Electrical Theory and Code Knowledge
This domain tests the math and code framework behind DC electrical work: Ohm's law and power in DC circuits, series and parallel behavior, conductor sizing, voltage drop, box fill, and the NEC's structure and definitions. Questions often hide a code reference inside a calculation.
Core rules
- Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R.
- Series: current is common; resistances add; voltages add. Parallel: voltage is common; conductances add.
- DC voltage drop, two-wire: VD = 2 × K × I × L / CM, where L is one-way length in feet, CM is circular mils, K ≈ 12.9 for copper and 21.2 for aluminum.
- NEC 210.19(A) informational note suggests branch-circuit drop ≤3% and feeder-plus-branch ≤5% for reasonable efficiency.
- Ampacity comes from NEC 310.16; ambient temperature and conductor-count adjustments come from 310.15. Overcurrent protection is not simply “next size up” under 240.4.
- Box fill uses NEC 314.16 and Table 314.16(A): each 12 AWG insulated conductor = 2.25 in³, 14 AWG = 2.00 in³; one or more equipment grounds count as one conductor; clamps count as one; a device counts as two.
Worked example
A 24 V DC pump draws 5 A and is 40 ft from the source. Copper conductors, 3% drop allowed. Allowed VD = 0.03 × 24 = 0.72 V. CM = 2 × 12.9 × 5 × 40 / 0.72 = 5,160 / 0.72 = 7,167 CM. 10 AWG copper has 10,380 CM, so it satisfies voltage drop; check 310.16 ampacity for the installation conditions. Using 2 × L accounts for the outgoing and return conductors.
Traps
- Using one-way length in the voltage-drop formula.
- Treating 3% as 3 V rather than 3% of system voltage.
- Sizing by voltage drop but ignoring ampacity, termination rating, or derating.
- Counting equipment grounds as one per conductor in box fill; they count as one total.
- Mixing series and parallel rules.