Services, Feeders, and Branch Circuits
Services, feeders, and branch circuits
This domain ties load calculation to conductor and overcurrent sizing. Article 220 provides standard and optional calculation methods for dwellings and other occupancies. Article 230 governs service conductors, service disconnects, and service equipment; Article 215 governs feeders; Article 210 governs branch circuits. The core rule is that conductors and overcurrent devices must be sized for the load, with continuous loads increased by 125% unless the assembly is listed for 100% operation. Receptacle, lighting, and appliance loads are calculated separately, then combined under the applicable article.
Branch-circuit requirements include receptacle spacing in dwellings, GFCI protection in specified locations, AFCI protection for dwelling branch circuits, and permissible loads on 15 A and 20 A circuits. Service disconnects must be readily accessible and located at a readily accessible point nearest the point of entry, subject to the specific rules in 230.70.
Traps and a worked example
Traps include measuring the 6 ft receptacle spacing along the floor line rather than through doorways, forgetting that a wall space 2 ft or wider needs a receptacle, mixing continuous and noncontinuous loads, and applying the 125% factor to the load but not to the overcurrent device. Another trap is assuming a 20 A circuit can carry 20 A continuously.
Worked example: a 20 A, 120 V branch circuit has a continuous load. The circuit capacity is 20 A × 120 V = 2,400 VA. For a continuous load, the load must not exceed 80% of the rating unless listed for 100%: 2,400 VA × 0.80 = 1,920 VA, or 16 A. Rule of thumb: for continuous loads, size at 125% of load; for a 20 A circuit, the continuous load limit is 16 A.