Electrical Systems Design, Planning, and Estimating
Core concepts
Design questions combine load calculation (Article 220), service and feeder sizing (Article 230), branch-circuit layout (Article 210), and demand factors. Know the standard method versus optional method for dwelling units, general lighting loads (3 VA/ft²), small-appliance and laundry circuits, and the application of demand factors to ranges, dryers, and heating/cooling. Estimating involves takeoffs, material quantities, labor units, and contingency. Understand voltage drop (VD = 2 × K × I × L ÷ CM for single-phase), and the recommended 3% branch / 5% total limits. Consider future capacity, spare breakers, and panel scheduling.
Traps
Questions mix standard and optional methods, or apply demand factors where not permitted. Remember that the largest motor adds 25% to the load. Voltage drop is a recommendation, not a Code requirement (except informational notes). Estimating traps include forgetting connectors, supports, and waste, or confusing labor hours with clock hours.
Worked example
Voltage drop for a 120 V, 20 A load 150 ft away using 12 AWG (6,530 CM): VD = 2 × 12.9 × 20 × 150 ÷ 6,530 ≈ 11.85 V, or about 9.9%-too high. Upsize to 8 AWG (16,510 CM): VD ≈ 4.7 V, about 3.9%.
Rule of thumb
Keep total voltage drop under 5% (3% branch plus 2% feeder) for good performance, and add 25% for the largest motor in load calcs.