Free electrical calculator

kVA to amps, amps to kVA, and kW

Single- or three-phase, any voltage. Enter amps, kVA or kW and get the other two, plus a full-load amps table for standard transformer sizes.

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System
I know the

Three-phase: use the line-to-line voltage (208, 480) and the line current. Power factor only matters for kW: kVA is apparent power, kW is real power. Use the equipment nameplate's power factor; 1.0 is right for resistive loads such as heaters.

Apparent power-
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kVA-
Amps-
kW-
Full-load amps of common transformer sizes

Full-load amps = kVA × 1,000 ÷ V (single-phase) or ÷ (1.732 × V) (three-phase). Sizes are those in the federal efficiency tables for low-voltage dry-type distribution transformers. Size a transformer and its overcurrent protection under the NEC (Article 450) and the maker's data; this is an estimate.

In short

Single-phase kVA = volts × amps ÷ 1,000; three-phase kVA = 1.732 × volts × amps ÷ 1,000, using line-to-line volts. A 480 V three-phase load drawing 100 A is 83.1 kVA. Turn it around for amps: a 75 kVA transformer at 208 V three-phase carries 208 A at full load. kW = kVA × power factor.

kVA to amps at common voltages
kVA240 V single-phase208 V three-phase480 V three-phase
1 kVA4.2 A2.8 A1.2 A
1.2 kVA5.0 A3.3 A1.4 A
3 kVA12.5 A8.3 A3.6 A
5 kVA20.8 A13.9 A6.0 A
7.5 kVA31.2 A20.8 A9.0 A
10 kVA41.7 A27.8 A12.0 A
15 kVA62.5 A41.6 A18.0 A
25 kVA104.2 A69.4 A30.1 A
30 kVA125.0 A83.3 A36.1 A
37.5 kVA156.2 A104.1 A45.1 A
45 kVA187.5 A124.9 A54.1 A
50 kVA208.3 A138.8 A60.1 A
58 kVA241.7 A161.0 A69.8 A
75 kVA312.5 A208.2 A90.2 A
100 kVA416.7 A277.6 A120.3 A
112.5 kVA468.8 A312.3 A135.3 A
150 kVA625.0 A416.4 A180.4 A
165 kVA687.5 A458.0 A198.5 A
225 kVA937.5 A624.5 A270.6 A
300 kVA1,250.0 A832.7 A360.8 A
500 kVA2,083.3 A1,387.9 A601.4 A

Amps = kVA × 1,000 ÷ V (single-phase) or ÷ (1.732 × V) with line-to-line volts (three-phase), at full load.

How it works

How to use the kVA calculator

1

Pick the system

Single- or three-phase, the voltage, and whether you know amps, kVA or kW.

2

Enter the value

Plus the power factor if you're working with kW.

3

Read the results

kVA, amps and kW, the next standard transformer size, and full-load amps by size.

What it does

What the kVA calculator does

Three ways in

Amps to kVA, kVA to amps, or kW and power factor to kVA and amps.

Single and three-phase

Uses √3 (1.732) for three-phase with line-to-line voltage.

Transformer table

Full-load amps of standard sizes at 120 and 240 V single-phase, 208 and 480 V three-phase.

Next size up

Points to the next standard kVA rating at or above your load.

Single-phase
kVA = V × A ÷ 1,000
Three-phase
kVA = 1.732 × V(line-to-line) × A ÷ 1,000
Real power
kW = kVA × power factor
Transformer sizes
Low-voltage dry-type ratings listed in 10 CFR 431.196
Not included
Transformer overcurrent protection and conductor sizing (NEC Article 450, 240.21(C))
Questions

kVA Calculator FAQ

How do I convert kVA to amps?

Multiply kVA by 1,000 and divide by the voltage for single-phase, or by 1.732 × the voltage for three-phase. A 50 kVA single-phase transformer at 240 V carries 208 A; a 112.5 kVA three-phase at 480 V carries 135 A.

What is the difference between kVA and kW?

kVA is apparent power, volts times amps. kW is real power, the part that does work, equal to kVA times the power factor. Resistive loads such as heaters have a power factor near 1; motors and some electronics are lower, so their kW is less than their kVA.

Why do transformers use kVA instead of kW?

Because the heat in a transformer depends on current and voltage, not on the load's power factor. A transformer rated in kVA can carry that many volt-amps whatever the load's power factor.

How many amps is a 75 kVA transformer?

On a 208 V three-phase secondary, 75,000 ÷ (1.732 × 208) = 208 A. On the 480 V primary side, 90 A.

Can I size a transformer from this?

It gives the load in kVA and the standard size above it. Sizing the transformer, its overcurrent protection and the secondary conductors follows NEC Article 450 and 240.21(C), the equipment data and the AHJ, so treat this as an estimate.

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Sources

Where the numbers come from

  1. 10 CFR 431.196: energy conservation standards for distribution transformers (kVA ratings)
  2. NIST Special Publication 811, Guide for the Use of the International System of Units

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