Occupying the "between space" in power distribution
Last month, we looked at branch-circuit requirements in Art. 210. The next logical step up from the branch circuit is the feeder circuit. Thus, Art. 215, which provides the requirements for sizing and installing feeders (see What's a Feeder? on page 62), follows Art. 210.
At first glance, the difference in length between Art. 215 and Art. 210 may seem puzzling — why have a separate Article just for feeders, anyhow? This is an object lesson in the value of Art. 100. A quick review of the definitions “branch circuit” and “feeder” helps us see why a separate Article is necessary and sheds light on why Art. 215 is so much shorter.
The main difference is that 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)].
Determine the minimum feeder conductor size, before you apply 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].
Here's a pop quiz to see if you can properly size a feeder. What size feeder conductor do you need for a 200A continuous load, if the terminals are rated 75°C (Fig. 1)? How do you determine the correct answer? Because the continuous load is 200A, the feeder conductors must have an ampacity at least 250A (200A × 1.25). Using the 75°C column of Table 310.16, you find that 250kcmil conductors are suitable because they have an ampere rating of 255A at 75°C. However, there is an exception to the 125% rule. Where the assembly and the OCPD are both listed for 100% continuous load operation, you can size the feeder conductors at 100% of the continuous load. 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.
Test your knowledge, with another pop quiz. What size grounded (neutral) conductor do you need for a feeder consisting of 250kcmil ungrounded conductors and one grounded (neutral) conductor protected by a 250A protection device, where the unbalanced load is only 50A, with 75°C terminals (Fig. 2)?
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.
You also have to consider the size of the service conductors when you size feeder conductors. Feeder conductors for individual dwelling units or mobile homes needn't be larger than service conductors sized per 310.15(B)(6). For the sake of efficiency, you should size the conductors to minimize voltage drop. Doing so is an engineering consideration, not an NEC requirement [215.2(A)(3) FPN No. 2].
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.”
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.
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. Keeping this in mind will help simplify feeder work on your next job.
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.” The clumsiness of this wording makes this definition hard to follow.
Here's an easier way to look at it: “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.”
Electricians often use the following color system for power and lighting conductor identification: