Branch Circuits, Feeders, and Service Calculations
Domain 3 is the quantitative core of the exam. Expect questions on mandatory branch circuits, demand-factor arithmetic, and the step from a computed load to a conductor and service size.
Branch-circuit rules
210.11 sets dwelling minimums: two or more 20 A small-appliance circuits, one 20 A laundry circuit, one 20 A bathroom circuit, plus garage and outdoor provisions. 210.19(A) sizes conductors at 100% of noncontinuous plus 125% of continuous load; 210.20 applies the same logic to the overcurrent device.
Service and feeder calculations
220.12 assigns 3 VA per square foot of dwelling floor area. 220.52 adds 1,500 VA for each small-appliance and laundry circuit. Table 220.42 applies the dwelling demand schedule. 220.53 permits 75% of the nameplate rating where four or more fixed appliances are involved. 220.54 sets dryers at 5,000 VA minimum. 220.55's table sets range demand - Column C gives 8,000 VA for one range of 12 kW or less, with 5% added per kW above 12. The 220.82 optional method allows 100% of the first 10 kVA plus 40% of the remainder and 125% of the largest motor. 230.79(C) sets a 100 A minimum service for a one-family dwelling.
Worked example
A dwelling has one 12 kW, 240 V range. Table 220.55 Column C makes the service contribution 8,000 VA, not 12,000 - a demand current of 33.3 A at 240 V. The branch circuit is a 50 A circuit, and 210.19(A)(3) permits 8 AWG copper conductors (40 A ampacity) on a 40 or 50 A range circuit, because the range's cycling is addressed by the table demand.
Traps
Using nameplate kW instead of the 220.55 demand; applying 220.53's 75% to only three appliances; dropping the laundry circuit from the general load; omitting the 125% multiplier on a continuous load; assuming Table 220.42 percentages apply to range or dryer loads.