General Electrical Knowledge and Theory
This domain checks whether you can predict what a low-voltage circuit will actually do before you install it: Ohm's law, series and parallel networks, power versus VA, conductor resistance, and voltage drop over long Class 2 runs. Expect plug-in numbers, not essay answers.
Formulas you must be able to use cold
- Ohm's law: V = I x R; P = V x I = I²R = V²/R.
- Series: one current path, resistances add, voltages divide in proportion to resistance.
- Parallel: voltage is common to all branches, currents add, R_total = 1/(1/R1 + 1/R2 + ...).
- AC values: V_peak = 1.414 x V_RMS; true power in watts = VA x power factor.
- Conductor resistance: R = K x L / CM, with K = 12.9 for copper and 21.2 for aluminum, L in feet, and CM in circular mils (14 AWG = 4110, 12 = 6530, 10 = 10380, 8 = 16510).
- Single-phase voltage drop: VD = 2 x K x I x L / CM. The 2 is the round trip out and back.
Worked example
A 24 VDC camera draws 1.5 A at the end of a 150 ft run. With 18 AWG (CM = 1620): VD = 2 x 12.9 x 1.5 x 150 / 1620 = 5805 / 1620 = 3.58 V. The camera sees 24 - 3.58 = 20.4 V, a 15% drop. If the datasheet requires 21.5 V minimum, that fails. Reworking with 16 AWG (CM = 2580) gives 5805 / 2580 = 2.25 V drop, or 21.75 V at the load, which is acceptable. The design check is the manufacturer's minimum operating voltage, not the 3% figure in the informational notes to 210.19(A), which addresses branch circuits rather than Class 2 power runs.
Traps that cost marks
Dropping the round-trip factor of 2; exchanging watts and VA while sizing a transformer or power supply; adding parallel resistances as if they were in series; and assuming a 24 V nominal supply holds 24 V under load.