CSLB C-20 HVAC ContractorControls, Electrical and SafetyEasy
On a standard 24-volt residential thermostat, which terminal wire typically energizes the compressor contactor coil to start cooling?
- AC
- BR
- CY
- DG
Show answer & explanationAnswer & explanation
Correct answer: C. Y
The Y terminal carries 24V to the contactor coil in the outdoor condensing unit, closing it to start the compressor. R is the hot 24V feed from the transformer, C is the common return, and G energizes the blower relay for fan operation.
Why the other options are wrong
- A. C is the common wire that completes the circuit back to the transformer.
- B. R is the powered leg from the transformer, not the cooling call.
- D. G controls the indoor blower fan relay, not the compressor.
Thermostat Terminal Y
The Y terminal on a low-voltage thermostat sends a 24VAC signal to energize the outdoor unit's compressor contactor, starting the cooling cycle.
- R = 24V hot from transformer
- Y = cooling call to contactor
- G = fan relay call
- C = common return leg
Memory trick: 'Y' is for 'Yes, cool it now' — Y calls the compressor.