Transformer Size Calculator
This transformer size calculator finds the kVA rating you need for a load and picks the next standard transformer size. Enter the load in kW, the power factor, and a growth margin, and the transformer size calculator returns the required kVA, the standard size, the loading percentage, and the rated secondary current.
Results
The Transformer Size Calculator is part of our AC & Three Phase collection. Browse more ac & three phase to size, convert, and check your work alongside this tool.
What the transformer size calculator does
Transformers are rated in kVA, the apparent power they can deliver, while most loads are listed in kW. The transformer size calculator divides the kW by the power factor to get the load kVA, adds your margin for growth and safety, and rounds up to the next standard rating. It also shows how heavily the chosen unit will be loaded and the full load current on the secondary.
Transformer kVA calculation
Convert real power to apparent power, add margin, and round up:
Standard three phase sizes run 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1,000 kVA and up; single phase sizes run 5, 10, 15, 25, 37.5, 50, 75, 100, 167 kVA and up.
Transformer size calculator example
A 100 kW three phase load at 0.9 power factor with a 25% margin and a 480 V secondary:
The next standard size is 150 kVA, loaded to 74.1% at full load, with a rated current of 180.4 A at 480 V. A 30 kW single phase load at 0.85 with 20% margin needs 42.4 kVA, so a 50 kVA unit with 208.3 A at 240 V.
How to use the transformer size calculator
Add up the connected or demand load in kW. Enter the load power factor, 0.8 to 0.9 for typical mixed loads, and a growth margin, often 20 to 25%. Choose single or three phase and enter the secondary voltage to see the rated current. Press Calculate to read the standard transformer size and its loading.
Reading the transformer sizing result
The standard size is the smallest stock rating that covers the required kVA. Loading between about 60 and 80% at full load leaves room for growth and keeps the unit cool and efficient. Motor starting, harmonic loads from drives and electronics, and high ambient temperatures may call for a larger unit or a K-rated transformer. The rated current sets the secondary conductor and breaker sizes.
When to use a kVA transformer calculator
- Sizing a transformer for a new building or shop
- Choosing a step-down unit for 208 V or 240 V equipment
- Checking whether an existing transformer has room for more load
- Comparing single and three phase options
- Planning feeders from the rated secondary current
Common transformer size calculator mistakes
- Using kW as kVA. Divide by the power factor first.
- Sizing for connected load. Use demand load where you can.
- Ignoring harmonics. Nonlinear loads may need a K-rated unit.
- No growth margin. Replacing a transformer later costs far more.
Tips for transformer sizing
- Aim for 60 to 80% loading at peak.
- Check voltage dip if one large motor dominates the load.
- Confirm the secondary voltage matches every piece of equipment.
How to size a transformer for real loads
Connected load overstates what a building draws, because not everything runs at once. Electrical codes and designers use demand factors to turn connected load into a realistic peak, and that peak is what you should enter here. For existing sites, a utility demand reading or a few weeks of logged data is better still. Harmonic loads such as computers, LED drivers, and variable speed drives heat a transformer more than their kW suggests, so offices and data rooms often use K-rated or oversized units. Ambient temperature matters too: ratings assume a set ambient, and a hot electrical room reduces capacity. Once you pick a unit, check its short circuit duty with the fault current calculator, since a larger transformer with low impedance raises the fault current downstream.
Standard transformer sizes and secondary current
Sizes come in fixed steps, so the margin you choose often matters less than the step you land on. A load of 120 kVA fits a 150 kVA unit whether you add 10% or 25%. The rated current then drives the rest of the design: the secondary conductors and main breaker are sized for it, with the 125% factor for continuous loads where it applies. Convert any kVA to current quickly with the kVA to amps calculator.
Transformer full load current chart
Rated current of standard three phase transformers at 208 V and 480 V:
| kVA | Current at 208 V | Current at 480 V |
|---|---|---|
| 15 | 41.6 A | 18.0 A |
| 30 | 83.3 A | 36.1 A |
| 45 | 124.9 A | 54.1 A |
| 75 | 208.2 A | 90.2 A |
| 112.5 | 312.3 A | 135.3 A |
| 150 | 416.4 A | 180.4 A |
| 225 | 624.5 A | 270.6 A |
| 300 | 832.7 A | 360.8 A |
| 500 | 1,387.9 A | 601.4 A |
| 750 | 2,081.8 A | 902.1 A |
| 1,000 | 2,775.7 A | 1,202.8 A |
Transformer Size Calculator FAQ
What size transformer do I need?
Divide the load kW by the power factor, add 20 to 25% margin, and choose the next standard kVA size. 100 kW at 0.9 with 25% margin needs 138.9 kVA, so a 150 kVA transformer.
How do I convert kW to transformer kVA?
Divide kW by the power factor. At 0.8 power factor, 80 kW is 100 kVA.
Should a transformer run at full load?
It can, but many designers aim for 60 to 80% at peak to leave room for growth and reduce heat and losses.
Do motors need a bigger transformer?
Large motors draw several times their running current at start, which can dip voltage on a small transformer. Check the starting kVA when one motor is a large share of the load.