Electrical Power, Controls, and Low-Voltage Wiring
This domain tests high-voltage power wiring, overcurrent protection, disconnects, and low-voltage control wiring for heat pumps. Expect questions on nameplate MCA and MOP, thermostat terminals, reversing-valve logic, defrost controls, and lockout settings.
Core rules and concepts
- The nameplate gives minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP). Conductors must have ampacity at least equal to MCA; the breaker must not exceed MOP.
- A disconnecting means must be within sight of and readily accessible from the outdoor unit, per code and manufacturer requirements.
- Standard 24V terminals: R is power, C is common, Y is compressor, G is indoor fan, O/B is reversing valve, and W/E/AUX is auxiliary or emergency heat.
- O energizes the reversing valve in cooling; B energizes it in heating. A heat-pump thermostat must be configured for the valve type used.
- Defrost boards control the reversing valve, outdoor fan, and auxiliary heat; some use temperature, time, or pressure termination.
- Communicating systems use proprietary data wires and usually require manufacturer-specific controls and configuration.
Worked example
A nameplate lists MCA 24.5 A and MOP 40 A. A 10 AWG copper conductor with 30 A ampacity satisfies the MCA, and a 40 A breaker is permitted because it does not exceed MOP. The installer must not install a 50 A breaker “for headroom,” because that exceeds the listed MOP and may not protect the equipment as intended. The disconnect must be sized for the circuit and located within sight of the unit.
Traps that cost marks
Reversing O and B on a heat-pump thermostat; using a gas-furnace thermostat that cannot energize the reversing valve; omitting the C wire on a system that needs it; and oversizing the breaker beyond MOP.