Service and Feeder Conductors

NOTE This standard governs two things. First, the service conductors complete, because no other standard carries them: the conductor or cable that runs from the service point to the service disconnecting means, its material, its size, the temperature basis it is sized on, the splices and terminations made on its line side, and the tests that release it for connection. Second, the rules that apply to a feeder because it is a feeder: how it is sized to the load it carries, how it is protected, how a parallel set is arranged, how its neutral is sized, how much voltage it is allowed to lose, where it is tapped, and what is measured on it after energization. (1.1)

NOTE The conductor as a product is specified once, under Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables. That standard carries the material selection for feeders and branch circuits, the insulation types, the stranding, the color coding, the ampacity correction and adjustment factors, the connector and splice hardware, the pulling limits, and the insulation resistance test of building wire. This standard is a layer over it: a feeder is built of the conductor product selected there and applied as this standard requires, and nothing in that standard is restated here. The service conductors are the exception, because Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables excludes them, so their product, sizing, terminations, and tests are carried here in full. (1.2)

NOTE Service conductors run from the service point to the service disconnecting means and have no overcurrent protection on the premises side; the serving utility's protection is the only thing that clears a fault on them. Feeders run from the service equipment, a separately derived system, or another feeder's overcurrent device to the final branch circuit overcurrent device, and each is protected at its supply end. The difference in protection is why the service conductors are sized and routed under rules of their own and why the feeder rules of this standard rest on the overcurrent device at the feeder's source. (1.3)

NOTE The following are governed elsewhere and are outside this standard: (1.4)

NOTE The product data and the voltage drop calculations for the feeder conductors as a product are submitted under Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables and are not repeated here. (3.1.3)

NOTE A connector on the line side of the service equipment sits where no premises overcurrent device protects it; the line-side marking means the connector has been tested to carry and withstand fault current with only the utility's protection ahead of it. (4.2.3)

NOTE A service raceway on a sunlit wall or a roof runs well above the 30 °C basis of the ampacity table for much of the day, and the service conductors in it carry the whole load of the building; a service sized at the table ampacity without the correction is the service that runs hot in the first summer. (5.1.3)

NOTE An underground raceway is a wet location over the life of the installation because groundwater enters through the joints, and a duct bank with several loaded circuits heats the earth around it so that no circuit in it runs at the free-raceway ampacity; the duct bank itself is built under Underground DuctbankUnderground Electrical Duct BanksResolves to the current adopted revision.sync/underground-ductbank, and the ampacity of the conductors inside it is a consequence of how many circuits it carries. (5.2.3)

NOTE THHN/THWN-2 is the thermoplastic building wire most raceway services are pulled with, at the smallest diameter and the lowest cost; XHHW-2 is a thermoset insulation that does not soften under sustained heat and absorbs less moisture, which suits a service raceway that runs hot or wet and a large aluminum service where thermal margin at the lugs matters, at a slightly larger diameter; RHW-2 is a thermoset type with a separate jacket that serves the same locations where the Owner's standard calls for it; USE-2 is the wet-location and direct-burial type that carries an underground service from the utility's terminals and is also listed for the raceway run inside; Type SE cable is the cable assembly of most dwelling and small commercial services and costs least where the serving utility and the Authority Having Jurisdiction accept it; a direct-buried Type USE cable assembly removes the raceway and cannot be replaced without re-excavation; Type MC cable listed for service-entrance use serves an indoor run between a transformer vault and the service equipment where a raceway cannot be routed. The wiring method of the service, raceway, cable, or busway, is selected under Electrical Service EntranceElectrical Service EntranceResolves to the current adopted revision.sync/electrical-service-entrance, and where that selection is busway the bars of the busway are the service conductors, under BuswayBuswayResolves to the current adopted revision.sync/busway, and the conductor selections of this article do not apply. (6.1.2)

NOTE Copper carries more current per unit of cross-section, terminates on every listed lug without a compound, and is less subject to creep at the terminal, at the highest material cost; aluminum weighs about half as much and costs a fraction of copper for the same ampacity at the sizes a service uses, and it is what most utilities run on their own side of the service point, at the cost of a larger conductor and raceway, lugs listed for aluminum, and the oxide-inhibiting practice at every termination; copper-clad aluminum weighs and costs near aluminum and is terminated as aluminum, and serves where the Owner wants a copper surface at the strand without the weight of copper. A service point at the utility's aluminum lugs takes any of the three. (6.2.2)

NOTE Aluminum 1350 conductor remains in use on the utility's side of the service point and in utility-furnished lateral conductors, and NFPA 70 310.3(B) does not accept it for conductors the Contractor furnishes, because its creep and its thermal expansion loosen a bolted terminal over the years in a way the 8000 series alloy was developed to resist. (6.2.5)

NOTE The grounded service conductor carries the unbalanced load in service and the whole fault current when a service conductor faults to the enclosure, and the second duty is why NFPA 70 sizes it from the ungrounded conductors as well as from the load; the grounding electrode conductor and the main bonding jumper it lands beside are sized under Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding. (6.3.8)

NOTE The temperature rating of the lowest-rated termination on the run, the meter socket, the current transformer cabinet, the line-side splice, or the service disconnect, is the rating that governs, and 75 °C is the marking on nearly every meter socket and service disconnect above 100 A; a 60 °C basis applies to a small service whose socket or disconnect is marked for 60 °C, and a 90 °C basis is claimed only where every termination on the run is marked for 90 °C, which few service equipment listings are. (6.4.2)

NOTE A compression splice crimped with the listed die and covered with a heat-shrink or molded cover makes a permanent joint with the lowest resistance and is what a splice inside a wireway or a pull box takes; an insulated mechanical multi-tap connector takes several conductors of a range of sizes under set screws in an insulated block and is what a tap to grouped disconnects or a transition between conductor sizes takes, at the cost of a torque check; a sealed underground splice kit listed under UL 486D is the only connector that serves a splice in a handhole or a direct-buried run, where the joint sits in water. (6.5.3)

NOTE A conductor forced into a lug below its size range, or two conductors landed in a lug listed for one, is a termination that is not listed, and it is the one the infrared survey finds in the first summer; the lug range is fixed when the equipment is ordered, so the conductor size and the lug range are reconciled on the submittals, not in the field. (6.5.6)

NOTE The 125 % factor on the continuous load exists to keep an ordinary molded-case device, which is tested to carry its rating in open air, from tripping on heat inside an enclosure under a three-hour load; a device listed for 100 % operation has been tested in the assembly it is installed in, and the factor is already in the listing. (7.1.4)

NOTE The rating and the settings of each feeder overcurrent device are established under Protective Coordination StudyProtective Device Coordination StudyResolves to the current adopted revision.sync/protective-coordination-study, and this standard sizes the feeder to carry what that device lets through; a feeder whose ampacity falls short of a device setting changed during the study is the coordination that was never checked against the conductor. (7.2.4)

NOTE A conductor above 500 kcmil gains little ampacity for its added copper, because the current crowds toward the surface of a large conductor, and it is stiff enough that a 750 kcmil or 1000 kcmil conductor needs a larger pull box, a larger bending radius, and a lug that most distribution equipment does not furnish as standard; 500 kcmil is the size most equipment lugs accept and most contractors keep on hand, 350 kcmil suits a project that wants lighter pulls at the cost of more parallel sets, and 600 kcmil and above suit a project with few raceways available and equipment ordered with lugs to match. (7.3.2)

NOTE Running one conductor of each phase and the neutral in each raceway keeps the magnetic fields of the set balanced inside every raceway, which NFPA 70 300.3(B)(1) and 300.20 require in a ferrous raceway and which keeps every raceway of the set at the same impedance so the conductors share current; an isolated-phase arrangement, with all the conductors of one phase in one raceway, is permitted only in nonmetallic raceways run underground as NFPA 70 300.3(B)(1) Exception No. 2 and 300.5(I) Exception No. 2 provide, and it suits a long underground duct bank where it simplifies the pull, at the cost of unequal reactance between phases and of a raceway that is never converted to metal. (7.4.3)

NOTE A parallel set shares current by impedance, and the impedance of each conductor is its length; a set whose conductors differ by a few feet in a short run divides its current unevenly, and the shorter conductor carries more than its share and runs hotter, which is why the length is measured and recorded rather than assumed from the raceway length. (7.4.7)

NOTE A full-size neutral costs one more conductor at the phase size and never has to be revisited when the load changes, which is why most projects carry it on every feeder; a neutral reduced to the unbalanced load under NFPA 70 220.61 saves copper on a feeder whose load is balanced three-phase equipment, and the reduction is lost the day a nonlinear load is added, since 220.61(C) forbids it where the load is largely nonlinear; a 200 % neutral serves a feeder to a panelboard whose load is electronic, where the third-harmonic currents of the three phases add in the neutral rather than cancel, and it is paired with the 200 % neutral bus selected under PanelboardsPanelboardsResolves to the current adopted revision.sync/panelboards. (7.5.2)

NOTE NFPA 70 215.2(A) carries an informational note that a feeder voltage drop of 3 % and a combined feeder and branch circuit drop of 5 % provide reasonable efficiency of operation, and several editions of the energy code enforced 2 % on feeders and 3 % on branch circuits as a requirement; a tighter feeder criterion buys motor starting torque and steady voltage at electronic loads at the cost of a larger conductor on every long feeder, and a looser one is accepted on a short feeder to a lighting load whose voltage the drivers tolerate. (7.6.3)

NOTE The combined feeder and branch circuit voltage drop criterion is selected under Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables, and the feeder criterion here is the portion of that total the feeder is allowed to consume. (7.6.4)

NOTE A tap lets one feeder device serve several pieces of equipment through short unprotected conductors, which removes a device and an enclosure from every tap and is how a transformer, a gutter feeding grouped disconnects, and a motor control center lineup are commonly supplied; forbidding taps puts a device at the head of every conductor, which costs devices and space and is what an Owner chooses when its maintenance staff want every conductor protected at its source and no length of feeder that relies on a device ten times its ampacity. (7.7.3)

NOTE ANSI/NETA ATS Table 100.1 assigns 1000 V dc and a 100 MΩ minimum to 600 V class conductors, and a 500 V dc test at the same minimum serves a conductor whose attached equipment the tester cannot isolate; the 2500 V dc setpoint serves a 1000 V rated conductor or an Owner that wants a stressed test of a long underground service, and the higher minimum values suit a new conductor in a dry raceway, where a reading near 100 MΩ on a short run is already a sign of moisture or damage. (8.1.3)

NOTE The test is performed before the conductors are connected because once they are landed on the utility's terminals and the service disconnect, the premises side can no longer be isolated without the utility, and a defect found then costs an outage; Electrical Service EntranceElectrical Service EntranceResolves to the current adopted revision.sync/electrical-service-entrance holds the service from the utility's connection until this test is recorded. (8.1.5)

NOTE A parallel set whose conductors are cut to the same length and terminated alike shares current within a few percent, and a conductor that carries noticeably more than its share is the one with a loose termination, a shorter length, or a different route; the tolerance is set to catch those without failing a set for the small differences that lug resistance alone produces, and a tighter tolerance suits a set near its ampacity where any imbalance leaves one conductor overloaded. (8.2.3)

NOTE A conductor hanging in a riser carries its own weight on the termination at the top unless it is supported, and a large conductor in a tall riser weighs enough to pull out of a lug or to deform its insulation at the top bushing; the table intervals shorten as the conductor grows for that reason. (9.2.2)