General Electrical Knowledge and Theory
Domain 1 tests the theory the rest of the exam leans on: Ohm's law and power in single- and three-phase circuits, series and parallel behavior, conductor resistance, voltage drop, power factor, and the Chapter 9 tables that turn theory into conductor-and-raceway answers.
Core relationships
- E = I × R and P = I × E = I²R. Balanced three-phase: P = √3 × E × I × PF, so I = VA ÷ (√3 × E).
- Series: one current path, resistances add. Parallel: same voltage, R_total = 1 ÷ (1/R₁ + 1/R₂ …); n equal resistors give R/n.
- Conductor resistance rises with temperature. Chapter 9 Table 8 lists DC resistance at 75°C; Table 9 lists AC resistance and reactance, which is what you use for feeders and large conductors.
- Single-phase voltage drop: VD = 2 × I × R × L, with L the one-way length. Three-phase: VD = √3 × I × R × L.
- In a balanced multiwire circuit the neutral carries only the unbalanced current; triplen harmonics add instead of cancel.
Worked example
A 120 V, 20 A branch circuit runs 100 ft one-way in 12 AWG copper. Table 9 gives 1.93 Ω per 1,000 ft, so R = 0.193 Ω for the run. VD = 2 × 20 × 0.193 = 7.72 V, which is 7.72 ÷ 120 = 6.4%. That exceeds the 3% branch-circuit recommendation in the 210.19(A) informational note, so the correct response is to upsize the conductors or shorten the run - not to change the breaker.
Traps
- Using 2 instead of √3, or the reverse, when the circuit is the other type.
- Forgetting the return-path factor of 2 in single-phase drop.
- Using Table 8 DC resistance for a large AC feeder where reactance matters.
- Mixing kVA and kW: 30 kVA at 0.8 PF is 24 kW, but the current follows the kVA.