HVAC Electrical Controls 5

Question 1

A thermostat wired with an 'R', 'C', 'Y', 'G', and 'W' terminal set: which terminal, when energized alone, activates indoor fan only?

Accepted Answer

G

Answer

The G terminal energizes the indoor (air handler) fan relay, running the fan independently of heating or cooling calls.

Question 2

What is the purpose of a Time-Delay Relay (TDR) in a multi-stage commercial HVAC startup sequence?

Accepted Answer

Stage the starting of multiple loads to reduce peak demand current

Answer

TDRs introduce a delay between starting multiple large loads (compressors, fans) so their inrush currents don't overlap, reducing peak demand charges and stress on wiring.

Question 3

An electronic expansion valve (EEV) is controlled by which type of signal from the controller?

Accepted Answer

Pulse-width modulated (PWM) or stepper motor pulses

Answer

EEVs use a stepper motor driven by PWM pulses from the controller, allowing precise modulation of refrigerant flow in small incremental steps.

Question 4

Which regulation requires HVAC electrical work to follow specific wiring methods, box fill calculations, and grounding rules?

Accepted Answer

NFPA 70 (National Electrical Code)

Answer

NFPA 70 (NEC) governs all electrical installations including HVAC equipment wiring, disconnect sizing, conductor ampacity, and grounding requirements.

Question 5

A technician measures 26V AC at the transformer secondary but only 18V AC at the contactor coil. The most likely cause is:

Accepted Answer

Overloaded control circuit drawing excessive current through wiring resistance

Answer

Excessive current draw in the control circuit (from added loads or a partially-shorted coil) creates voltage drop across the wiring, lowering voltage at the load.

HVAC Practice Test

HVAC Practice Test

HVAC Electrical Controls 5