Transformer kVA to Amps Calculator
Enter the nameplate kVA and both voltages, and get the full-load amps for the primary and the secondary at the same time — the two numbers every transformer protection and cable decision starts from.
Balanced load; nameplate kVA applied to both windings, before losses.
Why one transformer has two currents
A transformer passes power, not current. The same 112.5 kVA that arrives as a trickle of 5.21 A at 12,470 V leaves as 135.32 A at 480 V — the ratio between the currents is the ratio between the voltages. That is why the general kVA to amps calculator is only half the tool for transformer work: every protection and sizing question needs the current on the side it applies to.
Full-load amps (FLA) is each winding's current at the full nameplate kVA. Primary FLA sizes the high-voltage fuse or breaker; secondary FLA sizes the low-voltage main breaker and the feeder. Both come from the same identity the rest of this site uses — apparent power divided by voltage, with √3 for three-phase — applied once per winding.
The formulas
Apply it twice: once with the primary voltage for primary FLA, once with the secondary voltage for secondary FLA. This covers the great majority of distribution transformers.
Pole transformers and small control transformers are usually single-phase; the identity is the same without the √3.
Nameplate first, always
Use the kVA and voltages printed on the transformer's own plate — not the building's nominal service voltage. A unit plated 480 V on a system that actually runs at 470 V will draw proportionally more current at full kVA, and impedance (%Z on the plate) governs fault current, which is a separate calculation from full-load amps.
Worked examples
Primary: 112,500 ÷ (√3 × 12,470) = 5.21 A. Secondary: 112,500 ÷ (√3 × 480) = 135.32 A. A 150 A-class secondary main covers full load with the usual margin.
Primary: 500,000 ÷ (√3 × 11,000) = 26.24 A. Secondary: 500,000 ÷ (√3 × 400) = 721.69 A — serious low-voltage current, which is why 400 V secondaries at this size feed switchboards rather than ordinary panels.
Primary: 25,000 ÷ 2,400 = 10.42 A. Secondary: 25,000 ÷ 240 = 104.17 A — roughly one house's 100 A service, which is exactly the job these units do.
Transformer standard ratings — full-load amps
Standard distribution ratings with full-load amps at common secondary voltages, plus an 11,000 V primary column to show the high-side scale. Your transformer's own voltages go in the calculator above.
| Rating (kVA) | 240V · single-phase | 208V · three-phase | 480V · three-phase | 11,000V · three-phase |
|---|---|---|---|---|
| 25 | 104.17 A | 69.39 A | 30.07 A | 1.31 A |
| 45 | 187.50 A | 124.91 A | 54.13 A | 2.36 A |
| 75 | 312.50 A | 208.18 A | 90.21 A | 3.94 A |
| 112.5 | 468.75 A | 312.27 A | 135.32 A | 5.90 A |
| 150 | 625.00 A | 416.36 A | 180.42 A | 7.87 A |
| 225 | 937.50 A | 624.54 A | 270.63 A | 11.81 A |
| 300 | 1,250.00 A | 832.72 A | 360.84 A | 15.75 A |
| 500 | 2,083.33 A | 1,387.86 A | 601.41 A | 26.24 A |
| 750 | 3,125.00 A | 2,081.79 A | 902.11 A | 39.36 A |
| 1000 | 4,166.67 A | 2,775.72 A | 1,202.81 A | 52.49 A |
| 1500 | 6,250.00 A | 4,163.58 A | 1,804.22 A | 78.73 A |
| 2000 | 8,333.33 A | 5,551.44 A | 2,405.63 A | 104.97 A |
Frequently asked questions
How many amps is a 112.5 kVA transformer?
On its 480 V three-phase secondary, a 112.5 kVA transformer carries 135.32 A at full load. On a 208 V secondary it carries 312.28 A, and on its 12,470 V primary it draws just 5.21 A. All three are the same 112.5 kVA — the current changes because the voltage does.
Why is the primary current so much smaller than the secondary?
A transformer trades voltage for current while passing (nearly) the same power through. Step 12,470 V down to 480 V — a ratio of about 26:1 — and the current rises by the same ratio, minus a few percent of losses. That is the whole point of high-voltage transmission: the same power at a fraction of the current.
What is transformer full-load amps?
Full-load amps (FLA) is the current a winding carries when the transformer delivers its full nameplate kVA at its rated voltage: nameplate kVA × 1000 ÷ (√3 × volts) three-phase, or ÷ volts single-phase. Each winding has its own FLA — the primary figure uses primary volts, the secondary figure secondary volts.
Should I size protection from the primary or secondary current?
Both sides get protected, but differently: the NEC (Article 450.3) sets maximum overcurrent device ratings as percentages of each winding’s full-load current, and primary-only protection is common for smaller units while larger installations protect both sides. The percentages and the exceptions matter, so treat the FLA figures here as the input to that code table — and have the final selection checked by a licensed electrician.
Does a transformer draw full-load amps all the time?
No. Secondary current follows the load: an unloaded transformer draws only a small magnetising current on its primary. The exception is the first moment of switch-on, when inrush current can reach several times the full-load figure for a few cycles. That surge is a protection-coordination question, separate from the steady-state FLA this calculator gives.
Sources and references
Full-load current follows the standard identities below; nameplate and rating practice follows the IEEE C57 series. Derivations are on the methodology page.
- NFPA 70 — National Electrical Code (NEC) — the code electricians size conductors and protection against
- IEEE Std 141 — Recommended Practice for Electric Power Distribution (Red Book) — apparent power and load-current practice for industrial and commercial systems
- IEC 60038 — Standard Voltages — the standard voltage levels (120, 208, 240, 400, 480 V) used across this site
- NIST — SI unit of electric current (the ampere) — the definition of the ampere behind every result
- IEEE C57.12.00 — Standard for Liquid-Immersed Distribution and Power Transformers — nameplate ratings and full-load current basis for transformers