Electrical Theory and Load Calculations
Theory you must be able to use
Ohm law and power relationships drive almost every calculation: E = I x R, P = E x I, and P = I squared x R. In series circuits current is common and voltage divides; in parallel circuits voltage is common and current divides. Alternating current adds impedance and power factor, so true power is E x I x PF for single-phase and 1.732 x E x I x PF for balanced three-phase. For a balanced three-phase load, current equals VA divided by (1.732 x voltage). Voltage drop is estimated as 2 x K x I x L divided by circular mils for single-phase, with K near 12.9 for copper and 21.2 for aluminum.
Load calculation method
Article 220 provides a standard method and an optional method; never mix them in one calculation. General lighting comes from Table 220.12 (3 VA per square foot for dwellings). Dwelling units require two or more 20 A small appliance circuits at 1,500 VA each and a laundry circuit at 1,500 VA. Table 220.42 applies demand: 100 percent of the first 3,000 VA and 35 percent of the remainder for dwelling general lighting and receptacles. Apply 220.53 for four or more fixed appliances, 220.55 for ranges, 220.54 for dryers, and 220.60 when heating and cooling loads cannot run together. Add 25 percent of the largest motor where 430.24 requires it.
Worked example
A 2,000 square foot dwelling: lighting 6,000 VA plus small appliance 3,000 VA plus laundry 1,500 VA equals 10,500 VA. Demand gives 3,000 + 0.35 x 7,500 = 5,625 VA. Add fixed appliances, range, dryer, and the larger of heating or cooling, then divide by 240 V for service amps.
Traps
Watch continuous-load multipliers, VA versus watts with power factor, and ranges calculated by the wrong column of Table 220.55.