Electrical Components, Controls, Motors, and Wiring for HVAC Systems
Load side
Use the equipment nameplate, not rules of thumb. Minimum circuit ampacity (MCA) sets minimum conductor ampacity; maximum overcurrent protective device (MOP) sets the largest breaker permitted. A unit with 24.5 MCA and 40 MOP can use 10 AWG copper on a 40 ampere HACR breaker - conductors may be larger than MCA, but the breaker may not exceed MOP. The disconnecting means must be within sight of and accessible from the equipment, and NEC 210.63 requires a service receptacle within 25 feet.
Controls
Residential control circuits are Class 2, 24 volts, from a transformer sized in VA to the connected load. Know the terminals: R hot, C common, W heat, Y cooling, G fan, O/B reversing valve, and second-stage W2 and Y2. Components include contactors, relays, sequencers for electric heat, defrost boards, high- and low-pressure switches, float switches, and limits.
Motors
PSC motors use a run capacitor; ECM and variable-speed motors are electronically commutated and often constant-airflow programmed. Replace capacitors with the same microfarad rating within tolerance - a wrong value overheats the motor. Check start components, potential relays, and windings before condemning a compressor. Keep line and low voltage separated.
Worked example
A condenser nameplate reads 24.5 minimum circuit ampacity and 40 maximum fuse. The installer proposes 8 AWG and a 50 ampere breaker. That fails: the breaker exceeds MOP and no longer protects the compressor as listed. Correct is 10 AWG copper with a 40 ampere HACR breaker.
Traps
Sizing the breaker to MCA; upsizing the breaker for voltage drop instead of the conductor; no disconnect; Class 2 wiring run in line-voltage conduit; forgetting the C wire for a smart thermostat.