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HVAC NEC 2023

HVAC Certification Practice Test 3

This test covers HVAC control transformers and 24V control circuits. You will practice VA sizing calculations, primary and secondary winding roles, and troubleshooting a control circuit that has lost power.

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HVAC electrical exams cover electrical theory (Ohm's Law, circuits), transformers, single-phase and three-phase motors, control wiring, safety, and troubleshooting.

Requirements vary by state. HVAC technicians typically need EPA 608 certification and may need a separate electrical license for line voltage work.

Exam Tip: To check if a transformer can handle a load, multiply the secondary voltage (24V) by the total current, or add up the VA of every connected device, and compare the total to the transformer's VA rating.

25

Questions

25 min

Time

70%

Pass

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HVAC Certification Practice Test 3 — Question List

Question 1: What is the main job of a control transformer in HVAC equipment?

Convert AC power to DC power
Increase incoming voltage for the compressor
Step down line voltage (240V/208V/120V) to 24V for the control circuit
Store backup power during outages

Question 2: A control transformer's VA rating tells you what?

The transformer's physical size in inches
The voltage output only, with no relation to load
The number of thermostat wires needed
The maximum load, in volt-amps, the secondary can safely supply

Question 3: What happens to the control circuit if the control transformer fails completely?

The whole control circuit loses power, and nothing can call for heat, cooling, or fan
Only the thermostat display stops working
The compressor keeps running normally
The condenser fan speeds up

Question 4: In a control transformer, which winding connects to line voltage, such as 240V, 208V, or 120V?

The secondary winding
The primary winding
The common (C) terminal
The neutral bus

Question 5: Which winding of a control transformer supplies 24V to the thermostat circuit?

The primary winding
The grounding conductor
The secondary winding
The neutral bus

Question 6: A thermostat shows a blank display, and no 24V is measured at the R terminal. The line-voltage breaker to the equipment is on. What is a likely cause?

The condenser fan capacitor is weak
The reversing valve is stuck
The evaporator coil is frozen
The control transformer or its fuse has failed

Question 7: Which statement is true about the input (primary) side of a control transformer?

It receives AC line voltage, such as 240V, 208V, or 120V
It receives DC voltage from a battery
It always receives 24V AC
It only works with three-phase power

Question 8: A technician accidentally shorts two 24V thermostat wires together while working. What is the most likely result?

The line-voltage breaker trips instead
The transformer's fuse blows, or the transformer overheats and fails, protecting the circuit
Nothing happens because 24V is too low to cause damage
The compressor immediately locks out on high pressure

Question 9: A 24V transformer is rated for 75VA. About how much current can it safely supply?

1.8A
75A
About 3.1A
0.32A

Question 10: A 24V transformer is rated for 100VA. About how much current can it safely supply?

2.4A
100A
0.24A
About 4.2A

Question 11: On a typical 24V control transformer secondary, what are the two terminals usually labeled?

R and C
L1 and L2
Y and G
Hot and Neutral

Question 12: A control transformer is humming loudly and running hot, even though the wiring looks correct. What does this suggest?

The transformer is undersized in physical dimensions only, with no electrical issue
The transformer is likely overloaded, drawing more VA than it is rated for
The transformer needs a higher voltage primary
The condenser fan is wired backward

Question 13: A control circuit has a contactor coil drawing 15VA, a humidistat drawing 10VA, and a zone control board drawing 20VA, all powered from the same 24V transformer. The transformer is rated 40VA. Is the transformer adequately sized?

Yes, because 45VA is close enough to 40VA
Yes, because VA ratings only matter for line voltage
No, the total load of 45VA exceeds the transformer's 40VA rating
No, but only because of the humidistat alone

Question 14: Many control transformers include a fuse or a resettable breaker on the secondary side. What is its purpose?

To step the voltage down further to 12V
To convert the output to DC
To increase the VA rating of the transformer
To protect the transformer from damage if the 24V circuit is shorted or overloaded

Question 15: Is a control transformer a step-up or step-down transformer?

Step-down, because it reduces line voltage to a lower 24V control voltage
Step-up, because it raises voltage for the compressor
Neither, because voltage stays the same
It depends on the season

Question 16: A technician adds a new zone control board that increases the total 24V load beyond what the existing 40VA transformer can supply. What is the correct fix?

Add a second thermostat to share the load
Replace the transformer with one rated for a higher VA, such as 75VA or 100VA
Increase the primary voltage instead
Remove the fuse from the secondary

Question 17: What is the typical role of the C terminal on a 24V control transformer secondary?

It supplies line voltage directly to the compressor
It is the ground wire for the whole system
It provides the common (return) side of the 24V circuit
It only powers the condenser fan

Question 18: A 40VA transformer feeds a 24V control circuit where the connected loads draw a total of 1.5A. Is this within the transformer's rating?

No, because 1.5A alone exceeds the rating
Yes, but only because current does not matter
No, because 36VA is too close to 40VA to be safe
Yes, because 24V × 1.5A = 36VA, which is under the 40VA rating

Question 19: Does a control transformer supply power directly to line-voltage loads, like compressor motors?

No, it only powers the low-voltage control circuit, not line-voltage loads
Yes, it powers both the control circuit and the compressor
Yes, but only in three-phase systems
No, it only powers the outdoor unit

Question 20: Several 24V loads, such as relay coils, are wired in parallel across the R and C terminals. How does total current draw behave?

Only the largest load's current counts
The individual currents add together to form the total current drawn from the transformer
Current stays the same no matter how many loads are added
Current splits evenly regardless of each load's actual draw

Question 21: What is a common long-term effect of consistently overloading a control transformer beyond its VA rating?

The transformer automatically increases its VA rating
The transformer converts to a higher voltage output
The transformer runs hot and can fail prematurely
The transformer becomes more efficient over time

Question 22: Which statement is correct about what a control transformer powers?

It powers the compressor motor directly
It powers both line-voltage and low-voltage loads equally
It only powers emergency backup systems
It powers small control loads, like thermostats and relay coils, not large loads like compressor motors

Question 23: A technician replaces a failed transformer with one that has the correct VA rating but the wrong secondary voltage output, 120V instead of 24V. What is the likely result?

The 24V control components, like the thermostat and relay coils, will likely be damaged by the excess voltage
Nothing will happen, since VA rating is all that matters
The equipment will simply run at reduced capacity
The transformer will automatically adjust the output voltage

Question 24: A control circuit has a thermostat drawing 3VA, a contactor coil drawing 12VA, a defrost timer drawing 5VA, and a zone valve drawing 18VA, all from the same 40VA transformer. Is the transformer adequately sized?

No, because 38VA exceeds 40VA
Yes, because the total load of 38VA is under the transformer's 40VA rating
Yes, but just barely over the limit
No, because four loads is too many for one transformer

Question 25: A thermostat calls for cooling, but nothing happens anywhere in the system: no compressor, no condenser fan, no indoor blower. The line-voltage breaker is on. What is the most likely single point of failure?

Only the condenser fan motor has failed
Only the reversing valve has failed
The control transformer has failed, cutting power to the entire 24V control circuit
Only one zone valve has failed

Key NEC References for This Test

Article	What It Covers
Art. 440	Air Conditioning Equipment — use nameplate MCA and MOCP, not FLA alone
Art. 430	Motors — general motor rules that Art. 440 modifies

Study Tip: Practice reading ladder diagrams. Trace each rung from L1 to the load: contacts open → circuit dead. Contacts closed → current flows. This is how HVAC electrical troubleshooting works.

EXAM CARD HVAC

Questions25

Time limit25 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 Art. 440 & electrical theory