CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
A 24-volt, 40 VA control transformer powers a thermostat and relay circuit. What is the maximum continuous current this transformer can safely supply?
- A24 amps
- B1.67 amps
- C0.6 amp
- D40 amps
Show answer & explanationAnswer & explanation
Correct answer: B. 1.67 amps
VA (volt-amps) equals voltage multiplied by current, so current equals VA divided by voltage: 40 VA ÷ 24 V = 1.67 amps. Loading the secondary beyond this value will overheat the transformer and can trip its internal overcurrent protection or cause premature failure.
Why the other options are wrong
- A. This confuses the primary line voltage figure with the secondary current.
- C. This is roughly half the correct capacity and understates the transformer's rating.
- D. This is the VA rating itself, not the current in amps.
Transformer VA Rating
VA (volt-amps) describes a transformer's power capacity; dividing VA by secondary voltage gives the maximum safe continuous current.
- Formula: I = VA ÷ V
- Overloading causes overheating and nuisance transformer failures
- Common HVAC control transformers are rated 40 VA at 24V
Memory trick: VA over Volts gives you Amps.