Feeders

Feeder vs branch circuit

A feeder is every conductor between the service equipment — or another power source such as a generator or transformer — and the final branch-circuit overcurrent device. The branch circuit is what comes after that last overcurrent device: the conductors running to the outlets and loads. Drawing that line correctly is the first thing the exam checks, because feeder rules (Article 215) and branch-circuit rules (Article 210) are scored separately.

In a typical house, the conductors from the main panel out to a detached garage subpanel are a feeder; the circuits leaving that subpanel are branch circuits. Get the boundary right and the rest of the problem — sizing, protection, grounding — falls into place.

On the journeyman exam, feeder questions tend to test four things:

• telling a feeder apart from a branch circuit or service conductor;
• applying the 125 percent factor to the continuous portion of the load;
• matching the overcurrent device and disconnect rules; and
• recognizing the voltage-drop recommendation on long runs.

Sizing feeder conductors

Feeder conductors must have an ampacity at least equal to the load they serve, calculated under Article 220. The governing rule in 215.2 is that the minimum feeder ampacity is the noncontinuous load plus 125 percent of the continuous load, before any adjustment or correction factors. Once you have that minimum ampere figure, you pick a conductor from the ampacity tables at the appropriate termination temperature column.

Worked example: a feeder serves a 160-amp noncontinuous load and a 40-amp continuous load. The minimum conductor ampacity is 160 + (40 × 1.25) = 210 amps. You would then choose a conductor rated at least 210 amps in the 75°C column, assuming 75°C terminations.

Overcurrent protection and the disconnect count

The feeder overcurrent device protects the feeder at its ampacity, with the standard next-size-up allowance of 240.4(B) available where the feeder does not supply the loads that bar it. Just as a service is limited to six disconnecting means (230.71), a feeder that supplies a separate building or structure is limited to six disconnects at that building under 225.33 — the same "six operations of the hand" idea applied past the service. Knowing that the six-disconnect concept reaches feeders to outbuildings is a common exam distinction.

Voltage drop, grounding, and common mistakes

The NEC does not enforce voltage drop as a hard rule, but an informational note to 215.2 recommends sizing feeders so voltage drop does not exceed 3 percent, with the combined drop of the feeder plus its branch circuits held to 5 percent. On long runs you upsize the feeder conductors to stay within that recommendation — and remember that upsizing the phase conductors forces the equipment grounding conductor to grow proportionally under 250.122(B).

A feeder also needs an equipment grounding conductor, and it is sized from Table 250.122 using the feeder overcurrent device rating — the same logic as a branch circuit, just at a larger scale. Feeder tap conductors are a separate advanced topic: 240.21(B) lets a tap run a limited distance without overcurrent protection at the tap point, under the well-known 10-foot and 25-foot tap rules. Those are worth recognizing even if the journeyman exam keeps the heavy tap math light.

The mistakes that cost points on feeder questions are predictable:

• applying the 125 percent factor to the whole load instead of only the continuous portion;
• confusing feeder rules (Article 215) with service rules (Article 230); and
• forgetting that a feeder to a separate building carries its own disconnect and grounding requirements.