Services, Feeders, and Branch Circuits
Service equipment
Service conductors run from the service point to the service disconnecting means. Know the overhead clearance rules (typically 10 ft above sidewalks, 12 ft above driveways, 18 ft above roads), the requirement that the service disconnecting means be installed at a readily accessible location nearest the point of entrance, and the “six throws” rule: no more than six service disconnects, grouped, each marked as service equipment. A separate building or structure generally needs its own disconnect.
Feeders and branch circuits
Feeder conductors must have ampacity at least 125% of continuous loads plus 100% of noncontinuous loads, with the overcurrent device protecting the conductor. Branch circuits are rated 15, 20, 30, 40, or 50 A; general-purpose receptacle circuits in dwellings are typically 20 A. Small-appliance and laundry circuits must be 20 A and cannot serve other outlets.
Receptacle spacing traps
Dwelling receptacle spacing is measured along the floor line of wall space; no point may be more than 6 ft from a receptacle, which yields a 12 ft maximum spacing. Wall spaces wider than 2 ft need a receptacle. The 6 ft is measured along the floor line, not straight-line through a doorway.
Worked example
A 20 A, 120 V branch circuit supplies a continuous 16 A load. Continuous loads are limited to 80% of the circuit rating: 20 × 0.80 = 16 A, so this is exactly at the limit and acceptable. If the load were 17 A, the circuit would need to be 30 A or the load reduced.