2021 IRC Chapter 37
A branch circuit is the wiring downstream of the last overcurrent device, the wire that actually reaches a receptacle, a light, or an appliance. A feeder is everything upstream of that, from the service or a subpanel's main breaker down to the branch-circuit panelboard. This lesson covers which branch circuits a dwelling is required to have, how a conductor's size and the breaker protecting it have to match, and the handful of sizing rules, continuous loads, multiwire circuits, next-size-up breakers, that come up on nearly every rough-in inspection. None of this requires a full load calculation to check in the field; it requires knowing a small set of numbers cold.
IRC E3703.2 requires at least two 20 A small-appliance branch circuits to serve the wall, floor, and countertop receptacle outlets, and the refrigerator receptacle, in the kitchen, pantry, breakfast room, and dining room (E3901.3). Under E3901.3.1 these circuits may serve no other outlets at all, no lighting, no bathroom receptacles, nothing outside that group of rooms, with only two narrow exceptions: a receptacle for an electric clock, and receptacles that power supplemental equipment and lighting on gas-fired ranges, ovens, and cooking units. They exist to keep countertop appliances like a microwave and a coffee maker from competing with the rest of the house's lighting for capacity. E3703.3 requires at least one 20 A circuit for the laundry receptacle outlet, and it may serve only receptacle outlets located in the laundry area. E3703.4 requires at least one 20 A circuit for bathroom receptacles. That bathroom circuit can serve receptacles in more than one bathroom and nothing else, or it can be dedicated to a single bathroom, in which case it is then allowed to also serve that bathroom's other equipment, its light and exhaust fan, for instance. Mixing this up, feeding a dining room light off a small-appliance circuit, or landing a bathroom receptacle on general lighting, is one of the most common rough-in findings GCHS inspectors write up. E3703.5 adds one more required circuit: at least one 120-volt, 20 A branch circuit for the receptacle outlets in attached garages and in detached garages with electric power, with no other outlets except readily accessible outdoor receptacles.
Every conductor has an ampacity from the standard tables, but for the three sizes used constantly in dwelling branch circuits, E3705.5.3 (Table E3705.5.3) overrides the ampacity table with a flat rule: 14 AWG copper is limited to a 15 A overcurrent device, 12 AWG copper to 20 A, and 10 AWG copper to 30 A, regardless of what a table lookup might otherwise permit for that conductor in a particular installation. Small conductors do not have enough thermal mass to safely ride out a moderate overload for as long as a typical breaker takes to trip, so the code sets a hard ceiling rather than trusting the table alone. This is also why swapping a 15 A breaker for a 20 A breaker to stop a nuisance trip on 14 AWG wiring is not a shortcut, it is a straightforward violation that leaves the conductor able to run chronically overloaded before the larger breaker ever notices.
A continuous load is one where the maximum current is expected to continue for 3 hours or more. Under E3701.2.1, the conductors of a branch circuit supplying one must have an ampacity of not less than the noncontinuous load plus 125 percent of the continuous load, and the breaker is sized to the same figure (NEC 210.20(A)), which is the same as saying the continuous load itself cannot exceed 80 percent of the device's rating, because a breaker held near its full rating for hours runs hot and drifts toward nuisance tripping. E3702.10 sizes the branch circuit conductors and overcurrent device for a fixed storage-type water heater of 120 gallons or less at not less than 125 percent of the heater rating: a common 4,500 W, 240 V element draws 18.75 A, and at 125 percent that is 23.4 A, which rounds up to the next standard breaker size of 25 A at minimum. In practice these heaters are very often installed on a 30 A circuit with 10 AWG copper, which comfortably clears the 125 percent calculation with room to spare.
A multiwire branch circuit uses two ungrounded conductors on opposite legs of the 120/240 V supply sharing one neutral, three wires doing the work that would otherwise take four, and it shows up constantly in kitchens and in older houses feeding split-wired receptacles or a shared circuit to a garage subpanel. Because both hot conductors share that one neutral, opening only one of them still leaves the neutral carrying current from the other, live conductor, current an electrician working on what looks like a dead circuit does not expect. IRC E3701.5.1 (NEC 210.4(B)) requires each multiwire branch circuit to have a means that disconnects all of its ungrounded conductors simultaneously where the circuit originates, in practice a two-pole breaker or two single-pole breakers joined with an identified handle tie, precisely so that circuit cannot be only half de-energized. E3701.5.2 also requires the conductors to be grouped together by wire markers, cable ties, or similar means in at least one location within the panelboard, unless the circuit arrives in its own cable or raceway where the grouping is already obvious.
A feeder carries the combined load of everything downstream of it to a panelboard, most often the conductors from the main service disconnect to a subpanel, or from a meter-main to the service equipment. Feeders are sized from a load calculation covering everything the subpanel will serve (E3704.2), not from any single branch circuit's rating, and like branch circuits they still have to clear both an ampacity check for the conductor and the small-conductor-rule style limits that apply at their own sizes. A detached garage or accessory dwelling unit fed by a subpanel is the case GCHS runs into most: the feeder has to be sized for the receptacles, lighting, and any dedicated equipment circuits planned for that structure, and it needs its own equipment grounding conductor run alongside it (E3704.6 and E3607.3.1) exactly like any other feeder, not a bond to a local ground rod standing in for one.
Overcurrent devices only come in a defined list of standard ampere ratings, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125 amperes and larger steps above that (Table E3705.6). A conductor's calculated ampacity often does not land exactly on one of those numbers; 6 AWG copper at the 60 degree C column, for example, has an ampacity of 55 A, which is not a standard breaker size at all. Where that happens and the circuit does not supply multiple receptacles for cord-and-plug-connected portable loads, E3705.5.2 permits the next higher standard rating, so 6 AWG copper is routinely protected at 60 A, common on a range feeder or a subpanel feeder, provided that next rating does not exceed 400 A. This next-size-up allowance is narrow by design, it exists to bridge an odd ampacity to the nearest standard device, not to justify undersizing a conductor for a load it will actually have to carry.
A worked example ties the small-conductor rule and the next-size-up rule together. A subpanel feeder uses 6 AWG copper conductors with an ampacity of 55 A at the 60 degree C column. Because 55 A is not a standard breaker rating, and this feeder does not serve multiple cord-and-plug receptacle loads, the next higher standard rating, 60 A, is the permitted overcurrent device. Compare that with a 20 A kitchen small-appliance circuit in 12 AWG copper: 12 AWG's table ampacity may be higher still, but the small-conductor rule caps it at 20 A regardless, so there is no next-size-up question at all for that circuit. The two rules answer two different situations, an odd ampacity looking for the nearest standard size, and a small conductor being capped outright no matter what the table says.
In the field
On rough-in inspections of additions and rehabs, GCHS most often catches two branch-circuit problems together: a 20 A breaker landed on 14 AWG cable feeding general-use receptacles, almost always because an electrician grabbed the wrong spool mid-job, and a dining or breakfast room light tied onto one of the small-appliance circuits because it was the closest home run. Both get flagged before rock goes on the walls, since both are far cheaper to correct with the cable still exposed than after drywall, insulation, and paint are in place.
1. Under E3703.2 and E3901.3, a dwelling's kitchen and dining area receptacle outlets must be served by:
2. An electrician connects a dining room ceiling light to one of the two small-appliance circuits. This is:
3. Under E3703.3, the required laundry branch circuit must be rated:
4. A dwelling has two bathrooms. Which arrangement satisfies E3703.4's bathroom receptacle circuit requirement?
5. Under E3705.5.3, 14 AWG copper conductors are limited to an overcurrent device rated:
6. An inspector finds 14 AWG copper NM cable landed on a 20 A breaker feeding general-use receptacles. This is:
7. A branch circuit supplies a continuous load. Under E3701.2.1, the conductor ampacity must be not less than the noncontinuous load plus what portion of the continuous load?
8. A fixed storage-type water heater of 120 gallons or less with a 4,500 W, 240 V element draws 18.75 A. Applying E3702.10, the minimum standard breaker size is:
9. A multiwire branch circuit is best described as:
10. Under IRC E3701.5.1, the ungrounded conductors of a multiwire branch circuit must:
11. A feeder is best defined as:
12. A 6 AWG copper conductor has a 60-degree-C ampacity of 55 A, which is not a standard breaker rating. On a feeder that does not supply multiple cord-and-plug receptacle loads, it may generally be protected at:
13. Which is NOT a standard ampere rating for a fuse or inverse-time circuit breaker?
14. A household electric range rated 12 kVA requires a branch circuit of at least: