Practice: transformer current
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Formula
| Symbol | Meaning | Unit |
|---|---|---|
| kVA | Transformer rating | kVA |
| Ep, Es | Primary and secondary line-to-line voltage | V |
| Ip, Is | Primary and secondary full-load current | A |
| Np, Ns | Primary and secondary turns | turns |
Worked example
A 250 kVA single-phase transformer has a 480 V primary and a 120/240 V secondary. What is its full-load current on the secondary side?
- Single-phase transformer current: I = VA ÷ E, using the voltage of the side you want.
- VA = 250 kVA × 1,000 = 250,000 VA.
- The secondary voltage is 240 V, the full winding voltage, not 120 V.
- I = 250,000 ÷ 240 = 1,041.7 A.
Answer: 1,041.7 A
Transformer current, explained
A transformer passes nearly the same kVA from its primary to its secondary; only a small amount is lost as heat. So the side with the lower voltage carries the higher current. Divide the kVA (times 1,000) by the voltage of the side you want, and for a three-phase transformer also divide by 1.732.
The voltage ratio equals the turns ratio. A winding with half as many turns has half the voltage. Current goes the other way: the low-voltage winding with fewer turns carries more current. A 480 V to 240 V transformer has a 2 to 1 ratio, so its secondary current is twice its primary current.
To pick a transformer for a set of loads, convert each load to kVA (three-phase: E × I × 1.732 ÷ 1,000; single-phase: E × I ÷ 1,000), add them, and choose the smallest standard size that is at least that large. The rated currents you find here are the starting point for transformer overcurrent protection and for sizing the secondary conductors.
How to solve it on the exam
- Note whether the transformer is single-phase or three-phase, and which side (primary or secondary) the question asks about.
- Convert kVA to VA by multiplying by 1,000.
- Divide by that side's line-to-line voltage, and also by 1.732 if the transformer is three-phase.
- For turns questions, set up the ratio Ep ÷ Es = Np ÷ Ns and solve for the unknown.
- To size a transformer, convert each load to kVA, add them, and pick the smallest standard size at or above the total.
Common mistakes
- Using the primary voltage to find the secondary current, or the other way around.
- Using 120 V instead of 240 V for a 120/240 V secondary. Full-load current is figured on the full winding voltage.
- Leaving out 1.732 on a three-phase transformer, or adding it on a single-phase one.
- Turning the turns ratio upside down, which puts more turns on the low-voltage side.
- Adding amps from loads at different voltages instead of converting each one to kVA first.
Exam tips
- The low-voltage side always has the higher current. If your secondary current on a step-down transformer is smaller than the primary current, you swapped the voltages.
- Quick check numbers: 1 kVA single-phase is 4.17 A at 240 V and 2.08 A at 480 V; 1 kVA three-phase is 2.78 A at 208 V and 1.20 A at 480 V.
Transformer current calculator
Questions people ask
How do you calculate transformer full-load current?
Multiply the kVA rating by 1,000 and divide by the voltage of the side you want; for a three-phase transformer, also divide by 1.732. A 75 kVA, 480 V to 208Y/120 V transformer has 75,000 ÷ (208 × 1.732) = 208.2 A on the secondary.
What is the turns ratio of a transformer?
The turns ratio is primary turns divided by secondary turns, and it equals primary voltage divided by secondary voltage. A 480 V to 240 V transformer has a 2 to 1 ratio.
Is the current higher on the primary or the secondary of a step-down transformer?
The secondary. The kVA is about the same on both sides, so the lower-voltage secondary carries more current. Current changes in the opposite ratio to voltage.
What size transformer do I need for a load?
Convert each load to kVA, add them, and pick the smallest standard transformer at least that large. Common three-phase dry-type sizes include 15, 30, 45, 75, 112.5, 150 and 225 kVA.
Sources
- TDLR: Electrician examination information
- EC&M: Applying NEC Art. 450 requirements for distribution transformers
About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.