kVA to amps calculator and chart

Calculator

Enter the rating and the voltage, pick the phase, and the full-load current is worked out below with the arithmetic shown. Switch it round to find the kVA a known current amounts to.

The formulas

Apparent power is volts times amps. For three-phase the line-to-line voltage is used, which brings in the square root of three (1.732):

Single-phase: amps = kVA × 1,000 ÷ volts

Three-phase: amps = kVA × 1,000 ÷ (1.732 × volts)

And the other way:

Single-phase: kVA = volts × amps ÷ 1,000

Three-phase: kVA = 1.732 × volts × amps ÷ 1,000

Worked through for the most-searched size: a 75 kVA three-phase transformer at 480 V is 75 × 1,000 ÷ (1.732 × 480) = 75,000 ÷ 831 = 90.2 A. The same unit at 208 V is 75,000 ÷ (1.732 × 208) = 208 A: the lower the voltage, the higher the current for the same power.

Three-phase kVA to amps chart

Full-load line current for the standard three-phase ratings, at the secondary voltages used in buildings and plants (208, 240, 480 and 600 V) and the primary voltages they’re fed at (2,400 to 13,800 V).

Amps = kVA × 1,000 ÷ (1.732 × V). Rounded; the calculator gives the exact figure for any size or voltage.

Single-phase kVA to amps chart

Single-phase ratings, at the secondary voltages of a single-phase service (120, 240 and 277 V) and the primary voltages of a pole-mount or single-phase pad-mount (2,400 to 14,400 V line-to-neutral).

Amps = kVA × 1,000 ÷ V. For a 120/240 V secondary the 240 V column is the full-load current; each 120 V leg can carry that much.

Common sizes, one sentence each

The sizes people ask about most, at the two secondary voltages a plant or a building runs on, and on a 12,470 V primary.

• A 45 kVA three-phase transformer draws 54.1 A at 480 V, 125 A at 208 V, and 2.1 A on a 12,470 V primary. Find a 45 kVA unit for sale.
• A 75 kVA three-phase transformer draws 90.2 A at 480 V, 208 A at 208 V, and 3.5 A on a 12,470 V primary. Find a 75 kVA unit for sale.
• A 112.5 kVA three-phase transformer draws 135 A at 480 V, 312 A at 208 V, and 5.2 A on a 12,470 V primary. Find a 112.5 kVA unit for sale.
• A 150 kVA three-phase transformer draws 180 A at 480 V, 416 A at 208 V, and 6.9 A on a 12,470 V primary. Find a 150 kVA unit for sale.
• A 225 kVA three-phase transformer draws 271 A at 480 V, 625 A at 208 V, and 10.4 A on a 12,470 V primary. Find a 225 kVA unit for sale.
• A 300 kVA three-phase transformer draws 361 A at 480 V, 833 A at 208 V, and 13.9 A on a 12,470 V primary. Find a 300 kVA unit for sale.
• A 500 kVA three-phase transformer draws 601 A at 480 V, 1,388 A at 208 V, and 23.1 A on a 12,470 V primary. Find a 500 kVA unit for sale.
• A 750 kVA three-phase transformer draws 902 A at 480 V, 2,082 A at 208 V, and 34.7 A on a 12,470 V primary. Find a 750 kVA unit for sale.
• A 1,000 kVA three-phase transformer draws 1,203 A at 480 V, 2,776 A at 208 V, and 46.3 A on a 12,470 V primary. Find a 1,000 kVA unit for sale.
• A 1,500 kVA three-phase transformer draws 1,804 A at 480 V, 4,164 A at 208 V, and 69.4 A on a 12,470 V primary. Find a 1,500 kVA unit for sale.
• A 2,000 kVA three-phase transformer draws 2,406 A at 480 V, 5,551 A at 208 V, and 92.6 A on a 12,470 V primary. Find a 2,000 kVA unit for sale.
• A 2,500 kVA three-phase transformer draws 3,007 A at 480 V, 6,939 A at 208 V, and 116 A on a 12,470 V primary. Find a 2,500 kVA unit for sale.

What kVA is, and how it differs from kW

kVA stands for kilovolt-amperes: a thousand volts times amps. It’s apparent power, the product of the voltage across a piece of equipment and the current through it, which is what its conductors, insulation and core have to carry whatever the load does with it.

kW is real power, the part that does work and shows on the bill. The two are linked by the power factor: kW = kVA × power factor. A load of heaters has a power factor of 1, so its kW and kVA are the same; a plant full of motors runs at about 0.8 to 0.9, so a 1,000 kVA transformer carries about 800 to 900 kW of it. Transformers are rated in kVA because their losses depend on volts and amps alone; generator sets are rated in kW with a kVA figure beside it, at 0.8 power factor by convention, which is why a 100 kW set is a 125 kVA set.

Primary amps and secondary amps

A transformer carries the same kVA on both sides, less a percent or so of losses, so the current on each side follows its voltage: the higher-voltage side carries less. A 1,000 kVA unit rated 12,470 V to 480 V draws 46 A on its primary and delivers 1,203 A on its secondary, a ratio of 26, the same as its turns ratio.

On a nameplate the full-load amps for each winding are often printed beside its voltage, which is the quickest check that a listing’s figures are consistent. Our guide to step-up and step-down transformers covers the turns ratio itself.

Using the amps: conductors, protection and sizing

Full-load amps are the figure everything else is sized from. The NEC’s transformer rules (Article 450.3) set the overcurrent protection as percentages of it: for a transformer over 1,000 V, primary fuses or breakers up to 300% or 250% of primary full-load amps depending on the impedance, and secondary protection at 125% (or 250% with primary protection) of secondary full-load amps; for a 600 V-class unit with both primary and secondary protection, 250% on the primary and 125% on the secondary. The conductors on each side are sized to carry those amps continuously, and the secondary main breaker on a 1,000 kVA, 480 V unit is commonly 1,600 A (1,203 A × 1.25 = 1,504 A, rounded up to the next standard size).

For sizing a new or replacement transformer, work the other way: add up the load in kVA (kW ÷ power factor), allow for the motors that start together and for growth, and pick the next standard rating. A load of 650 kW at 0.85 power factor is 765 kVA, and a 750 kVA transformer at its 65 °C rating would be fully loaded; most engineers would specify 1,000 kVA, or a 750 kVA unit with fans (a 750/1,000 kVA rating) if the load peaks only occasionally.

When you’ve settled on the size, describe it to Magic Search with its voltages and connection, and it finds the units for sale that fit.