Electrical Systems and General Electrical Knowledge
Core relationships
Questions in this domain test fundamentals more than memorized tables. Be fluent with Ohm's law (E = I x R), the power formulas (P = I x E for resistive loads; P = 1.732 x E x I x PF for three-phase), and the distinction between kW (real power) and kVA (apparent power). Know series behavior (current constant, voltage divides, resistances add) versus parallel behavior (voltage constant, current divides, resistances combine through reciprocals).
Systems you should recognize
- 120/240 V single-phase, three-wire: dwellings, two ungrounded legs plus a grounded neutral.
- 120/208 V and 277/480 V three-phase wye: commercial and industrial; 277 V is line-to-neutral of 480 V.
- 240/480 V delta: no usable neutral for line-to-neutral loads unless a center tap exists.
Understand why a neutral carries only the unbalanced current in a balanced three-phase system, and why a shared neutral in a multiwire branch circuit becomes dangerous when the neutral opens. Recognize that voltage drop is a performance/design issue, not a code maximum: the NEC recommends 3 percent on a branch circuit and 5 percent total.
Traps
- Using 2 instead of 1.732 for three-phase calculations, or applying single-phase formulas to a wye system.
- Treating kVA and kW as interchangeable when power factor is not 1.
- Assuming the grounded conductor always carries full load current.
- Confusing ampacity (conductor heating) with voltage drop (equipment performance).
Worked example
A 480 V three-phase motor draws 30 A at 0.90 power factor. Apparent power = 1.732 x 480 x 30 = 24,940 VA, about 24.9 kVA. Real power = 24.9 x 0.90, about 22.4 kW. If the question asks for conductor ampacity, use the 30 A with the applicable Article 430 adjustments, not the kW figure.