SFB-5 — Branch-Circuit Ratings, Conductors, and Loads

Exam checkpoints

Find the final overcurrent device

Article 100 defines a branch circuit as the circuit conductors between the final overcurrent device protecting the circuit and the outlet or outlets. An outlet is a point on the wiring system where current is taken to supply utilization equipment; it is not limited to a receptacle. Conductors ahead of that final device are feeders or service conductors, depending on their source.

Under 210.3, the branch-circuit rating is determined by the maximum permitted ampere rating or setting of the branch-circuit overcurrent device. Common classifications include 15, 20, 30, 40, and 50 A, but other ratings can be permitted. The connected load does not rename a 20 A circuit as a 16 A circuit.

Coordinate load, conductor, and protection

Under 2017 NEC 210.19(A)(1), branch-circuit conductors generally need ampacity at least equal to the maximum load and at least the noncontinuous load plus 125 percent of the continuous load. If a branch circuit supplies two or more receptacles or outlets, conductor ampacity also cannot be less than the branch-circuit rating. The listed 100-percent-rated assembly exception applies only when its conditions are met.

Section 210.20(A) generally sizes branch-circuit overcurrent protection at the noncontinuous load plus 125 percent of the continuous load, with a corresponding 100-percent-rated exception. Other articles can impose different or additional rules for motors, HVAC, appliances, welders, and similar equipment.

Example: assume a 120 V load draws 16 A continuously for three hours or more, with no noncontinuous load, no special equipment rule, and no 100-percent-rated assembly. The conductor and overcurrent load basis is 16 A × 1.25 = 20 A. Real power for a resistive load at power factor 1.00 is 120 V × 16 A = 1,920 W. A 20 A branch circuit can supply that continuous load under these assumptions. For ordinary copper conductors, 12 AWG is protected at not more than 20 A under 240.4(D), even if a higher-temperature ampacity table shows a larger number.

Do not reverse the calculation and claim every 20 A circuit must carry 20 A continuously. Under the ordinary 125 percent relationship, 20 A of protection corresponds to at most 16 A of continuous load when there is no other load.

Apply small-conductor and terminal rules

Section 240.4(D) generally limits 14 AWG copper to 15 A, 12 AWG copper to 20 A, and 10 AWG copper to 30 A, unless a listed specific application permits otherwise. These limits override a tempting higher ampacity from the conductor's 75°C or 90°C insulation column for ordinary circuits.

Terminal limitations in 110.14(C) still apply. Equipment rated 100 A or less or marked for 14 AWG through 1 AWG conductors generally uses the 60°C provisions unless listed and identified for another allowed temperature. Conductor insulation with a higher rating can be useful for correction and adjustment, but it does not erase terminal or small-conductor limits.

The voltage-drop language associated with 210.19 is an Informational Note. The familiar 3 percent branch and 5 percent combined guidance supports reasonable operation but is not a general mandatory maximum by itself. Some equipment rules can independently require adequate voltage at terminals.

Match receptacles to the circuit

Under 210.21(B)(1), a single receptacle on an individual branch circuit must have an ampere rating not less than the branch-circuit rating. For two or more receptacles or outlets, Table 210.21(B)(3) applies. A 15 A circuit uses 15 A receptacles; a 20 A circuit permits 15 A or 20 A receptacles. A duplex receptacle counts as two receptacles, so a listed 15 A duplex can be used on a 20 A multioutlet circuit. A single 15 A receptacle cannot be the only receptacle on an individual 20 A circuit.

Receptacle rating does not determine conductor ampacity by itself, and a 20 A receptacle does not authorize a 20 A load on a 15 A circuit. Check circuit rating, number of receptacles, load, conductor, and protection together.

Check permissible loads

Section 210.23 governs loads on branch circuits. For 15 A and 20 A circuits supplying lighting or other loads, the rating of any one cord-and-plug-connected utilization equipment not fastened in place generally cannot exceed 80 percent of the circuit rating. Where lighting, cord-and-plug equipment not fastened in place, or both are also supplied, utilization equipment fastened in place cannot exceed 50 percent of the circuit rating.

Thus, assume a 20 A branch circuit also supplies lighting. One fastened-in-place appliance is generally limited to 20 A × 0.50 = 10 A. One cord-and-plug appliance not fastened in place is generally limited to 20 A × 0.80 = 16 A. These limits address load arrangement; they do not replace required individual circuits or appliance instructions. Sections 210.23(B), (C), and (D) provide different permissions for 30 A, 40 A, and 50 A circuits.

Recognize multiwire branch circuits

A multiwire branch circuit has two or more ungrounded conductors with voltage between them and a grounded conductor with equal voltage between it and each ungrounded conductor, with the neutral connected to the system neutral. Under 210.4(B), a common-trip device or identified handle ties must simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates. Section 210.4(D) requires grouping of the ungrounded and grounded conductors at the panelboard or other point of origin, subject to its exception.

The grounded conductor carries imbalance only when the ungrounded conductors have the required phase relationship. Miswiring both ungrounded conductors to the same leg can make neutral currents add instead of cancel. The simultaneous-disconnect rule does not make the neutral an equipment grounding conductor and does not permit neutral-ground bonding downstream. Finish every branch-circuit problem by checking rating, load type, continuous duration, conductor and terminal limits, overcurrent protection, permitted outlets, and equipment-specific rules.