Branch circuits

A branch circuit is the final link in an electrical system: the conductors between the last overcurrent device and the outlets, where lights, receptacles and equipment connect. Most of an electrician's working day is spent installing, extending and troubleshooting branch circuits, and most of the code questions that arise on site are about them: how many receptacles may go on one circuit, what rating of receptacle suits a 20 A circuit, how much continuous load a circuit may carry, and what is required when two circuits share a neutral. This lesson introduces the rules that govern branch circuits; Lesson 4 then sizes their conductors, and ELEC-130 applies them to dwellings.

In the National Electrical Code, branch circuits are covered by Article 210 (ratings, outlets required, GFCI and AFCI protection, multiwire circuits) and the load calculation methods of Article 220; receptacles are in Article 406.

Branch circuits, feeders and services

The code distinguishes three kinds of circuit by where they sit in the system:

• The service: conductors and equipment that bring power from the utility to the building and contain its main disconnecting means.
• Feeders: circuits from the service equipment (or another power source, such as a transformer) to a panelboard or other distribution point.
• Branch circuits: circuits from the final overcurrent device to the outlets.

Branch circuits are further described by what they supply: a general-purpose branch circuit supplies several outlets for lights and appliances; an appliance branch circuit supplies outlets for appliances; an individual branch circuit supplies only one piece of equipment.

The rating of a branch circuit

A branch circuit is rated by the rating of its overcurrent device, not by its conductors. A circuit protected by a 20 A breaker is a 20 A circuit even if it is wired with conductors that could carry more, for example oversized for voltage drop. Standard ratings for circuits supplying several outlets are 15 A, 20 A, 30 A, 40 A and 50 A; individual branch circuits can have any rating.

The conductors must have an ampacity at least equal to the load, and at least 125% of any continuous load (Lesson 4), and the overcurrent device must protect them. The receptacles and other devices on the circuit must be rated to match it.

Continuous loads

A continuous load is one expected to run at its maximum current for three hours or more, such as lighting in a store, an electric water heater or a server. Standard breakers and fuses are not designed to carry their full rating continuously, so continuous loads are limited to 80% of the device rating (equivalently, the device must be at least 125% of the continuous load).

Receptacles on branch circuits

Receptacles must match the circuit: a receptacle rated less than the circuit could be overloaded by a plug that the circuit would allow, and one rated more could accept a plug for equipment that the circuit cannot supply. The standard North American configurations are defined by CSA C22.2 No. 42 and NEMA WD 6: the 5-15R is the common 15 A, 125 V receptacle, and the 5-20R, with one T-shaped slot, accepts both 15 A and 20 A plugs.

In US dwellings, kitchen countertop receptacles must be supplied by at least two 20 A small-appliance branch circuits (210.11(C)(1) and 210.52(B)), and bathroom receptacles by a 20 A circuit (210.11(C)(3)).

How much may a circuit supply?

The NEC does not limit the number of outlets on a branch circuit as such; instead, the calculated load must not exceed the circuit rating. In occupancies other than dwellings, each general-use receptacle outlet is counted at 180 VA for load calculations (220.14(I)), which effectively limits how many may share a circuit. In dwellings, general lighting and receptacle load is calculated per square foot of floor area (220.41), and the number of circuits follows from that load.

Cord-and-plug and fixed equipment

When a branch circuit supplies several outlets, a single piece of cord-and-plug connected equipment should not use more than 80% of the circuit rating, leaving room for other loads. Equipment fastened in place (such as a built-in dishwasher or a wall heater) on a circuit that also supplies lighting or other receptacles is limited to 50% of the circuit rating, so that the other outlets remain usable.

Multi-wire branch circuits

A multi-wire branch circuit uses two or three ungrounded (hot) conductors that share one grounded (neutral) conductor. On a 120/240 V system, a two-hot multi-wire circuit supplies two 120 V circuits with three conductors instead of four, saving copper and space; the neutral carries only the difference between the two hot currents (ELEC-110, Lesson 6).

Multi-wire circuits bring three hazards, which the code addresses:

• The hot conductors must be on opposite phases (or different phases on a three-phase system). If both were on the same phase, the neutral would carry the sum of the two currents and could be overloaded without any breaker tripping.
• All ungrounded conductors must be disconnected simultaneously, by a two-pole breaker or by single-pole breakers with an identified handle tie, so that a worker who switches off one circuit does not find the shared neutral still carrying current from the other.
• The neutral must not depend on a device for continuity: it is spliced with a pigtail rather than run through the terminals of a receptacle, so that removing a receptacle cannot open the neutral and put the remaining loads in series across 240 V.

The NEC requirements for multiwire branch circuits are in 210.4: simultaneous disconnection of all ungrounded conductors, grouping of the conductors of each circuit, and limitation to line-to-neutral loads unless all ungrounded conductors are opened together.

Identification and labeling

Circuits must be identifiable. Every panelboard needs a legible directory showing what each circuit supplies, specific enough that a person can find the right breaker without guesswork: "Kitchen counter receptacles, east wall" rather than "Kitchen" or "Plugs". Conductors are identified by color (Lesson 2), and the conductors of each multi-wire circuit should be grouped so that their neutral can be traced.

Planning branch circuits

Good branch-circuit design goes beyond the minimum:

• Separate circuits for large or sensitive loads: refrigerators, sump pumps, freezers, network equipment and medical devices.
• Lighting and receptacles in an area on different circuits, so that a fault in one does not leave people in the dark.
• Spare capacity, both in the circuit loads and in panel spaces, for future additions.
• Voltage drop checked for long runs (Lesson 4), especially for 120 V circuits to distant outbuildings or parking areas.