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.