Electrical Theory and Practical Wiring Competency
Electrical theory questions on this exam are practical: they check whether you can predict what a circuit will do before you pull wire, and diagnose it after it fails.
Core relationships
- Ohm's law: E = I × R. Power: P = E × I = I²R. For AC, replace resistance with impedance, and remember long runs add inductance as well as resistance.
- Series: current is common, voltage divides, resistances add. Parallel: voltage is common, current divides, and total resistance is always below the smallest branch.
- Transformers trade voltage for current; VA is roughly conserved, so a 24 V, 75 VA transformer can supply about 3.1 A.
- Conductor resistance comes from Chapter 9, Table 8 at 75 °C, which is why long 18 AWG runs lose more volts than a bench test predicts.
Practical wiring competency
Torque terminations to the manufacturer's values (110.14(D)); support and secure cable per its article; keep Class 2 and 3 conductors out of power raceways; install an intersystem bonding termination (250.94) so communications and alarm systems have a grounding point. Test with a low-impedance meter when phantom voltage is suspected.
Worked example
Six zone valves draw 0.32 A each at 24 VAC. Each is 24 × 0.32 = 7.7 VA; six total 46 VA, plus 5 VA of controls = 51 VA. A 75 VA transformer gives about 68 percent loading, leaving margin for inrush - a 50 VA unit would be marginal. Total current is 1.92 A, within the Table 11(A) allowance for 18 AWG at 24 V.
Traps
Reading 24 V at the transformer and assuming 24 V at the load; sizing a transformer on amps without VA; trusting a digital meter's ghost reading on an open conductor; confusing equipment grounding with circuit conductors.