Branch Circuits, Feeders, and Service Calculations
Articles 210, 215, 220, and 230 cover what the exam calls load calculations: receptacle and lighting requirements for branch circuits, demand factors for feeders, and the minimum service size. Both the standard method in 220 Parts III and IV and the optional method in 220.82 and 220.84 appear on the exam.
Branch-circuit rules
- 210.52(A): wall receptacles spaced so no point is more than 6 ft from one; kitchen countertops so no point is more than 24 in from one.
- 210.11(C): two or more 20 A small-appliance circuits, one laundry circuit, and a 20 A bathroom circuit.
- 210.8(A) lists the dwelling locations needing GFCI protection, and 210.8(D) covers specific appliances such as dishwashers.
Worked example: 2,400 ft² dwelling, 120/240 V
- General lighting: 2,400 × 3 VA = 7,200 VA.
- Small appliance: 2 × 1,500 = 3,000 VA; laundry: 1,500 VA. Subtotal 11,700 VA.
- 220.42 demand: first 3,000 at 100%, remainder 8,700 at 35% = 3,045; total 6,045 VA.
- Range 12 kW: 8,000 VA (Table 220.55, Column A, one range). Dryer: 5,000 VA minimum (220.54). Water heater: 4,500 VA.
- Total 23,545 VA ÷ 240 V = 98 A, so a 100 A service (240.6). Reasoning note: the demand factor applies only to the general lighting and appliance subtotal, and Table 220.55 applies to a single dwelling range - a commercial range uses nameplate per 220.56.
Traps
- Charging the small-appliance circuits at their actual load instead of 1,500 VA each.
- Adding 25% for continuous loads twice, once at the branch and again at the feeder.
- Using the optional method on a dwelling that does not meet 220.82's conditions.