General Electrical Knowledge and Theory
General Electrical Knowledge and Theory
This domain tests the physics behind the Code: Ohm's law, power, series and parallel circuits, AC values, conductor resistance, and voltage drop. Expect calculation items, not just definitions.
Core relationships
- Ohm's law: E = I × R; power: P = E × I = I²R = E²/R.
- In series, current is common and resistances add; in parallel, voltage is common and currents add.
- AC RMS values are the effective values used for power and Code calculations; peak = RMS × √2 for a sine wave.
- Conductor voltage drop: VD = 2 × K × I × L / CM for single-phase; use √3 for three-phase. K ≈ 12.9 for copper, 21.2 for aluminum.
Worked example
A 120 V, 20 A branch circuit supplies a 1,440 W resistive load. Current = 1,440/120 = 12 A, so the load is 60% of the circuit rating. If the same load is 240 V, current = 6 A-half-showing why higher voltage reduces conductor loss for the same power. On a 150 ft one-way 12 AWG copper circuit at 16 A, VD ≈ 2×12.9×16×150/6,530 = 9.5 V, about 7.9% of 120 V-above the common 3% branch recommendation.
Traps
- Using peak voltage in power calculations.
- Adding parallel resistances directly instead of using reciprocals.
- Ignoring power factor on inductive loads; kW ≠ kVA.
- Forgetting the return-path factor of 2 in single-phase voltage drop.