This specification covers the materials, configuration, installation, and field testing of the grounding and bonding systems for low-voltage electrical distribution rated 1000V and below. Grounding and bonding shall comply with NFPA 70 Article 250 (National Electrical Code) and shall be installed so that the completed system provides a low-impedance path for fault current, limits voltage imposed by lightning and line surges, stabilizes voltage to earth during normal operation, and connects normally non-current-carrying metallic parts of equipment to the source so that overcurrent devices clear ground faults reliably.
Grounding and bonding is not a single product but a coordinated system that touches nearly every other electrical scope. The Contractor shall treat the requirements of this standard as applying to the service equipment, every separately derived system, every panelboard and distribution assembly, every raceway and cable system, and every piece of utilization equipment on the project. Coordinate raceway and conductor work with Raceways And Conduit and Conductors And Cables. Coordinate service and distribution equipment grounding with Low Voltage Switchgear and Electrical Rooms.
Two distinct functions are addressed and shall not be conflated. Grounding is the intentional connection of a system or equipment to earth. Bonding is the connection of metallic parts to each other to establish electrical continuity and conductivity. Earth itself is not an effective fault-clearing path; the effective ground-fault current path that operates overcurrent devices is created by bonding, not by the connection to earth. This distinction governs every requirement that follows.
This standard does not cover lightning protection systems. Where a lightning protection system is installed, it shall be bonded to the building grounding electrode system in accordance with NFPA 780 and NEC 250.106, but the design and installation of the lightning protection system itself is a separate scope.
Equipment, materials, and installation shall comply with the latest adopted edition of the following standards and codes. Where the contract documents, the adopted building code, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
| Standard | Title |
|---|---|
| NFPA 70 | National Electrical Code (Article 250 — Grounding and Bonding) |
| 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 |
| UL 96 | Lightning Protection Components |
| UL 869A | Reference Standard for Service Equipment |
| IEEE 142 | Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) |
| IEEE 1100 | Recommended Practice for Powering and Grounding Electronic Equipment (Emerald Book) |
| IEEE 81 | Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System |
| IEEE 837 | Standard for Qualifying Permanent Connections Used in Substation Grounding |
| 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 |
Contractor shall submit the following for the Engineer's review prior to procurement and installation. Installation shall not proceed on any portion of the grounding system until the corresponding submittals are reviewed and returned.
Contractor shall provide the following at substantial completion before the grounding system is accepted.
Grounding and bonding work shall be performed by electricians experienced in the installation of grounding systems and supervised by a licensed electrical contractor. Personnel making exothermic welded connections shall be trained by the connection-system manufacturer in the specific mold and process being used, and shall demonstrate competency on sample connections before performing production work.
Permanent grounding connections that are buried, encased in concrete, or otherwise inaccessible after installation shall use a connection method qualified by testing. Exothermic welded connections and compression connections used in inaccessible locations should be qualified in accordance with IEEE 837, which subjects connections to electrical, thermal, and corrosion testing that an ordinary mechanical lug cannot pass. Mechanical connections are not permitted in locations that become inaccessible.
The grounding electrode system, including all electrodes and the grounding electrode conductor connections, shall be available for inspection by the Authority Having Jurisdiction before being concealed by backfill, concrete, or finishes. Contractor shall coordinate inspection timing and shall not conceal any portion of the grounding electrode system until it has been inspected and released.
All grounding and bonding equipment — connectors, clamps, bushings, ground bars, and ground rods — shall be listed and labeled to UL 467 by a Nationally Recognized Testing Laboratory. Connectors shall be listed for the specific conductor materials, conductor sizes, and surfaces on which they are used. Connectors used in direct contact with earth or concrete shall be additionally listed for direct burial or concrete encasement.
The performance of a grounding electrode system depends heavily on the soil in which it is installed. Soil resistivity varies by orders of magnitude with moisture content, temperature, mineral content, and compaction. The Engineer shall establish the design soil resistivity from a site investigation, and the Contractor shall report any field condition that differs materially from the design basis.
Where a measured ground resistance value must be achieved and the design soil resistivity is high, the Engineer shall evaluate supplemental measures early in design. Driving electrodes deeper, increasing electrode spacing, extending a ground ring, or adding chemically enhanced or ground-enhancement-material electrodes are all more cost-effective when planned before excavation than when added as remedial work.
In soils with low resistivity, high chloride or sulfate content, low pH, or significant dissimilar buried metals, accelerated corrosion of grounding electrodes and connections shall be expected. In corrosive soils, the Contractor shall use connection methods and electrode materials selected for corrosion resistance, and shall avoid creating galvanic couples between dissimilar buried metals.
Copper and copper-bonded steel are appropriate for the majority of soil conditions. Galvanized steel electrodes shall not be used where they would be electrically connected through the earth or through buried conductors to copper electrodes, because the resulting galvanic cell consumes the zinc and then the steel. Stainless steel electrodes should be used where the soil is highly corrosive to copper.
Where the installation is subject to seasonal freezing, electrode design shall account for the increase in soil resistivity that occurs when soil freezes. Driven electrodes shall extend below the local frost depth so that the lower portion remains in unfrozen, conductive soil year-round. Frost depth shall be as indicated on the civil drawings or determined from the local building code.
The grounding electrode system is the set of electrodes that connect the electrical system to earth. NEC 250.50 requires that all grounding electrodes present at a building or structure be bonded together to form a single grounding electrode system. A common field error is to install a driven rod and ignore the concrete-encased electrode, the metal water pipe, or the building steel that are also present; all qualifying electrodes shall be bonded together.
Every electrode listed above that exists at the building shall be bonded into the grounding electrode system. The selection above records which electrodes the project relies on; it does not relieve the Contractor of the obligation to bond any qualifying electrode that is present, whether shown or not.
Where a building has a concrete foundation or footing in direct contact with earth, a concrete-encased electrode shall be provided in accordance with NEC 250.52(A)(3). The electrode shall consist of at least 20 ft of either bare copper conductor not smaller than 4 AWG, or one or more bare zinc-coated or uncoated steel reinforcing bars not smaller than 1/2 in. diameter, encased by at least 2 in. of concrete and located within and near the bottom of a foundation or footing that is in direct contact with the earth.
The concrete-encased electrode is the most effective and most durable electrode available on most projects because the concrete itself behaves as a large, moisture-retaining, conductive mass. The Contractor shall coordinate the installation of the concrete-encased electrode connection point with the structural and concrete trades so that a connection tail extends out of the foundation before concrete is placed. Retrofitting a concrete-encased electrode after the foundation is poured is not feasible.
Where a ground ring is provided, it shall encircle the building or structure, be in direct contact with earth, and consist of at least 20 ft of bare copper conductor not smaller than 2 AWG, in accordance with NEC 250.52(A)(4). A ground ring shall be buried at least 30 in. below grade per NEC 250.53(F). Ground rings are commonly specified for buildings with sensitive electronic systems, data centers, and structures where a low and stable ground resistance is required.
Rod and pipe electrodes shall be not less than 8 ft in length and shall be installed so that at least 8 ft of length is in contact with the soil, in accordance with NEC 250.53(G). Rod electrodes shall be not less than 5/8 in. in diameter unless listed. Where rock bottom is encountered, the electrode shall be driven at an oblique angle not exceeding 45 degrees from vertical, or buried in a trench at least 30 in. deep, as permitted by NEC 250.53(G).
A single rod, pipe, or plate electrode that does not have a resistance to earth of 25 ohms or less shall be supplemented by one additional electrode, in accordance with NEC 250.53(A)(2). Where two rods are used, they shall be spaced at least 6 ft apart; spacing of at least one rod length is recommended because closely spaced rods share the same volume of soil and add little. The 25-ohm threshold is a code minimum, not a performance target — many projects require a substantially lower resistance and the project value shall be taken from the testing requirements of this standard.
A 3/4 in. rod is recommended over the 5/8 in. minimum because the larger diameter resists bending during driving, couples more reliably for sectional rods, and provides greater corrosion margin over the life of the installation.
A metal underground water pipe in direct contact with earth for 10 ft or more shall be used as a grounding electrode where present, in accordance with NEC 250.52(A)(1). A metal water pipe electrode shall always be supplemented by at least one additional electrode because water utilities increasingly insert nonmetallic pipe sections and dielectric fittings that can interrupt the electrode without notice. The bonding connection to the water pipe shall be made within the first 5 ft of the pipe's entrance into the building, and any insulating fitting or water meter shall be bypassed with a bonding jumper sized per NEC 250.53(D).
The connection of the grounding electrode conductor to a grounding electrode shall be accessible, except that connections to concrete-encased electrodes, driven rods, and other buried or encased electrodes are permitted to be inaccessible, in accordance with NEC 250.68(A). Where a connection to an electrode will be buried or encased, the connection method shall be exothermic welding or a listed irreversible compression connector; mechanical lugs and bolted clamps shall not be buried or encased.
The grounding electrode conductor connects the grounded conductor of the service or the equipment grounding conductor, or both, to the grounding electrode system. It is the conductor that establishes the system's reference to earth.
Copper shall be used for the grounding electrode conductor in most applications. Aluminum or copper-clad aluminum grounding electrode conductors shall not be used where in direct contact with masonry or earth, where subject to corrosive conditions, or installed outdoors within 18 in. of earth, in accordance with NEC 250.64(A). Because grounding electrode conductors so frequently run to buried or near-earth electrodes, copper is the practical default.
The grounding electrode conductor shall be sized in accordance with NEC Table 250.66, based on the size of the largest ungrounded service-entrance conductor or equivalent area for parallel conductors. The Contractor shall not size the grounding electrode conductor from the equipment grounding conductor table — confusing Table 250.66 with Table 250.122 is one of the most common grounding errors and produces a conductor of the wrong size.
NEC 250.66 also caps the required size for connections to specific electrode types. The portion of the grounding electrode conductor that is the sole connection to a concrete-encased electrode is not required to be larger than 4 AWG copper. The portion that is the sole connection to a ground ring is not required to be larger than the ring conductor. The portion that is the sole connection to a rod, pipe, or plate electrode is not required to be larger than 6 AWG copper. Where one continuous conductor serves multiple electrodes, the full Table 250.66 size applies to the common portion.
The grounding electrode conductor size shall be as indicated on the grounding riser diagram and verified against NEC Table 250.66.
The grounding electrode conductor shall be installed in one continuous length without a splice or joint, except where spliced by irreversible compression connectors listed for the purpose, by exothermic welding, or where busbars are connected together, in accordance with NEC 250.64(C). Bolted, screwed, or otherwise removable splices in the grounding electrode conductor are not permitted. The intent is that the path to the grounding electrode system cannot be casually opened.
A grounding electrode conductor of 6 AWG copper or larger that is free from exposure to physical damage is permitted to run along the surface of the building without metal covering, in accordance with NEC 250.64(B). Conductors smaller than 6 AWG, and any conductor exposed to physical damage, shall be protected in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor.
Where a grounding electrode conductor is installed in a ferrous metal raceway, the raceway shall be electrically continuous from the cabinet or enclosure to the electrode, and the conductor shall be bonded to the raceway at both ends, in accordance with NEC 250.64(E). A ferrous raceway around a grounding conductor acts as a choke and increases the impedance of the path; bonding both ends cancels this effect. This requirement is frequently missed in the field and shall be specifically verified during inspection.
The equipment grounding conductor bonds the normally non-current-carrying metal parts of equipment, raceways, and enclosures together and back to the source. It is the conductor that carries ground-fault current back to the source so that the overcurrent device operates. The equipment grounding conductor — not the earth — is the effective ground-fault current path.
NEC 250.118 recognizes several wiring methods as equipment grounding conductors, including rigid metal conduit, intermediate metal conduit, and electrical metallic tubing. A wire-type equipment grounding conductor installed in every raceway is nonetheless recommended for all feeders and for branch circuits serving sensitive, life-safety, or critical loads, because it provides a continuous, predictable path that does not depend on the integrity of every coupling, connector, and locknut. Where a metallic raceway is relied upon as the sole equipment grounding conductor, every connection in the raceway run is part of the safety path and shall be made wrench-tight.
Insulated equipment grounding conductors of 6 AWG and smaller shall be identified by a continuous green color or green with yellow stripes, in accordance with NEC 250.119. Conductors larger than 6 AWG are permitted to be re-identified green at terminations.
The equipment grounding conductor shall be sized in accordance with NEC Table 250.122, based on the rating of the overcurrent device protecting the circuit conductors. The equipment grounding conductor is never required to be larger than the circuit conductors it accompanies.
Where ungrounded circuit conductors are increased in size for voltage drop or any other reason, the wire-type equipment grounding conductor shall be increased in size proportionally to the increase in circular mil area of the ungrounded conductors, in accordance with NEC 250.122(B). This proportional upsizing is routinely missed when a designer enlarges feeder conductors for voltage drop and leaves the ground at the table-minimum size; the Contractor shall verify that every upsized feeder carries a correspondingly upsized equipment grounding conductor.
Where a single equipment grounding conductor serves multiple circuits in the same raceway, it shall be sized for the largest overcurrent device protecting any conductor in that raceway, in accordance with NEC 250.122(C). Equipment grounding conductors installed in parallel sets of conductors shall be sized in each raceway for the full overcurrent device rating, in accordance with NEC 250.122(F).
Bonding establishes electrical continuity between metallic parts so that an effective ground-fault current path exists and so that no objectionable difference of potential develops between accessible metal parts. Bonding is what makes the grounding system actually clear faults.
The main bonding jumper connects the equipment grounding conductor, the service-equipment enclosure, and the grounded (neutral) conductor together at the service, in accordance with NEC 250.24(B) and 250.28. There shall be exactly one main bonding jumper for the service. The main bonding jumper is the single, intentional point at which the neutral and the ground are tied together for a service.
The main bonding jumper shall be sized in accordance with NEC 250.28(D), generally not smaller than the values in Table 250.102(C)(1) based on the size of the service-entrance conductors. Where service conductors exceed 1100 kcmil copper, the main bonding jumper shall be not less than 12.5 percent of the area of the largest ungrounded conductor.
The grounded (neutral) conductor shall not be connected to equipment grounding conductors, enclosures, or the equipment grounding bus on the load side of the service disconnecting means, except as specifically permitted by NEC 250.142. Re-bonding the neutral to ground at a downstream panelboard creates parallel neutral paths that put normal load current onto raceways, equipment grounding conductors, and building steel. The Contractor shall verify that every downstream panelboard has its neutral bus isolated from the enclosure and that bonding screws or straps are removed at every panel except the service.
The grounding and bonding system shall be arranged so that it does not carry objectionable current under normal operating conditions, in accordance with NEC 250.6. Objectionable current on grounding conductors is almost always a symptom of an improper neutral-to-ground connection downstream of the service, multiple service disconnects without proper treatment, or a wiring error. Where objectionable current is detected during testing, the Contractor shall locate and correct the wiring error; the current shall not be addressed by removing or interrupting grounding connections.
The supply-side bonding jumper bonds metallic enclosures and raceways on the line side of the service overcurrent device — where no overcurrent device exists between the bonding point and the source — and shall be sized per NEC 250.102(C). The load-side equipment bonding jumper bonds enclosures and raceways downstream of an overcurrent device and shall be sized per NEC 250.102(D), generally per Table 250.122. The distinction matters because supply-side jumpers must carry fault current with no upstream protection and are therefore sized more conservatively. The Contractor shall confirm for each bonding jumper whether it is on the supply side or load side and size it accordingly.
Metal raceways, cable armor, and enclosures containing service conductors shall be bonded together by a method that does not rely on locknuts and bushings alone, in accordance with NEC 250.92. Acceptable methods include bonding jumpers around concentric or eccentric knockouts, bonding-type locknuts, bonding bushings, and threaded hubs. Standard locknuts alone shall not be relied upon for service-conductor bonding because they cannot be assured to carry available service fault current.
Metal water piping systems, metal gas piping, and structural metal that is interconnected to form a building frame shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or the grounding electrodes, in accordance with NEC 250.104. The bonding jumper for the metal water piping system shall be sized per NEC Table 250.102(C)(1) based on the service conductors. Bonding of metal building frame and interior piping is sized per NEC 250.104 based on the circuit likely to energize the piping.
An intersystem bonding termination shall be provided at the service equipment and at any disconnecting means for buildings supplied by a feeder, in accordance with NEC 250.94. The intersystem bonding termination provides an accessible point with at least three terminals for bonding communications, CATV, antenna, and other systems to the building grounding electrode system. The Contractor shall install the intersystem bonding termination so that it remains accessible after all other equipment is in place; it shall not be located behind equipment or finishes.
A separately derived system has no direct electrical connection of circuit conductors to conductors of another system. The secondary of an isolation or dry-type distribution transformer, the output of an on-site generator with a transfer switch that switches the neutral, and the output of certain uninterruptible power supplies are separately derived systems. Each separately derived system is, in effect, a new source and shall be grounded and bonded at the source in accordance with NEC 250.30.
A system bonding jumper shall be installed for each separately derived system, connecting the equipment grounding conductor of the derived system to the grounded (neutral) conductor, in accordance with NEC 250.30(A)(1). The system bonding jumper shall be installed at a single point, either at the source itself or at the first system disconnecting means, but not at both. Installing a system bonding jumper at two points re-creates the same parallel-neutral problem as a downstream neutral-to-ground bond and shall not be done.
The system bonding jumper shall be sized per NEC 250.30(A)(1) and Table 250.102(C)(1), based on the derived phase conductors.
Each separately derived system shall be connected to a grounding electrode by a grounding electrode conductor, in accordance with NEC 250.30(A)(4) and (A)(5). The grounding electrode shall be the nearest of an effectively grounded structural metal member or the nearest effectively grounded metal water pipe; where neither is available, another electrode of the building grounding electrode system shall be used. The grounding electrode conductor shall be sized per NEC Table 250.66 based on the derived phase conductors and connected at the same single point as the system bonding jumper.
Where there are multiple separately derived systems, a common grounding electrode conductor with taps to each system is permitted by NEC 250.30(A)(6) and is often the cleanest installation in a building with many dry-type transformers. The common grounding electrode conductor shall be sized for the largest derived system it serves.
Whether an on-site generator is a separately derived system depends on the transfer switch. Where the transfer switch switches the neutral, the generator is a separately derived system and shall have its own system bonding jumper and grounding electrode connection. Where the transfer switch does not switch the neutral — the neutral is solid through the transfer switch — the generator is not a separately derived system and shall not have a neutral-to-ground bond at the generator, because the service main bonding jumper already provides it. The Contractor shall confirm the transfer switch neutral arrangement before bonding the generator and shall coordinate this with Emergency And Standby Power.
Every panelboard, switchboard, and distribution assembly shall be provided with an equipment grounding bus or terminal bar to which all equipment grounding conductors entering the enclosure are terminated, in accordance with NEC 408.40. Each equipment grounding conductor shall terminate in an individual terminal; more than one conductor per terminal is permitted only where the terminal is listed for multiple conductors. The equipment ground bus shall be bonded to the enclosure.
Where the project includes telecommunications rooms, equipment rooms, or sensitive-equipment areas, a dedicated ground bar shall be provided and bonded to the building grounding electrode system. Telecommunications bonding shall be coordinated with ANSI/TIA-607 and with Telecommunications Bonding. The electrical Contractor's responsibility under this standard is the bonding conductor from the building grounding electrode system to the primary telecommunications ground bar; the telecommunications bonding backbone itself is a separate scope.
The integrity of the grounding system depends entirely on the quality of its connections. A grounding conductor that is correctly sized but poorly terminated provides no protection.
Connections that are buried in earth, encased in concrete, or otherwise inaccessible after construction shall be made by exothermic welding or by listed irreversible compression connectors. Both methods create a permanent connection that cannot loosen, corrode at a mechanical interface, or be disturbed; mechanical connections shall not be used in inaccessible locations.
In accessible locations, listed mechanical connectors and lugs are permitted. Mechanical connectors shall be listed for the conductor material and size and shall be tightened to the manufacturer's specified torque using a calibrated torque tool.
Connection of a grounding electrode conductor or bonding jumper to an electrode or to equipment shall be made by listed lugs, listed pressure connectors, exothermic welding, or other listed means, in accordance with NEC 250.70. Connections shall not depend on solder, because solder melts at fault-current temperatures. No more than one conductor shall be terminated under a single connector unless the connector is listed for multiple conductors.
Surfaces to be connected shall be cleaned of paint, enamel, mill scale, and coatings to bare metal at the point of connection, and the connection shall be made directly on the cleaned conductive surface. Where a coating must be restored after connection, a corrosion-inhibiting compound listed for the purpose shall be applied.
Connectors used in direct contact with the earth or in concrete shall be listed for direct burial or concrete encasement, in accordance with NEC 250.70. Connectors not so listed shall not be buried or encased.
Grounding and bonding conductors shall be copper conforming to ASTM B3 and ASTM B8, except where aluminum is specifically permitted by NEC 250.64(A) and selected on the contract documents. Bare grounding conductors used as electrodes or buried shall be soft-drawn copper.
Ground bars and busbars shall be copper conforming to ASTM B187. Tin plating shall be provided where specified to reduce contact-surface oxidation and to ease future re-termination.
Ground rod electrodes shall conform to NEMA GR 1 and shall be listed to UL 467. Copper-bonded steel rods shall have a copper bonding thickness sufficient to survive driving without exposing the steel core and to provide corrosion life appropriate to the soil conditions.
All connectors, clamps, bushings, and bonding devices shall be listed and labeled to UL 467 and shall be of materials compatible with the conductors and surfaces they contact to avoid galvanic corrosion. Connectors of dissimilar metal to the conductor or the contacted surface shall not be used in damp, wet, or buried locations.
An isolated (insulated) equipment grounding conductor is an additional, insulated equipment grounding conductor that bypasses the normal raceway-and-enclosure grounding path and connects a receptacle's grounding terminal directly back to an upstream point, in accordance with NEC 250.146(D). The intent is to reduce coupling of electrical noise into sensitive electronic equipment by avoiding the shared, multi-point grounding of the conduit system.
Where isolated ground receptacles are used, the following shall apply. The isolated equipment grounding conductor is permitted to pass through panelboards and enclosures without bonding to them, but it shall ultimately terminate at the applicable derived-system or service grounding point. It is not permitted to terminate the isolated ground at earth alone, and it is not permitted to omit the connection back to the source — an isolated ground is still a grounding conductor and shall provide an effective fault-clearing path. The metal raceway and any normal equipment grounding conductor shall still be installed and bonded; the isolated ground is in addition to, never in place of, the standard grounding path.
Isolated grounding is frequently specified where it provides no benefit and occasionally installed in a way that defeats its own purpose. The Engineer should confirm that the connected equipment actually requires an isolated ground before specifying it; modern electronic equipment with properly designed power supplies generally does not. Where isolated ground receptacles are installed, they shall be identified by the orange triangle marking required by NEC 406.3(E).
Certain occupancies impose grounding and bonding requirements beyond the general rules of NEC Article 250. Where the project includes any of the spaces below, the Contractor shall apply the additional requirements of the corresponding NEC article.
In health care facilities, patient care spaces require redundant grounding and equipotential bonding per NEC Article 517; the impedance of the equipment grounding path is limited and shall be tested. Swimming pools and water features require equipotential bonding of all metal within the prescribed distance per NEC Article 680. Hazardous locations require bonding by methods that do not rely on locknuts or double-locknut-and-bushing connections per NEC 250.100, regardless of voltage. Where any of these occupancies are present, their grounding requirements govern over the general requirements of this standard.
Grounding products do not receive factory acceptance testing of the type applied to assembled equipment; the meaningful verification of a grounding system happens in the field after installation. The Contractor shall engage a qualified testing agency to perform the field tests below, and shall correct and re-test any item that does not meet the acceptance criteria.
The resistance to earth of the grounding electrode system shall be measured after installation and before the system is placed in service, in accordance with IEEE 81 and ANSI/NETA ATS. The test method shall be appropriate to the electrode configuration and site conditions.
The fall-of-potential method is the reference method and shall be used for acceptance testing of the grounding electrode system where the electrode can be temporarily isolated and where space permits placement of test electrodes. The clamp-on method may be used for periodic verification of installed electrodes that have a parallel return path, but it shall not be used as the sole acceptance test of an isolated electrode because it requires a parallel path to produce a reading.
The maximum acceptable ground resistance shall be as indicated on the contract documents. A value of 5 ohms is a common target for commercial buildings and provides margin below the 25-ohm code threshold for a single supplemented electrode. Facilities with sensitive electronic systems, data centers, and critical operations frequently require lower values; the Engineer shall set the project value based on the connected systems. Where a measured value exceeds the acceptance criterion, the Contractor shall add electrodes, extend the ground ring, or apply ground-enhancement measures and re-test until the criterion is met.
The Contractor shall verify electrical continuity of the equipment grounding system from representative points of utilization back to the service or derived-system grounding point. The continuity of bonding jumpers at service equipment, around water meters and insulating fittings, and across expansion or vibration fittings in raceway runs shall be confirmed.
Accessible mechanical grounding connections shall be checked for tightness against the manufacturer's torque specifications using a calibrated torque tool. Exothermic welded connections shall be inspected visually for full fusion, absence of porosity, and proper conductor coverage; a connection that can be moved or that shows incomplete fusion shall be cut out and remade.
The Contractor shall verify, with the building under normal load, that grounding conductors and bonding jumpers do not carry objectionable current per NEC 250.6. Measurable current on a grounding electrode conductor or on an equipment grounding conductor under normal operation indicates an improper neutral-to-ground connection or a wiring error, which the Contractor shall locate and correct.
All test results shall be recorded on the testing agency's report forms, including the test method, instrument identification and calibration date, ambient conditions, measured values, acceptance criteria, pass/fail determination, and the corrective action taken for any failed item. Reports shall be included in the closeout submittals.
The Contractor shall sequence grounding work so that no portion of the grounding electrode system is concealed before it is inspected. The connection tail for the concrete-encased electrode shall be installed and protected before foundation concrete is placed. Ground ring and electrode trenches shall be left open, or the connections left accessible, until inspected and released by the Authority Having Jurisdiction.
Grounding electrode conductors and bonding jumpers shall be routed as directly and as short as practical. Sharp bends shall be avoided; the conductor shall not be bent to a radius smaller than that which would damage the conductor or its insulation. Conductors shall be supported and protected so that they are not subject to physical damage or to use as a handhold or support.
Driven electrodes shall be installed by driving, not by excavating and backfilling, except where rock or refusal makes driving impossible and the oblique or buried alternatives of NEC 250.53(G) are used. Electrodes shall be installed clear of the building footprint and clear of other utilities by the spacing required by the utility owners.
Trenches for ground rings and buried conductors shall be backfilled with the excavated native soil, compacted, and free of rock, debris, and corrosive fill in contact with the conductor. Where ground-enhancement material is specified, it shall be installed around the electrode in accordance with the manufacturer's instructions before backfill.
Grounding conductors and connections left exposed during construction shall be protected from damage by other trades. Temporary grounding installed for construction power shall be removed where it is not part of the permanent installation, and shall not be left to create unintended parallel paths.
Wire-type equipment grounding conductors shall be identified by green or green-with-yellow-stripe insulation, or shall be bare, in accordance with NEC 250.119. Grounded (neutral) conductors shall be identified white or gray and shall not be confused with equipment grounding conductors.
The grounding electrode conductor and its connection points shall be labeled where the label aids future maintenance — for example, at the connection to a concrete-encased electrode or at a ground ring access point. The intersystem bonding termination shall be labeled to identify it as the point for bonding other systems. Isolated ground receptacles shall carry the orange triangle identification required by NEC 406.3(E), and isolated ground circuits shall be identified at the panelboard.
Grounding and bonding materials shall be delivered in the manufacturer's original packaging with listing marks intact. Conductors and connectors shall be stored indoors in a clean, dry location and protected from corrosive atmospheres until installed. Ground rods shall be stored so that the bonding coating and the driving end are not damaged. Exothermic weld molds and weld material shall be stored dry, because moisture in the mold or charge produces porous, defective welds.
Grounding and bonding products that carry a manufacturer warranty against defects in materials and workmanship shall be warranted to the Owner. The Contractor shall warrant the installation, including all connections and the achievement of the specified ground resistance, for the project warranty period.
Where the achieved ground resistance is marginal relative to the acceptance criterion, the Engineer may require a re-test during the warranty period to confirm that seasonal soil changes have not raised the resistance above the acceptance value.