Branch Circuits, Feeders, and Service Calculations
What the exam tests
This domain covers branch-circuit ratings, receptacle and lighting loads, feeder and service demand factors, and the 125% continuous-load rule in Articles 210, 215, 220, and 230.
Core rules
- Continuous loads are calculated at 125% for branch-circuit and feeder sizing.
- Dwelling general lighting is 3 VA per sq ft; small-appliance branch circuits are 1,500 VA each; laundry is 1,500 VA.
- Table 220.42 applies demand to dwelling lighting and receptacle loads: first 3,000 VA at 100%, the remainder at 35%.
- Standard branch-circuit ratings include 15, 20, 30, 40, and 50 A; receptacle outlets on 15/20 A circuits must match conductor and OCPD ratings.
- Service and feeder conductors must carry the calculated load and have an OCPD rated at least the calculated load.
Worked example
A 2,000 sq ft dwelling has general lighting 2,000 × 3 = 6,000 VA; two small-appliance circuits 2 × 1,500 = 3,000 VA; laundry 1,500 VA. Total = 10,500 VA. Demand: first 3,000 at 100% = 3,000; remaining 7,500 at 35% = 2,625; total 5,625 VA. At 240 V, 5,625 / 240 = 23.4 A. Add the range, dryer, and heating loads separately before sizing the service.
Traps
Applying Table 220.42 before adding all lighting and receptacle loads; forgetting the 125% factor for a continuous water-heater or EV load; using 3 VA/sq ft for a store; and sizing the service from the unadjusted connected load.