General Electrical Knowledge and Theory
General Electrical Knowledge and Theory is the calculations-and-fundamentals domain. Expect Ohm's law, power, series and parallel circuits, AC values, conductor resistance, voltage drop, and balanced three-phase relationships, usually as short word problems rather than code lookups.
Core relationships
- Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R.
- Series: current is common and resistances add. Parallel: voltage is common and reciprocals add; equivalent resistance is always less than the smallest branch.
- AC sine wave: RMS = 0.707 × peak. RMS is the value used by meters and by Code calculations.
- Three-phase power: P = 1.732 × VL × IL × power factor. Single-phase: P = V × I × PF.
Conductors and voltage drop
- R = K × I × L / CM, with K about 12.9 for copper and 21.2 for aluminum at 75°C.
- Single-phase two-wire VD = 2KIL/CM. Three-phase VD = 1.732KIL/CM.
- L is the one-way length; the factor 2 accounts for the return conductor.
Worked example
A 120 V, 20 A load is 100 ft from the panel on 12 AWG copper (6,530 cmil). VD = 2 × 12.9 × 20 × 100 / 6,530 = 7.9 V, or 6.6% of 120 V. To hold 3%, the maximum drop is 3.6 V, so required CM = 2 × 12.9 × 20 × 100 / 3.6 = 14,333 cmil. 8 AWG (16,510 cmil) is the next standard size.
Traps that cost marks
- Using the round-trip length and then multiplying by 2 again.
- Mixing peak and RMS values, or using 120 V where a 240 V load requires 240 V.
- Applying the single-phase formula to a three-phase feeder.
- Ignoring power factor when converting kW to kVA: kVA = kW/PF.
- Treating a balanced neutral as a current-carrying conductor when the load is truly balanced.