How do I calculate what size transformer I need?

Multiply the load voltage by the load current, and by 1.732 on three phase, then divide by 1,000 for kVA. Add any allowance for growth and take the next standard rating up. An 80 A load at 208 V three phase is 28.82 kVA; with 25% added it needs a 45 kVA transformer.

How many amps is a 75 kVA transformer?

On three phase, 75,000 divided by (208 × 1.732) is 208.2 A at 208 V, and 75,000 divided by (480 × 1.732) is 90.2 A at 480 V. On single phase at 240 V it is 312.5 A.

How do you calculate transformer fault current?

Divide the secondary full-load current by the nameplate impedance as a decimal, or multiply it by 100 and divide by the percent impedance. A 45 kVA, 208 V three-phase unit carries 124.9 A at full load, so at 4.5% impedance it can deliver about 2,776 A into a fault at its terminals, assuming an unlimited supply.

How do you convert kW to kVA?

Divide kW by the power factor. 60 kW at a power factor of 0.85 is 70.6 kVA, which needs a 75 kVA three-phase transformer.

What is the turns ratio of a transformer?

The ratio of its primary to secondary voltage. 480 to 208 V is 2.31 to 1, and 480 to 240 V is 2 to 1. Current goes the other way, so the secondary carries that many times more amps than the primary.

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Transformer Sizing Calculator

DOE 10 CFR 431.196 ratings and minimum efficiency

A transformer is sized in kVA: volts times amps, times 1.732 on three phase, divided by 1,000. The size you can buy comes from a short list, and the federal efficiency standard is the one public place that list is printed with a number attached to every rating.

Low-voltage dry-type distribution transformer, sized from the load

Phase

What do you know about the load

Secondary voltage (the load side)

Primary voltage (0 to skip)

Load current (amps mode)

Load (kW mode)

Power factor (kW mode)

Allowance for growth

Nameplate impedance (0 to skip fault current)

Ratings and efficiencies are the DOE table for low-voltage dry-type units made from 2016 until April 23, 2029.

Specification summary

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Entered

Result

Load against the transformer rating

Worked example

An 80 A three-phase load at 208 V, fed from a 480 V system, with 25% added for growth and a transformer nameplate reading 4.5% impedance.

Load. 208 × 80 × 1.732 / 1,000 = 28.82 kVA, or 36.03 kVA with the 25% allowance.
Rating. The DOE three-phase list runs 15, 30, 45, 75 kVA. 30 is too small, so 45 kVA, loaded to 80.1%.
Currents. 45,000 / (208 × 1.732) = 124.9 A on the secondary and 54.1 A on the 480 V primary.
Efficiency. The standard requires at least 98.40% at 35% load, which allows no more than 256 W of loss at that load.
Fault current. 124.9 × 100 / 4.5 = 2,776 A available at the secondary terminals.

The growth allowance is what moved this job from 30 kVA to 45. Without it, 28.82 kVA fits a 30 kVA unit at 96% loading, which is a transformer that runs hot every day it is used.

The formula

Four relationships, all arithmetic on the nameplate:

kVA = V × A × √3 / 1,000 (three phase) | A = kVA × 1,000 / (V × √3) | I_fault = I_{full load} × 100 / %Z

√3: 1.732, used on three phase only; single phase drops it
kVA from kW: kW divided by power factor
%Z: the impedance printed on the nameplate, in percent
loss: 35% of the rating in watts × (1 / efficiency − 1), the most the DOE minimum allows at the test load

The fault current formula assumes the utility side can deliver unlimited current, which Schneider Electric describes as the worst case.

Where the list of sizes comes from

Transformer ratings are a short list, and the list is federal law. 10 CFR 431.196 sets a minimum efficiency for every standard low-voltage dry-type rating: 15, 25, 37.5, 50, 75, 100, 167, 250 and 333 kVA single phase, and 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 and 1,000 kVA three phase. This page picks from that list.

The fault current method is Schneider Electric's published one. Its FAQ FA105006 divides the secondary full-load current by the impedance and states that it assumes an infinite source, the worst case. It also notes impedance labels are not required below 15 kVA.

The efficiency table changes in 2029. Units made on or after April 23, 2029 must meet a higher table. This page uses the one in force for transformers made now.

Why the growth allowance decides the size

A transformer's rating is continuous: it will carry that kVA all day. What it will not do is carry it all day in a hot room with nowhere for the heat to go, and the load you measure today is rarely the load in five years. The allowance here is yours, not a code figure. Enter 0 to size to the load exactly and read the loading percentage before you choose.

Primary current and the turns ratio

Power in equals power out, less a percent or two, so current scales inversely with voltage. A 480 to 208 V transformer has a ratio of 2.31 to 1, and its primary carries 2.31 times less current than its secondary. That is the whole of a turns ratio calculation for a distribution transformer: the ratio of the nameplate voltages.

Frequently asked questions

How do I calculate what size transformer I need?: Multiply the load voltage by the load current, and by 1.732 on three phase, then divide by 1,000 for kVA. Add any allowance for growth and take the next standard rating up. An 80 A load at 208 V three phase is 28.82 kVA; with 25% added it needs a 45 kVA transformer.
How many amps is a 75 kVA transformer?: On three phase, 75,000 divided by (208 × 1.732) is 208.2 A at 208 V, and 75,000 divided by (480 × 1.732) is 90.2 A at 480 V. On single phase at 240 V it is 312.5 A.
How do you calculate transformer fault current?: Divide the secondary full-load current by the nameplate impedance as a decimal, or multiply it by 100 and divide by the percent impedance. A 45 kVA, 208 V three-phase unit carries 124.9 A at full load, so at 4.5% impedance it can deliver about 2,776 A into a fault at its terminals, assuming an unlimited supply.
How do you convert kW to kVA?: Divide kW by the power factor. 60 kW at a power factor of 0.85 is 70.6 kVA, which needs a 75 kVA three-phase transformer.
What is the turns ratio of a transformer?: The ratio of its primary to secondary voltage. 480 to 208 V is 2.31 to 1, and 480 to 240 V is 2 to 1. Current goes the other way, so the secondary carries that many times more amps than the primary.

Check these numbers yourself

10 CFR 431.196, Energy conservation standards and their effective datesUS Department of Energy, Electronic Code of Federal Regulations. Read 2026-09-15. Paragraph (a)(2): kVA ratings and minimum efficiencies at 35 percent load for low-voltage dry-type distribution transformers made from January 1, 2016 until April 23, 2029.
How do you calculate the maximum possible Available Fault Current at the secondary for single-phase and three-phase Transformers?Schneider Electric FAQ FA105006, published 7/29/2003, last modified 6/22/2026. Read 2026-09-15. Secondary full-load current divided by impedance, assuming an infinite source.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-15. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Sizing a transformer is part of an electrical design. Overcurrent protection, conductor sizing and grounding for the transformer are set by NEC Article 450 and your inspector, and none of them are on this page. The fault current is the transformer's maximum; the actual figure is lower once utility and conductor impedance are counted.