How do I calculate kVA?

For a single-phase supply, kVA is volts times amps divided by 1,000. For three-phase, multiply by the square root of three as well: kVA = √3 × volts × amps ÷ 1,000. Enter the voltage and current here, pick the phase, and the tool returns the apparent power in kVA.

How do I find amps from kVA?

Rearrange the same formula. Single phase: amps = kVA × 1,000 ÷ volts. Three phase: amps = kVA × 1,000 ÷ (√3 × volts). Flip the tool to solve for current and it does exactly that, which is the usual question when you are sizing a feed from a transformer rating.

What is the difference between kVA and kW?

kVA is apparent power — the full product of volts and amps. kW is real power, the part that does work. They are linked by the power factor: kW = kVA × power factor. On a purely resistive load the two are equal; on motors and other inductive loads the power factor is below one, so the kW is less than the kVA.

Which voltage do I use for three phase?

Use the line-to-line voltage — 208, 480 or 600 in common systems — because the √3 formula is written around it. Entering a phase-to-neutral voltage by mistake is the most common error here, and it throws the result off by a factor of √3.

kVA calculator

These figures come straight from the voltage and current you enter. They do not account for power factor, harmonics or diversity, and they are not a service or feeder sizing — treat the kVA as a starting point and defer to the equipment nameplate, your code book and the authority that governs the job.

Enter the voltage and current and the tool gives the apparent power in kVA, or flip it to find the current a given kVA draws at your voltage. The single-phase and three-phase forms differ only by the √3 factor, which the phase switch handles.

Exemple détaillé

A three-phase load at 480 V drawing 100 A is √3 × 480 × 100 ÷ 1,000 ≈ 83.1 kVA. Going the other way, a 50 kVA three-phase load at 480 V pulls 50,000 ÷ (√3 × 480) ≈ 60.1 A per line. The same 100 A on a single-phase 240 V supply would be only 240 × 100 ÷ 1,000 = 24 kVA.

Sur le chantier

Sizing a rental generator off a clamp meter. 208 V three-phase, 150 A on the busiest leg: √3 × 208 × 150 ÷ 1,000 = 54 kVA. That is the demand you measured; the generator's rating is kVA (or kW at its stated power factor), so this is the number to hand the rental desk — with whatever margin your job wants on top.

Full-load current of a transformer. A 75 kVA three-phase unit at 208 V secondary: 75,000 ÷ (√3 × 208) = 208 A per line. Switch the tool to "amps from kVA" and it does the division. What protects that secondary is the code book's business, not this page's.

Single phase is just V × A. A 25 kVA single-phase pole pig at 240 V: 25,000 ÷ 240 = 104 A. No √3 — the 1.732 only appears when the volts are line-to-line on a three-phase system.

Exercez-vous

Quick referencefor kVA

Monophasé: kVA = V × A ÷ 1 000 · A = kVA × 1 000 ÷ V
Triphasé: kVA = 1,732 × V × A ÷ 1 000 · A = kVA × 1 000 ÷ (1,732 × V)
Amps per kVA: 120 V 1φ 8,33 · 240 V 1φ 4,17 · 208 V 3φ 2,78 · 480 V 3φ 1,20 · 600 V 3φ 0,96
kW: kVA × power factor — the nameplate gives the PF
√3: 1.732 — only with line-to-line volts on three phase

Mistakes to catchbefore you trust the number

Using the line-to-neutral voltage (120) for a three-phase calculation — the tool wants line-to-line (208 or 480), and the wrong one is off by √3.
Reading kVA as kW: the transformer is rated in kVA, and the load in kW is less by the power factor.
Typing amps for one phase as the total for three — the current field is per line.

What an inspector looks forthe electrical inspector

On this part of the job

Transformer primary and secondary protection sized by the rules that apply to each side — they are different rules, and the secondary one is the one people miss.
Conductor sizing on both sides against the calculated current, with the terminations rated for it.
The secondary system: how the transformer's neutral is grounded and bonded, and where the bonding jumper sits.
Clearances and ventilation around the transformer, and that the location suits its enclosure type.
The load calculation behind the kVA you picked, particularly whether the demand factors used are ones the code allows.

Have this ready

The transformer nameplate data.
The load calculation, in writing.
The one-line diagram with protection on both sides shown.

Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.

La formule

single phase: kVA = V × A ÷ 1000 · three phase: kVA = √3 × V × A ÷ 1000

Apparent power is simply the product of voltage and current, scaled to kilovolt-amps by dividing by a thousand. Three-phase power adds the √3 (about 1.732) that comes from the geometry of three line voltages 120° apart, using the line-to-line voltage.

Questions

How do I calculate kVA?: For a single-phase supply, kVA is volts times amps divided by 1,000. For three-phase, multiply by the square root of three as well: kVA = √3 × volts × amps ÷ 1,000. Enter the voltage and current here, pick the phase, and the tool returns the apparent power in kVA.
How do I find amps from kVA?: Rearrange the same formula. Single phase: amps = kVA × 1,000 ÷ volts. Three phase: amps = kVA × 1,000 ÷ (√3 × volts). Flip the tool to solve for current and it does exactly that, which is the usual question when you are sizing a feed from a transformer rating.
What is the difference between kVA and kW?: kVA is apparent power — the full product of volts and amps. kW is real power, the part that does work. They are linked by the power factor: kW = kVA × power factor. On a purely resistive load the two are equal; on motors and other inductive loads the power factor is below one, so the kW is less than the kVA.
Which voltage do I use for three phase?: Use the line-to-line voltage — 208, 480 or 600 in common systems — because the √3 formula is written around it. Entering a phase-to-neutral voltage by mistake is the most common error here, and it throws the result off by a factor of √3.

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