dissipative flooring ground connections
NOTE None of these systems is required by NFPA 70 to be bonded unless it is likely to become energized, and a project adds them for reasons of its own: metal duct and sprinkler piping to give a lightning-exposed or electronics-heavy building an equipotential structure, metal roof and wall panels where a lightning protection system under Lightning ProtectionLightning Protection SystemsResolves to the current adopted revision.sync/lightning-protection or a corporate standard asks for it, the doors, frames, ladders and racks of an electrical room under Electrical RoomsElectrical RoomsResolves to the current adopted revision.sync/electrical-rooms so that nothing a worker touches during a fault is at a different potential from the equipment, water and fuel tanks under Water Storage TanksWater Storage TanksResolves to the current adopted revision.sync/water-storage-tanks, Fuel Dispensing And Storage TanksFuel Storage TanksResolves to the current adopted revision.sync/fuel-dispensing-and-storage-tanks and Fuel Oil SystemsFacility Fuel-Oil SystemsResolves to the current adopted revision.sync/fuel-oil-systems for lightning and static, and the connections of a static-dissipative floor under Resinous FlooringResinous FlooringResolves to the current adopted revision.sync/resinous-flooring because the floor's maker requires a ground reference. Each adds conductors and clamps and buys an equipotential surface where the project values one. (12.2.2)
12.2.3 Where fuel storage tanks and fuel piping are selected, the tank, its fill and vent connections and its piping shall be bonded to the grounding electrode system, and a listed bonding terminal for the delivery vehicle's static bond shall be provided within 3 ft of the fill connection.
12.2.4 Where a cathodically protected tank or piping is bonded under this article, the bond shall be made through the decoupling device Cathodic ProtectionCathodic ProtectionResolves to the current adopted revision.sync/cathodic-protection furnishes and shall not be omitted.

12.3 Bonding Across Fittings and Meters

12.3.1 A bonding jumper shall be installed around every water meter, filter, strainer, dielectric fitting, flexible connector, expansion joint and other removable or insulating device in a bonded piping system, sized as the bonding jumper for that system is sized, long enough to permit removal of the device without disconnecting the jumper.
12.3.2 A bonding jumper shall be installed across every expansion fitting, telescoping section, flexible section and nonmetallic section of a metal raceway system, and across every joint in a raceway run that is not made up by a threaded or listed grounding coupling.
12.3.3 Where an underground duct bank enters a manhole, vault or building, the metallic conduits, the cable racks and the pulling irons of each personnel-accessible structure shall be bonded to a rod electrode or a ground bar in the structure that is bonded to the grounding electrode system, and the structure's grounding provisions furnished under Utility Manholes And HandholesUtility Manholes and HandholesResolves to the current adopted revision.sync/utility-manholes-and-handholes, or under Site Concrete StructuresCast-in-Place Site Concrete StructuresResolves to the current adopted revision.sync/site-concrete-structures for a structure cast in place, shall be connected under this standard.

12.4 Bonding in Hazardous (Classified) Locations

12.4.1 Every metal raceway, fitting, box and enclosure within a hazardous (classified) location, and every one in the raceway path from that location back to the service equipment or to the grounding point of the separately derived system that supplies it, shall be bonded by a bonding jumper with listed fittings, by a bonding-type locknut or bushing, or by a threaded connection made up wrench-tight, in accordance with NFPA 70 250.100, and never by a standard locknut and bushing alone, regardless of the voltage of the circuit.
12.4.2 The bonding means within the classified location and the seals, drains and wiring methods there are governed by Electrical Hazardous LocationsElectrical Installations in Hazardous (Classified) LocationsResolves to the current adopted revision.sync/electrical-hazardous-locations, and this standard governs the bonding of the raceway path outside the location back to the source.
NOTE A loose locknut outside a classified location is as much a source of ignition for the fault current passing through it toward the location as one inside, because the current from a fault in the wet well flows through every fitting between the wet well and the switchboard. That is why the bonding requirement runs all the way back to the source. (12.4.3)

13 Interfaces with Other Grounded Systems

13.1 Lightning Protection System

13.1.1 Where a lightning protection system is installed under Lightning ProtectionLightning Protection SystemsResolves to the current adopted revision.sync/lightning-protection, its grounding electrodes shall be bonded to the grounding electrode system of the building by a main-size conductor of that system connected at the point indicated on the grounding riser diagram, in accordance with NFPA 70 250.106 and NFPA 780, and the common bonding connection shall be made under this standard.
13.1.2 The connection between the two systems shall be exothermic welded or a listed irreversible compression connection, and shall be made to the grounding electrode conductor, to a ground bar bonded to the electrode system or to the ground ring, and never to a panelboard ground bus alone.
13.1.3 The lightning protection electrodes shall not be used as the grounding electrode system of the electrical service, and the electrical service electrodes shall not be used as the lightning protection electrodes.
NOTE A building with a lightning ground and an electrical ground that are not bonded has two earth references that can differ by tens of thousands of volts during a strike, and that difference appears across every circuit that touches both. Bonding them makes the whole building rise and fall together, which is the only condition under which nothing inside it is damaged. (13.1.4)

13.2 Telecommunications Bonding

13.2.1 The telecommunications bonding conductor from the primary bonding busbar shall be connected to the grounding electrode system at the service equipment ground bus, at the grounding electrode conductor or at a ground bar bonded to the electrode system, at the point indicated on the grounding riser diagram, and that connection shall be made under this standard.
13.2.2 The connection of the telecommunications bonding conductor at the electrode system shall be exothermic welded or a listed irreversible compression connection, shall be made to a terminal that is not shared with any other conductor, and shall be the only point at which the telecommunications bonding infrastructure is connected to the electrode system.
13.2.3 The size and routing of the telecommunications bonding conductor and everything downstream of the primary bonding busbar are governed by Telecommunications BondingTelecommunications Bonding and GroundingResolves to the current adopted revision.sync/telecommunications-bonding and ANSI/TIA-607, and the support and protection of that conductor where it runs through electrical spaces shall be as this standard requires for a grounding electrode conductor.

13.3 Cathodic Protection Interface

13.3.1 Where a cathodic protection system under Cathodic ProtectionCathodic ProtectionResolves to the current adopted revision.sync/cathodic-protection protects a structure that NFPA 70 Article 250 requires to be bonded, the bond shall be installed under this standard, the decoupling device in the bond shall be furnished and tested under that standard, and no electrical safety bond shall be omitted, removed or interrupted on account of the cathodic protection system.
13.3.2 The rectifier enclosure and the anode junction box of a cathodic protection system shall be bonded to the equipment grounding conductor of the rectifier branch circuit as electrical equipment under this standard, and the rectifier negative output shall not be connected to the equipment grounding conductor.

13.4 Direct-Current Control Systems

13.4.1 Each direct-current control system supplied by a station battery under DC Battery SystemsDC Battery SystemsResolves to the current adopted revision.sync/dc-battery-systems shall be grounded or left ungrounded as indicated in the datasheet, in accordance with NFPA 70 250.162 and 250.164.
Direct-Current Control System Groundingradio
● Ungrounded, with ground-fault detection
○ One pole solidly grounded
○ Battery midpoint grounded
NOTE An ungrounded DC control system keeps operating through its first ground fault, which is the property a protective-relay battery is installed for, and NFPA 70 250.162(A) permits it for an industrial system equipped with a ground detector; it is the arrangement of nearly every switchgear, generating station and plant control battery. A solidly grounded pole trips the circuit on the first ground fault and is chosen where the DC system serves equipment whose maker requires a grounded supply, such as some telecommunications rectifier plants; a grounded battery midpoint halves the voltage to ground of each pole and is the arrangement of a few high-voltage battery systems and some traction-style supplies. (13.4.2)
13.4.3 Where the DC system is grounded, the grounding electrode conductor shall be sized in accordance with NFPA 70 250.166 and connected to the grounding electrode system at the source, and the DC bonding jumper shall be sized in accordance with NFPA 70 250.168.
13.4.4 Where the DC system is ungrounded, a ground detector shall be furnished under DC Battery SystemsDC Battery SystemsResolves to the current adopted revision.sync/dc-battery-systems, and the battery rack, the battery cabinet, the charger enclosure, the DC panel enclosure and the raceway of the DC circuits shall be bonded to the equipment grounding conductor of the AC circuit supplying the charger and to the building grounding electrode system.

14 Special Occupancy Provisions

14.1 Where the project includes any of the occupancies indicated in the datasheet, the grounding and bonding of that occupancy shall meet the additional provisions of this article and of the NFPA 70 article that governs it, and those provisions shall govern over the general requirements of this standard where they differ.
Special Occupancy Grounding Provisionscheckbox
☐ Patient care spaces
☐ Swimming pools, spas, fountains and water features
☐ Agricultural buildings
☐ Information technology equipment rooms
14.2 In every patient care space, each branch circuit shall be installed in a metal raceway or cable armor that qualifies as an equipment grounding conductor and shall also contain an insulated copper equipment grounding conductor, so that the grounding path is redundant, in accordance with NFPA 70 517.13, and the receptacles and fixed equipment in the space shall be bonded through both paths; the essential electrical system that supplies those circuits is governed by Healthcare Essential Electrical SystemsHealthcare Essential Electrical SystemsResolves to the current adopted revision.sync/healthcare-essential-electrical-systems.
14.3 At every permanently installed swimming pool, spa, hot tub and fountain, the reinforcing steel or a copper grid in the shell and deck, the metal forming shells, ladders, rails, fittings and fences within the distances NFPA 70 680.26 states, the pump motors and the metal piping shall be bonded together by solid copper conductor not smaller than 8 AWG into one equipotential bonding grid, and the grid shall be connected to the equipment grounding conductor of the pool equipment and not to a separate electrode.
14.4 In every agricultural building with livestock, an equipotential plane shall be installed in the concrete floor of each area where livestock stand and electrical equipment is present, bonded to the equipment grounding conductor of the building's electrical system by solid copper conductor not smaller than 8 AWG, in accordance with NFPA 70 547.10.
14.5 In every information technology equipment room, the signal reference grid, the raised floor understructure under Raised Access FlooringRaised Access FlooringResolves to the current adopted revision.sync/raised-access-flooring, the equipment racks and the power distribution units shall be bonded to the room ground bar and to the equipment grounding conductors of the circuits in the room in accordance with NFPA 70 645.15 and IEEE 1100, and no isolated or separate electrode shall be installed for the room's equipment.
NOTE Each of these occupancies puts a person, an animal or a sensitive load in contact with grounded metal under conditions where a few volts of difference matters: a patient with a catheter in a vessel, a swimmer in water, a cow on wet concrete, a server whose signal reference drifts. The code answer in every case is the same, more bonding and never a separate ground, because a separate electrode is exactly what creates the difference in potential. (14.6)

15 Ground Bars

15.1 Ground bars shall be provided where indicated on the grounding riser diagram and wherever another standard requires one, and each shall be a copper bar conforming to ASTM B187, not less than 1/4 in. thick and 2 in. high, of the length the number of conductors landed on it requires with not fewer than four spare terminal positions, predrilled for two-hole compression lugs on NEMA spacing, and mounted on listed insulating standoffs.
15.2 Ground bars shall be finished as indicated in the datasheet.
Ground Bar Platingradio
● Electro-tin plated
○ Bare copper
NOTE A tin-plated bar presents a surface that does not oxidize under a lug, accepts aluminum and copper lugs without an antioxidant compound, and is what nearly every ground bar on a building project is; a bare copper bar is cheaper, is the bar a telecommunications standard sometimes names for its busbars, and needs its contact surfaces cleaned and treated at every termination. (15.3)
15.4 Every ground bar that serves as the grounding point or tap point of one or more separately derived systems shall be bonded to the grounding electrode system by a conductor sized in accordance with NFPA 70 Table 250.66 from the largest service or derived system it serves, and every ground bar that serves only to collect equipment grounding conductors shall be bonded by a conductor sized in accordance with NFPA 70 Table 250.122 from the largest overcurrent device supplying the equipment landed on it.
15.5 Each conductor landed on a ground bar shall terminate in its own two-hole compression lug or its own listed terminal, no terminal shall carry more than one conductor unless it is listed for that number, and the bar shall be labeled as the Identification article requires.
NOTE A ground bar in an electrical room, furnished where Electrical RoomsElectrical RoomsResolves to the current adopted revision.sync/electrical-rooms requires one, is the place the room's equipment grounds, the derived system taps and the bonding conductors of the room's doors, racks and trays are brought together and can be seen and tested; a bar that is connected to the electrode system by a conductor too small for the derived systems it serves defeats that purpose, which is why the bar's bonding conductor is sized from the largest system landed on it and not from the bar's own duty as an equipment ground. (15.6)

16 Connections and Terminations

16.1 Inaccessible Connections

16.1.1 Every connection that is buried in earth, encased in concrete, installed in a wall or above a ceiling that is not accessible, or otherwise inaccessible after construction shall be made by one of the methods indicated in the datasheet.
Connection Methods for Inaccessible Connectionscheckbox
Any of:
☑ Exothermic welded
☑ Listed irreversible compression connector
NOTE An exothermic weld fuses the conductors into one piece of copper that carries fault current without a contact interface, survives in earth as long as the conductor, and needs a trained operator, a dry mold and the heat and smoke a hot-work permit covers; an irreversible compression connector listed for direct burial does the same job with a hydraulic tool and a matched die, needs no hot work and no mold inventory, and is the method a project in a finished building, in a fuel area or under a no-hot-work rule uses. Most projects accept either, and the datasheet narrows the choice where an Owner's standard or a site condition excludes one. (16.1.2)
16.1.3 Mechanical connectors, bolted clamps and set-screw lugs shall not be installed in any location that becomes inaccessible.
16.1.4 Every exothermic weld shall be made with the mold and weld metal the connection system's manufacturer specifies for the conductor sizes and surfaces joined, in a clean, dry mold, on conductors cleaned to bright metal, and a weld showing porosity, slag inclusion, incomplete fill or a conductor that moves in the weld shall be cut out and remade.
16.1.5 Every compression connection shall be made with the connector, die and tool the connector's listing names for the conductors joined, with the crimp fully closed and the die's identification embossed on the connector, and a connection crimped with a die not listed for the connector shall be cut out and remade.

16.2 Accessible Connections

16.2.1 Every connection that remains accessible after construction shall be made by one of the methods indicated in the datasheet, and shall be made by a means NFPA 70 250.70 recognizes.
Connection Methods for Accessible Connectionscheckbox
Any of:
☑ Exothermic welded
☑ Listed irreversible compression connector
☑ Listed mechanical connector or clamp
NOTE An accessible connection can be inspected, tested and retightened, so a listed mechanical connector, a bolted clamp on a pipe or a column or a two-hole lug on a bar is acceptable and is what most accessible connections are; exothermic and compression connections are specified at accessible points where the Owner wants no connection in the system that can loosen, where the connection carries lightning current, or where a clamp would be exposed to vibration or tampering. (16.2.2)
16.2.3 Every connection to a ground bar, an enclosure or equipment shall be made with a listed two-hole compression lug or a listed terminal, and a single-hole lug shall be used only where the equipment provides a single tapped hole for it.
16.2.4 Solder shall not be used in any grounding or bonding connection, and a connection shall not depend on solder for its mechanical or electrical integrity.
16.2.5 Connections to a rod electrode shall be made by exothermic welding or by a listed ground rod clamp of a metal compatible with the rod, and a clamp shall be used for one conductor only unless listed for more.

16.3 Surface Preparation and Torque

16.3.1 Every surface a connection is made to shall be cleaned of paint, enamel, mill scale, rust, oxide and coating to bare bright metal over the full contact area before the connection is made, and where the connection joins aluminum to any other metal, a listed antioxidant compound shall be applied to the cleaned aluminum before the connection is closed.
16.3.2 Where a protective coating is removed to make a connection, the exposed metal around the completed connection shall be recoated with a corrosion-inhibiting coating compatible with the original finish, and a buried or embedded connection between dissimilar metals shall be coated with a listed corrosion-inhibiting compound over its entire surface.
16.3.3 Every bolted and mechanical grounding connection shall be tightened with a calibrated torque tool to the value the connector manufacturer publishes, or to the value ANSI/NETA ATS publishes for the bolt size where the manufacturer publishes none, and each torqued connection shall be witness-marked.
16.3.4 A connector between dissimilar metals shall be listed for that combination, and a copper connector or conductor shall not be placed in direct contact with galvanized steel, aluminum or a reinforcing bar in a damp, wet or buried location except through a listed bimetallic connector.

17 Testing

17.1 Testing Organization and Report

17.1.1 The field tests of this article shall be performed by the testing organization, with the instruments, under the safety practice and with the report that Electrical Acceptance TestingElectrical Acceptance TestingResolves to the current adopted revision.sync/electrical-acceptance-testing requires, and the grounding electrode system shall be an equipment class within that standard's acceptance test program.
17.1.2 This standard governs the method of the electrode resistance test, the value it is judged against, the scope of the continuity verification and the timing of each test, and that standard governs who performs it, the instrument calibration and the report.
17.1.3 Every test shall be performed after the electrode system and the connections under test are complete and before the system served is energized, and the electrode resistance test shall be repeated after any electrode is added.

17.2 Electrode Resistance Test

17.2.1 The resistance to earth of each grounding electrode system shall be measured by the method indicated in the datasheet, in accordance with IEEE 81.
Electrode Resistance Test Methodselect
Fall-of-potential, three-point, per IEEE 81
Clamp-on, induced frequency
Selective, clamp with current and potential probes
Slope method per IEEE 81
NOTE The fall-of-potential test disconnects the electrode system from the service, drives two test probes out into the earth and measures the resistance directly; it is the reference method, needs the system de-energized or the electrode isolated and open ground for the probes, and is the method nearly every acceptance test uses. A clamp-on tester measures the loop resistance through the electrode and every parallel path back through the utility neutral without disconnecting anything, in minutes, and reads nothing useful on an isolated electrode with no parallel path, which makes it a maintenance method and not an acceptance method for a new electrode system. The selective method combines a clamp with probes so that one electrode of a bonded system can be measured without disconnecting it, and is the method for a site where the electrode system cannot be isolated from the utility. The slope method is a fall-of-potential variant for a large electrode system, a ring or a grid, where the probe spacing a plain fall-of-potential test needs cannot be had on the site. (17.2.2)
17.2.3 Where the fall-of-potential or slope method is selected, the electrode system shall be isolated from the utility neutral and from every other grounded system during the test, the current probe shall be placed at a distance not less than five times the longest dimension of the electrode system, and readings shall be taken at not fewer than three potential probe positions to confirm that the measurement lies on the flat portion of the curve.
17.2.4 The electrode resistance test shall be performed with the soil at its natural moisture, not during rain and not within 48 hours after rainfall that wets the soil to the depth of the electrodes, and the date, the weather of the preceding 48 hours, the soil condition and the air temperature shall be recorded with each reading.
17.2.5 The test shall be performed on the complete electrode system and in addition on each rod, plate or electrolytic electrode individually before it is bonded into the system, and the individual readings shall be recorded.
17.2.6 A measured resistance exceeding the value in the Resistance to Earth Target article is a deficiency, to be corrected as that article requires.
NOTE A resistance measured the day after a storm is the lowest the system will ever read and says nothing about August. The individual rod readings are recorded because a system that meets the target with one rod carrying it has no margin when that rod corrodes, and because the readings are the only way to find a rod that was driven into a void. (17.2.7)

17.3 Bonding and Continuity Verification

17.3.1 The continuity and the resistance of the bonding and equipment grounding paths indicated in the datasheet shall be verified with a low-resistance ohmmeter applying not less than 10 A direct current, and every measured path shall read 0.1 Ω or less across each connection and 1 Ω or less from the point tested to the service or derived-system grounding point.
Bonding and Continuity Verification Scopecheckbox
☑ Main bonding jumper and service raceway bonding
☑ System bonding jumper and grounding electrode conductor of each separately derived system
☑ Piping and structural metal bonding jumpers
☑ Intersystem bonding termination
☑ Ground bar bonding conductors
☑ Equipment grounding path from representative outlets and equipment to the source
☐ Isolated-ground circuits from each receptacle to the source grounding point
☐ Lightning protection and telecommunications common bonds
17.3.2 Where the equipment grounding path from representative outlets and equipment is selected, not fewer than one outlet or item of equipment on each branch circuit panelboard and every item of equipment rated 100 A or more shall be tested, and where any tested path fails, every path on that panelboard shall be tested.
17.3.3 Every patient care space, where that occupancy is selected, shall in addition receive the grounding system test NFPA 99 requires, with the voltage and the impedance between the reference grounding point and every exposed conductive surface and receptacle grounding contact measured and recorded.
NOTE The line-to-ground fault current a short-circuit study under Short Circuit StudyShort-Circuit StudyResolves to the current adopted revision.sync/short-circuit-study calculates exists only if the bonding path it assumes exists, and the continuity test is what proves the path. A connection that reads 0.5 Ω is one that heats, and a path that reads 3 Ω is one on which a 20 A breaker sees a fault as a 40 A load. (17.3.4)

17.4 Objectionable Current

17.4.1 After energization and with the building under normal load, the current on the grounding electrode conductor, on the main bonding jumper, on the bonding jumper of each separately derived system and on the equipment grounding conductor of each feeder shall be measured with a clamp-on ammeter and recorded, and any reading exceeding 1 A, or any reading on a feeder equipment grounding conductor exceeding 10% of the neutral current of that feeder, shall be investigated.
17.4.2 Where the investigation finds a neutral-to-ground connection downstream of the service or of a system bonding jumper, a shared neutral between circuits on different equipment grounding paths, or a grounded conductor landed on a ground bar, the Contractor shall correct it and shall repeat the measurement.

17.5 Deficiencies and Retesting

17.5.1 A connection that cannot be moved by hand, that reads within the continuity limits and that shows the torque witness mark or a sound weld is accepted; a connection that fails any of these shall be remade and retested.
17.5.2 The cost of correcting a deficiency found under this article and of every repeated test shall be borne by the Contractor, except for the additional electrodes the Resistance to Earth Target article assigns to the Owner.
17.5.3 Where the Contractor and the testing organization disagree whether a result is a deficiency, the Engineer of Record shall make the determination.

18 Installation

18.1 Sequencing and Concealment

18.1.1 The concrete-encased electrode and its tail or stub shall be installed, inspected and protected before the footing concrete is placed, and the Contractor shall notify the Engineer of Record not less than two working days before each pour that contains an electrode.
18.1.2 Trenches for ground rings, buried conductors and rod electrodes shall be left open, or the connections left exposed, until the Inspection Before Concealment article has been satisfied.
18.1.3 Grounding connections at equipment shall be made before the equipment is energized, and no equipment shall be energized with its equipment grounding conductor or its bonding jumper unterminated.

18.2 Electrode Installation and Backfill

18.2.1 Rod electrodes shall be installed by driving with a driver that does not mushroom the head or strip the coating, and not by excavating and backfilling, except where rock makes driving impossible and the angled or trenched alternatives of NFPA 70 250.53(G) are used, with the alternative recorded on the electrode layout.
18.2.2 Rod electrodes shall be installed not less than 2 ft from the foundation wall, outside the drip line of the roof where practicable, and clear of every other underground utility by the separation its owner requires, and the top of each rod shall be not less than 12 in. below finished grade unless it is in a test well.
18.2.3 Trenches for ground rings and buried grounding conductors shall be backfilled with the excavated native soil free of rock, debris, cinders, slag and construction waste, placed and compacted so that the conductor is in full contact with soil, and ground enhancement material where used shall be placed around the electrode before the native soil.
18.2.4 Buried grounding conductors shall be installed at a depth of not less than 18 in., except that the ground ring shall be at the depth the datasheet requires, and a buried conductor that crosses a road, a utility or a structure footing shall be sleeved only where the sleeve is nonmetallic.

18.3 Test Wells

18.3.1 Test wells shall be provided as indicated in the datasheet, and each shall be a precast concrete, polymer concrete or heavy-duty polymer box with a bolted cover marked GROUND, set flush with grade over the electrode, with the connection to the electrode accessible inside it.
Ground Rod Test Wellsradio
○ None
○ At the electrode the resistance test is made on
○ At every rod electrode
NOTE A test well lets the resistance test be repeated over the life of the building without excavating, and lets a clamp-on tester be put around the conductor to the electrode. No test well is the arrangement of a building whose electrodes are the footing steel and whose rods are never expected to be retested; a well at the electrode the acceptance test is made on gives the maintenance program one place to repeat it; a well at every rod is the design of a communications site, a data center or a substation yard where the rods are retested on a schedule and the one that has corroded must be found. (18.3.2)

18.4 Routing and Support of Conductors

18.4.1 Grounding electrode conductors, bonding jumpers and ground bar bonding conductors shall be routed as directly as the structure permits, shall be supported at intervals not exceeding 5 ft and within 12 in. of every termination, shall not be used as a support or a handhold for anything, and shall be protected from physical damage where they are exposed to it.
18.4.2 A grounding or bonding conductor shall not be run through a metal sleeve, a metal plate or the wall of a ferrous enclosure unless it is bonded to the sleeve, plate or enclosure at that point, or unless the opening is slotted or fitted with a nonmetallic bushing so that the conductor is not encircled by ferrous metal.
18.4.3 Temporary grounding installed for construction power or for the protection of workers shall be removed when it is no longer needed unless it is part of the permanent installation, and no temporary ground shall be left in place where it creates a parallel path the permanent design does not intend.

19 Identification

19.1 The grounding system shall be identified at the points indicated in the datasheet, with labels of the materials and lettering Electrical IdentificationWiring System IdentificationResolves to the current adopted revision.sync/electrical-identification requires.
Grounding System Labelscheckbox
☑ Grounding electrode conductor at each end and at each connection to an electrode
☑ Main bonding jumper and each system bonding jumper
☑ Intersystem bonding termination
☑ Each ground bar, with the systems landed on it
☑ Each test well cover
☐ Common bond to the lightning protection system
☐ Common bond to the telecommunications bonding conductor
19.2 The intersystem bonding termination shall be labeled to identify it as the bonding point for communications systems, and the label shall remain legible and visible after the service equipment and its surroundings are finished.
19.3 Each ground bar shall carry a label naming the bar and listing the grounding electrode conductor, the derived systems and the bonding conductors landed on it, and each conductor landed on the bar shall carry a tag naming its origin.
19.4 Every grounding and bonding conductor that is bare or whose insulation is not green shall be identified green at each termination and at every accessible point, and the grounded conductor shall be identified white or gray and never green.
NOTE A ground bar with a dozen unlabeled conductors on it cannot be tested, because the technician cannot tell which conductor is the transformer tap and which is the door bond, and cannot be safely worked on, because a lifted conductor can be the only ground of a live derived system. (19.5)

20 Delivery, Storage, and Handling

20.1 Grounding and bonding materials shall be delivered in the manufacturer's original packaging with the listing marks and the connector size and conductor range markings legible.
20.2 Exothermic molds, weld metal and starting material shall be stored in a dry, heated space, a cartridge that has been wet shall not be used, and a mold that is cracked, eroded or carbon-fouled beyond the manufacturer's stated life shall be discarded.
20.3 Rod electrodes shall be stored off the ground, with the driving end and the copper bonding protected from damage, and a rod whose bonding is nicked to the steel over more than 1 in. of length shall not be installed.
20.4 Bare copper conductor shall be stored on reels or in coils off the ground, protected from contact with concrete, acid, fertilizer and road salt, and shall be installed clean.

21 Warranty

21.1 The Contractor shall warrant the grounding and bonding installation, including every connection and the measured resistance to earth of the electrode system, against defects in materials and workmanship for the period indicated in the datasheet from the date of substantial completion.
Installation Warranty Periodrange
Not less than
years
1235
21.2 Where the measured resistance to earth at acceptance was within 20% of the value in the datasheet, the Contractor shall repeat the electrode resistance test once during the warranty period in the driest month of the year at no cost to the Owner, and where the repeated measurement exceeds the value in the datasheet, the Contractor shall add electrodes under the warranty until it does not.
21.3 A connection found loose, corroded or open during the warranty period shall be remade at the Contractor's cost, together with any equipment damage the Engineer of Record attributes to the defective connection, and the remade connection shall carry a fresh warranty of the full period from the date it is remade.
21.4 The warranty on the grounding products themselves shall be the manufacturer's standard warranty, assigned to the Owner, and shall not reduce the Contractor's installation warranty.

SynC Standards are reference material provided for informational purposes only and as a guide. They are not engineering, architectural, or legal advice and are not a substitute for the judgment of a licensed design professional. It is the responsibility of the user to determine the applicability of any standard to a specific project and to verify all requirements against the governing codes, manufacturer data, and project conditions. SynC does not render professional services and forms no professional relationship by publishing this content. Provided "as is," without warranty of any kind, including fitness for a particular purpose. See our Terms of Use for the complete terms. Moderation log.

This standard is written and maintained by SynC and licensed under Creative Commons Attribution 4.0. You may share and adapt it, including commercially, provided you credit SynC Standards, link to the license, and indicate any changes. Each standard is drafted from cited sources under published authoring rules, reviewed before adoption, and corrected in the open. Keep the attribution and notice below with any copy; it carries the warranty notice the license asks you to retain.

Attribution and reuse notice. Keep this with any copy:
"Grounding and Bonding for Electrical Systems," SynC Standards, revision 9. Licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://synergyinconstruction.com/standards/general/grounding-and-bonding. Reference material only; not professional engineering advice and provided without warranty. Verify against governing codes and have a licensed professional review before use.