Electrical Theory for the Exam
Ohm's Law, Power, Series/Parallel, Three-Phase
NFPA 70 National Electrical Code, 2023 edition · Reviewed 2026-09-21
Theory items are the only questions you cannot look up in the code book, so these formulas have to be in your head. There are not many. Every calculations-portion question is one of these formulas dressed up in a scenario.
1. The two wheels
Ohm's law: E = I × R → I = E ÷ R → R = E ÷ I
Power (single-phase, resistive): P = E × I → I = P ÷ E → P = I² × R → P = E² ÷ R
Where E = volts, I = amperes, R = ohms, P = watts.
Worked example: a 240 V, 4800 W water heater draws
I = 4800 ÷ 240 = 20 A. Its element resistance isR = 240 ÷ 20 = 12 Ω. If the voltage sags to 208 V, the current becomes208 ÷ 12 = 17.3 Aand the power falls to208² ÷ 12 = 3605 W— resistive loads drop power with the square of the voltage. (Exam favourite.)
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