What does transformer kVA mean?

kVA is a transformer's apparent-power capacity — the volts times amps it can carry continuously without overheating. It's stated without a power factor because the transformer doesn't know or care what fraction of the load is reactive; it just has to carry the current.

What size transformer do I need for a 25 A, 240 V single-phase load?

25 × 240 ÷ 1,000 = 6 kVA minimum. You'd install the next standard size up — 7.5 kVA or 10 kVA — for margin against continuous loading, inrush, and growth.

Why does three-phase sizing use √3?

The three line currents are 120° apart, so line-to-line apparent power works out to 1.732 × V × I rather than a plain V × I. Using the single-phase formula on a three-phase load undersizes the transformer by about 42%.

Should I pick a transformer exactly equal to the calculated kVA?

No. The result is a bare minimum. Choose the next standard rating above it, and add headroom for continuous (3-hour-plus) loads, motor starting current, high ambient temperature, harmonic heating from electronic loads, and future expansion.

Does this account for power factor?

No, and it doesn't need to — transformers are rated in kVA precisely so power factor drops out. Power factor only matters when you're converting the load to kW.

What about inrush when the transformer energizes?

A transformer draws a large magnetizing inrush — often 8 to 12 times its rated current for a few cycles — when first switched on. That affects the upstream breaker's trip curve, not the transformer's continuous kVA sizing.

Transformer Sizing Calculator

Transformer sizing helps ensure that a transformer can supply an electrical load without overheating or operating beyond its rated capacity. This calculator estimates the minimum apparent power requirement in kilovolt-amperes (kVA) from the load current, voltage, and whether the system is single-phase or three-phase.

How to Calculate Transformer Size (step by step)

Step One: Identify the transformer phase

The phase configuration determines the multiplier used in the apparent power calculation. Single-phase systems use a factor of 1. Balanced three-phase systems use the square root of 3, approximately 1.732.

Phase factor = 1 for single-phase
Phase factor = √3 = 1.7320508075688772 for three-phase

With the default selection, the transformer is single-phase:

Transformer type = Single-phase
Phase factor = 1

Step Two: Multiply load current by voltage

First calculate the apparent power in volt-amperes by multiplying the load current by the load voltage.

Apparent power = Load current × Load voltage × Phase factor

Using the default inputs:

Apparent power = 25 A × 240 V × 1
Apparent power = 6,000 VA

Step Three: Convert volt-amperes to kVA

Divide the apparent power in volt-amperes by 1,000 to express the result in kilovolt-amperes.

Minimum transformer size = Apparent power ÷ 1,000

Using the default values:

Minimum transformer size = 6,000 VA ÷ 1,000
Minimum transformer size = 6 kVA

Transformer Sizing Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

What your results mean

The result is the minimum kVA the transformer must carry to serve the entered load continuously. With the defaults — 25 A, 240 V, single-phase — that’s 6 kVA.

It’s an apparent-power figure, nothing more. It says nothing about the transformer’s efficiency, its physical size, its impedance, or how it behaves during a fault. And 6 kVA is the floor, not the pick: a transformer loaded to exactly its nameplate runs hot, ages faster, and has no room for the load to grow.

Standard sizes and where the calculated load lands

Calculated minimum	Next standard single-phase rating
up to 3 kVA	3 kVA
3–5 kVA	5 kVA
5–7.5 kVA	7.5 kVA
7.5–10 kVA	10 kVA
10–15 kVA	15 kVA
15–25 kVA	25 kVA

A 6 kVA calculated load points to a 7.5 kVA transformer, or 10 kVA if the load is continuous or expected to grow.

What to add on top of the bare number

Continuous-load margin. For loads running three hours or more, common practice keeps the transformer at 80% of rating or below.
Motor inrush. A motor started across the line pulls locked-rotor current several times its running value; the transformer has to hold voltage through it.
Harmonics. Variable-frequency drives, LED drivers, and computer supplies distort the current waveform and add heating a plain kVA figure misses. K-rated transformers exist for this.
Ambient temperature and enclosure. A transformer in a hot, unventilated room is derated.
Future load. Adding a circuit later is far cheaper if the transformer already has spare capacity.

Common mistakes

Using the single-phase formula for a three-phase load — a 42% undersize.
Sizing to the calculated minimum with no margin.
Summing nameplate currents that never run together, which oversizes and wastes money — use a realistic demand factor.
Ignoring the secondary voltage — the kVA is the same on both windings, but the currents differ, so check both sides against their conductors.

For the current a transformer’s kVA rating implies, see the kVA to amps calculator. To convert the load to real power, use kVA to kW.

About the formula: The calculator multiplies voltage by current to find volt-amperes, applies the appropriate phase factor, and divides by 1,000 to convert VA to kVA. Single-phase loads use a factor of 1, while balanced three-phase loads use the square root of 3.