These NEC transformer calculation practice questions cover turns ratio, primary and secondary current, kVA sizing and Article 450 protection. Transformer questions reduce to a small number of relationships, and once you have those the arithmetic is straightforward. Volts times amps gives you VA on either side, and the power going in equals the power coming out, so a transformer that halves the voltage doubles the current. Everything else on this topic is built on that.
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Written by Kevin, a licensed
Michigan master electrician — 26 years in the trade and 17 of them as an electrical
inspector. Every answer below names the NEC section it comes from, so you learn where to
look it up rather than memorising it.
Exam tipWatch the difference between single-phase and three-phase. The √3 in the three-phase formula is the most common slip on this topic, and dropping it produces an answer that sits right there in the choices waiting for you.
Tap an answer to see whether you got it right, the code section it comes
from, and the reasoning worked out in full. Nothing to sign up for.
Question 1Transformer Ratio
A transformer has a primary of 400 turns and a secondary of 100 turns. If the output is 50V, what is the input?
Correct answer: A. 200V
NEC 4x50
Rule — transformer turns ratio
Voltage changes in the same proportion as the turns — whichever side has more turns has more volts.
Vp / Vs = Np / Ns
Step 1 — find the turns ratio
400 turns ÷ 100 turns = 4 : 1 (step-down)
Step 2 — apply it to the voltage
The primary has 4× the turns, so 4× the volts:
4 × 50 V = 200 V
Primary (input) = 200 V
How to do it
Set up the ratio of turns, then move the voltage the same direction: the high-turns side is the high-voltage side. 400:100 is 4:1, so 50 V on the 100-turn side becomes 200 V on the 400-turn side.
Sizing tip — VA in = VA out. A transformer's power is the same on both sides. If you know the kVA, current = VA ÷ voltage (÷ 1.732 for three-phase) — that's what sizes the conductors and overcurrent protection (450.3).
Common mistakes
Multiplying when you should divide. Low-turns → high-turns raises the voltage; the other way lowers it.
Mixing up primary and secondary. Read which side each turns count belongs to.
Question 2Transformer Ratio
A transformer has a primary input of 150 VA and an output of 140 VA. What is the power efficiency?
Correct answer: A. 93%
NEC Efficiency = output ÷ input (140÷150 = 0.93)
Rule — transformer efficiency
Efficiency is simply what you get out divided by what you put in.
Efficiency = output ÷ input
Step — plug in
140 VA ÷ 150 VA = 0.933 = 93%
93%
How to do it
Output over input, then turn the decimal into a percent. The missing 7% is lost as heat in the core and windings.
Common mistakes
Dividing input by output (that gives more than 100%).
Forgetting to convert the decimal to a percent.
Question 3Transformer Ratio
A transformer turns ratio is 10:1 and the primary is 120 V. What is the secondary voltage?
Correct answer: B. 12 V
NEC Transformer turns ratio (step-down)
Transformer turns ratio
Primary ÷ ratio = secondary.
120 V ÷ 10 = 12 V
Transformer turns ratio (step-down)
12 V
How to do it
Voltage follows the turns: the high-turns side is the high-voltage side. Divide by the ratio to step down, multiply to step up (Vp / Vs = Np / Ns).
Common mistakes
Multiplying when you should divide (step-down vs step-up).
Swapping primary and secondary.
Assuming the ratio changes the power — VA in = VA out.
Question 4Transformer Ratio
A transformer turns ratio is 4:1 and the secondary is 120 V. What is the primary voltage?
Correct answer: A. 480 V
NEC Transformer turns ratio
Transformer turns ratio
Secondary × ratio = primary.
120 V × 4 = 480 V
Transformer turns ratio
480 V
How to do it
Voltage follows the turns: the high-turns side is the high-voltage side. Divide by the ratio to step down, multiply to step up (Vp / Vs = Np / Ns).
Common mistakes
Multiplying when you should divide (step-down vs step-up).
Swapping primary and secondary.
Assuming the ratio changes the power — VA in = VA out.
Question 5Transformer Ratio
A transformer has a primary-to-secondary ratio of 1:2 with a 120 V primary. What is the secondary?
Correct answer: B. 240 V
NEC Transformer turns ratio (step-up)
Transformer turns ratio
A 1:2 ratio steps UP. Secondary = primary × 2.
120 V × 2 = 240 V
Transformer turns ratio (step-up)
240 V
How to do it
Voltage follows the turns: the high-turns side is the high-voltage side. Divide by the ratio to step down, multiply to step up (Vp / Vs = Np / Ns).
Common mistakes
Multiplying when you should divide (step-down vs step-up).
Swapping primary and secondary.
Assuming the ratio changes the power — VA in = VA out.
Question 6Transformer Ratio
A transformer has 500 W input and 485 W output. What is its efficiency?
Correct answer: B. 97 %
NEC Transformer efficiency
Transformer efficiency
Efficiency = power output / power input, expressed as a percent.
Step 1 — Divide output by input
485 W / 500 W = 0.97
Step 2 — Convert to percent
0.97 x 100 = 97%
Efficiency = 97%.
Transformer efficiency
97 %
How to do it
Efficiency = output ÷ input, then turn the decimal into a percent. The missing part is heat lost in the core and windings.
Common mistakes
Dividing input by output (that gives more than 100 %).
Forgetting to convert the decimal to a percent.
Question 7Transformer Current
A transformer supply is 480 V at 2 A; the load side is 120 V. What is the load current? (VA in = VA out)
Correct answer: B. 8 A
NEC Transformer: E × I (primary) = E × I (secondary)
Transformer current — VA in = VA out
VA in = VA out.
480 V × 2 A = 960 VA
960 VA ÷ 120 V = 8 A
8 A
How to do it
Power is the same on both sides. Find the VA on the known side (V × A), then divide by the OTHER side's voltage to get its current.
Common mistakes
Forgetting that VA is equal on both sides.
Dividing by the wrong side's voltage.
Confusing this with the turns-ratio (voltage) problem.
Question 8Transformer Current
Supply 240 V at 20 A, load side 120 V. What is the load current?
Correct answer: B. 40 A
NEC VA in = VA out
Transformer current — VA in = VA out
240 V × 20 A = 4,800 VA
4,800 VA ÷ 120 V = 40 A
40 A
How to do it
Power is the same on both sides. Find the VA on the known side (V × A), then divide by the OTHER side's voltage to get its current.
Common mistakes
Forgetting that VA is equal on both sides.
Dividing by the wrong side's voltage.
Confusing this with the turns-ratio (voltage) problem.
Question 9Transformer Current
Supply 480 V at 20 A, load side 120 V. What is the load current?
Correct answer: B. 80 A
NEC VA in = VA out
Transformer current — VA in = VA out
480 V × 20 A = 9,600 VA
9,600 VA ÷ 120 V = 80 A
80 A
How to do it
Power is the same on both sides. Find the VA on the known side (V × A), then divide by the OTHER side's voltage to get its current.
Common mistakes
Forgetting that VA is equal on both sides.
Dividing by the wrong side's voltage.
Confusing this with the turns-ratio (voltage) problem.
Question 10Transformer Current
The load side is 120 V at 80 A; the supply is 480 V. What is the supply current?
Correct answer: A. 20 A
NEC VA in = VA out
Transformer current — VA in = VA out
120 V × 80 A = 9,600 VA
9,600 VA ÷ 480 V = 20 A
20 A
How to do it
Power is the same on both sides. Find the VA on the known side (V × A), then divide by the OTHER side's voltage to get its current.
Common mistakes
Forgetting that VA is equal on both sides.
Dividing by the wrong side's voltage.
Confusing this with the turns-ratio (voltage) problem.
These are 10 of 803 questions
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