Electrical and controls for HVAC equipment
Domain 6 tests electrical safety, branch-circuit selection, motors and components, and low-voltage controls.
Power and protection
Use the equipment nameplate. Minimum circuit ampacity (MCA) sets the minimum conductor ampacity. Maximum overcurrent protective device (MOP) sets the largest breaker or fuse. A disconnect must be within sight of the equipment, and the equipment must be grounded. Motors have capacitors, contactors, overloads, and relays; check voltage imbalance and amp draw against nameplate. Always lock out and tag out before servicing.
Controls
Most residential systems use 24 V control circuits: R is power, C is common, W calls heat, Y calls cooling, G runs the indoor fan, and O/B energizes the reversing valve. The C wire powers smart thermostats. Check transformer VA before adding accessories. Control wiring must be sized for voltage drop on long runs.
Worked example
A unit nameplate lists MCA 24 A and MOP 30 A. Select conductors with ampacity at least 24 A; 10 AWG copper at 75°C is rated 35 A, so it is acceptable. The branch breaker must not exceed 30 A. Do not install a 40 A breaker to stop nuisance trips; find the cause, such as a failing capacitor, loose connection, or high head pressure.
Traps that cost marks
Do not size breakers by wire size alone, ignore MCA/MOP, undersize thermostat wire, miswire O/B on a heat pump, omit the C wire, or bypass safety controls. Never test a capacitor without discharging it.