Code Calculations and Reading Electrical Plans
What the exam targets
Article 220 calculations, conductor and OCPD selection from tables, box and conduit fill, voltage drop, and the ability to read a plan set: legend, one-line diagram, panel schedule, and load calculations.
Concepts to own
- Standard method for a dwelling: general lighting at 3 VA/ft²; demand factors in Table 220.42 (first 3000 VA at 100%, remainder at 35%); 1500 VA per small-appliance circuit and 1500 VA for laundry; appliances at 75% (220.53); ranges per Table 220.55; optional method in 220.82.
- Commercial: lighting VA/ft² by occupancy (Table 220.12), receptacles at 180 VA each, continuous versus noncontinuous loading.
- Conductor and OCPD selection from Table 310.16 and 240.6; voltage drop = 2 × K × I × L ÷ circular mils (K = 12.9 copper, 21.2 aluminum).
- Plan reading: match the symbol legend to the panel schedule; the one-line diagram gives service size, main device, and feeder sizes; check the revision block and the "issued for construction" stamp before pricing.
- Never scale from a photocopy; use the stated scale and a scale rule on the original or digital file.
Traps
Using only the lighting load and forgetting small-appliance and laundry circuits; applying demand factors line by line instead of to the combined total; using 180 VA per receptacle in a dwelling; reading a dimension that is a "nominal" or existing condition rather than new work.
Worked example
A 2,000 ft² dwelling: lighting 2,000 × 3 = 6,000 VA; two small-appliance circuits = 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. Add fixed appliances under 220.53 before converting to amps at 240 V.