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Transformer kVA Rating

A transformer's apparent power rating in kilovolt-amperes (kVA) is calculated by multiplying its rated voltage by its rated current, then dividing by 1000. For example, a transformer rated 400 V and 250 A has a capacity of 100 kVA. This rating tells you how much load the transformer can handle without overheating. It is independent of the power factor, unlike kW, because the transformer must carry both real and reactive power.

Written by the Vorzep Team

Power Systems Transformers

Transformer kVA Rating Calculator

Size a transformer from its apparent power: kVA = (V · I) / 1000 for single-phase, and √3 · V · I / 1000 for three-phase.

Inputs

Drag the handle (or focus it and press ↑/↓) to reorder. Scientific notation such as 1.1e4 is accepted. Pick any unit; conversion is automatic. Choose single-phase or three-phase first; for three-phase use line-to-line voltage and line current. Pick any unit; conversion is automatic.

Presets

Results

Enter values and press Calculate. Your answer will appear here.

Current vs. voltage for this rating

For a fixed kVA rating, current falls as voltage rises (I = kVA·1000 / V), a straight line on log-log axes. The dashed line is the other system type, which needs √3 times less or more current. The amber dot is your winding; the teal dot is the second winding if you entered its voltage.

What is being calculated

Common standard ratings

Series	Commonly available kVA ratings
US (ANSI)
IEC

Typical lists only; available sizes vary by manufacturer, region and phase type.

Full-load current, three-phase

Rating	At 400 V	At 11 kV

I = kVA · 1000 / (√3 · V). Higher voltage means proportionally lower current.

Unit conversions

Quantity	Conversion
Apparent power	1 MVA = 1,000 kVA = 10⁶ VA
Voltage	1 kV = 1,000 V
Current	1 kA = 1,000 A · 1 A = 1,000 mA
Real power	1 MW = 1,000 kW = 10⁶ W

Formula

kVA = (V · I) / 1000 3-phase: kVA = (√3 · V · I) / 1000

kVA: apparent power rating (kilovolt-amperes)
V: winding voltage (V); line-to-line for three-phase
I: winding current (A); line current for three-phase

Example: single-phase, V = 240 V and I = 50 A: kVA = 240 × 50 ÷ 1000 = 12 kVA. Three-phase, V = 400 V and I = 100 A: kVA = √3 × 400 × 100 ÷ 1000 ≈ 69.28 kVA.

Rearranged: I = kVA × 1000 / V (divide V by √3 for three-phase) and V = kVA × 1000 / I. If you know the load in kW, kVA = kW / power factor. A transformer is rated in kVA, not kW, because its heating depends on current and voltage regardless of the power factor. The calculator can add spare capacity and pick the next standard size. This is sizing guidance only: impedance, temperature rise, harmonics, inrush current, ambient conditions and local codes also affect the final choice, so confirm with a qualified engineer or the manufacturer.

References

Wikipedia, Transformer: windings, ratings and ratios
Wikipedia, Volt-ampere: apparent power and kVA
Wikipedia, Three-phase electric power: the √3 factor for line quantities

How to Use This Tool

Choose single-phase or three-phase, or pick a preset.
Enter the voltage V and the current I with their units.
In Advanced, find the current or voltage from a known rating, or start from the load in kW and power factor.
Optionally add the second winding voltage and spare capacity.
Click Calculate, then read the kVA rating, the next standard size and the chart.

What This Tool Offers

Basic and Advanced modes on one calculation engine
Single-phase and three-phase calculation with V, kV, A, kA, kVA, MVA, kW and MW units
Solve for kVA, current or voltage, or size from kW and power factor (Advanced)
Spare capacity, next standard size, loading and second-winding current (Advanced)
Current-vs-voltage chart and labelled transformer diagram
Reorderable inputs, presets and a mobile-friendly layout

Variables

Symbol	Quantity	Unit
kVA	Transformer apparent power rating	kVA
V	RMS voltage (primary or secondary winding)	V
I	RMS current (corresponding winding)	A

What it means

The kVA rating of a single‑phase transformer is the product of the primary (or secondary) voltage and current, divided by 1000: kVA = (V × I) / 1000. For a three‑phase transformer, it is kVA = (√3 × V_L × I_L) / 1000. This rating represents the apparent power the transformer can handle. It is used to size transformers for a given load. Example: A single‑phase transformer with primary voltage 440V and primary current 50A has a rating of 440 × 50 / 1000 = 22 kVA. This means it can deliver 22 kVA to the load. The actual kW output depends on the load’s power factor.

Worked example

Transformer kVA Rating – Practical Example

Real‑World

Scenario: A transformer has primary voltage 240 V and primary current 20.8 A. What is its kVA rating?

Parameter	Value
V	240 V
I	20.8 A
Formula	`kVA = (V × I) / 1000`

1V × I = 240 × 20.8 = 4992 VA

2kVA = 4992 / 1000 ≈ 5 kVA

Final Design 5 kVA ✓ Rating

Why: Transformer kVA rating is the product of voltage and current – it defines the maximum apparent power it can handle.

Common mistakes

Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.

Applications

Transformer kVA rating: kVA = (V × I)/1000 for single‑phase, or √3·V_L·I_L/1000 for three‑phase, gives the apparent power capacity. This is the primary rating for transformers. Engineers use it to select transformers for loads, to design substations, and to ensure capacity meets demand. Understanding kVA rating is essential for power system design.

Transformer selection and specification
Substation and distribution system design
Load capacity planning
Transformer protection and coordination
Educational understanding of transformer ratings

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Published July 10, 2026
Last updated August 25, 2026