Electrical and Controls
Electrical questions test safety and practical troubleshooting more than code minutiae. Know Ohm's law and the power equation (P = V × I), the difference between AC and DC, and how a basic circuit works: source, switch, load, return. Understand series versus parallel circuits - parallel is how buildings are wired so one load failing does not kill the rest. Know the function of fuses, breakers, contactors, relays, transformers (step-down to 24V control), and capacitors (start and run).
Controls knowledge matters: thermostats (conventional versus heat pump, and why heat pumps need auxiliary/emergency heat and defrost logic), low-voltage control circuits, and the sequence of operation for a furnace or packaged unit. Understand safeties: high-limit switches, flame sensors, rollout switches, pressure switches, and float switches. Know that a safety that keeps tripping is a symptom, not the disease.
Traps: jumping out a safety to "test" a unit; confusing a start capacitor with a run capacitor; and assuming a blown low-voltage fuse means the transformer is bad. Also watch for lockout boards that require a reset and for thermostat wiring mismatches on heat pumps.
Worked example: a 24V control circuit drawing 0.5A. Using P = V × I, that is 12 VA. A transformer rated 40 VA can comfortably carry it, but add the contactor coil and any accessories before declaring it adequate. Ohm's law: if a 240V heater draws 20A, resistance is 12 ohms and power is 4,800W.