Article 100 defines feeders as: “All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.”
It is better this way, “The circuit conductors that supply power to a branch-circuit overcurrent device or to a panel that contains such devices. The power may come from the service equipment, a separately derived system, or other power supply source.”
A feeder runs between an overcurrent protection device (OCPD) at the supply and a downstream OCPD (typically supplying a branch circuit), while a branch circuit runs between an OCPD and an outlet (or final load). In other words, a feeder supplies power to a branch-circuit OCPD — which, in turn, powers a branch circuit. However, you size that branch-circuit OCPD based on branch-circuit load calculations (and outlet requirements), not on feeder calculations.Because a feeder occupies this “between space” in power distribution, feeder requirements are simpler and fewer than branch requirements. Consequently, Art. 215 is much shorter than Art. 210.
Article 210 also devotes extensive space to dwelling-area branch circuits. But because they occupy the “between space” in power distribution, feeders have minimal requirements for dwellings [215.2(A)(3)].
Minimum rating
Determine the minimum feeder conductor size, before applying any conductor adjustment and/or correction factors, by adding the two following quantities: 125% of the continuous load [215.2] and 100% of the noncontinuous load. Once you have the total load, size the minimum conductor required to carry that load based on the terminal temperature rating ampacities as listed in Table 310.16 [110.14(C)]. Size the OCPDs based on this same ampacity [215.3 and 240.4].
Fig. 1. Feeder ampacity must be no less than 125% of the continuous load, plus 100% of the noncontinuous load (200A 2 1.25 4 250A).
Note that equipment suitable for 100% continuous loading is rarely available in ratings under 400A. The NEC requires the feeder grounded (neutral) conductor not to be smaller than the size listed in Table 250.122 (Table), based on the rating of the feeder OCPD.
Table 310.16 and 220.61 would permit an 8 AWG grounded (neutral) conductor rated 50A at 75°C to carry the 50A unbalanced load; however, Table 250.122 requires that the grounded (neutral) conductor not be smaller than 4 AWG.
The size of the grounded neutral conductor must not be smaller than specified in 250.122, based on the rating of the feeder overcurrent protection device.
On a 4-wire, delta-connected, 3-phase system — where the midpoint of one phase winding is grounded — the conductor with the highest phase voltage-to-ground (208V) is called the high-leg.
Panelboards supplied by a 4-wire, delta-connected, 3-phase system must have the high-leg conductor (208V) terminate to the “B” (center) phase of a panelboard [408.3(E)]. (This has been a rule since 1975). An exception to 408.3(E) permits the high-leg conductor to terminate to the “C” phase when the meter is located in the same section of a switchboard or panelboard.
Ensure this high-leg conductor is durably and permanently marked with an orange outer finish (see Color System) at each point a connection is made where the grounded (neutral) conductor is present [110.15]. The NEC says you can use “other means” but doesn't provide further detail. Get permission from the AHJ to use some “other means.”
Ground-fault protection of equipment
Article 100 tells us that “ground-fault protection of equipment” systems interrupt power to protect equipment, but at lower current levels than those required to protect conductors through the operation of a supply circuit overcurrent device.
Each solidly grounded wye electrical 277/480V feeder disconnecting means rated 1,000A or more must be provided with ground-fault protection of equipment, and the installation must comply with 230.95 or 240.13 [215.10].
If ground-fault protection is on the supply side of the feeder, you don't have to also provide it on the load side; nor do you have to provide it for emergency systems [700.26] or legally required standby systems [701.17].
Never apply ground-fault protection to fire pumps [695.6(H)], which must run no matter what. It makes no sense to save the pump but burn down the building.
Identification
Fig. 3. The grounded (neutral) conductor of a feeder must be identified per 200.6 [215.12(A)].
The grounded (neutral) conductor of a feeder must be identified per 200.6 [215.12(A)] (Fig. 3).
The equipment grounding conductor must be identified per the requirements of 205.119, but it's not really a grounding conductor. It's actually a bonding conductor. This conductor creates a low-impedance path between metallic objects. Because of the low impedance, the voltage differential between objects is low. The result is an equipotential plane between objects bonded by the equipment grounding (bonding) conductors. Equipment grounding (bonding) conductors:
Can be bare, or individually covered or insulated.
Sized 6 AWG and smaller that are insulated must have a continuous outer finish that is either green or green with one or more yellow stripes [250.119].
Larger than 6 AWG and insulated can be permanently reidentified with green marking (at the time of installation) at every point where the conductor is accessible [250.119(A)].
Where the premises wiring system contains feeders supplied from more than one voltage system, each ungrounded conductor (where accessible) must be identified. Identification can be by color-coding, marking tape, tagging, or other means approved by the AHJ. System identification must be permanently posted at each feeder panelboard or similar feeder distribution equipment.
Service conductors supply power to the service equipment, and feeders run from the service equipment to the branch OCPD. Feeders are the “between circuits” that occupy the “between space” in power distribution.
Sidebar: Color System
Electricians often use the following color system for power and lighting conductor identification:
- 120/240V single-phase — black, red, and white
- 120/208V, 3-phase — black, red, blue, and white
- 120/240V, 3-phase — black, orange, blue, and white
- 277/480V, 3-phase — brown, orange, yellow, and gray; or, brown, purple, yellow, and gray.
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