Transformer Size Calculator

Transformer size calculator formula

A transformer size calculator finds the kVA rating needed to carry a given load. Minimum kVA = connected load in watts × headroom factor ÷ 1,000. The default headroom factor is 1.25, which leaves 25 percent spare. After you have the figure, you choose the next standard size above it.

The formula in plain words

Add up the watts of everything the transformer will feed. Multiply by 1.25 to leave some room. Divide by 1,000 to change watts to kilovolt-amps. The result is the smallest rating that would work.

Worked examples

These match the calculator exactly.

1. Workshop load: 24,000 W with a headroom factor of 1.25

• With headroom: 24,000 × 1.25 = 30,000
• Minimum size: 30,000 ÷ 1,000 = 30 kVA
• At 240 V single-phase that is 30,000 ÷ 240 = 125 A

2. Small building: 12,000 W with a headroom factor of 1.25

• With headroom: 15,000
• Minimum size: 15 kVA

3. An odd result: 9,000 W with a headroom factor of 1.25

• Minimum size: 11.25 kVA
• Next standard single-phase size: 15 kVA

The third example shows why you round up. No 11.25 kVA unit is on the shelf, so you go to the next one.

Load to kVA at 1.25 headroom

Connected load	Minimum kVA
5,000 W	6.25
10,000 W	12.5
24,000 W	30
40,000 W	50
60,000 W	75

Common stock sizes are usually 10, 15, 25, 37.5, 50, 75 and 100 kVA for single-phase units. Round each result up to the next of these.

Power factor

Watts measure real power, while kVA measures apparent power. For many loads the two differ. An electric heater is close to equal, but motors and some electronics draw more current for the same watts. If the nameplate gives a power factor, divide the watts by it. A 24,000 W load at 0.8 power factor is 30,000 VA before headroom, and 37.5 kVA after the 1.25 factor. That is a size up from the first example.

Finding the load

Add up the nameplate watts of each piece of equipment that runs at the same time. For motors, use the starting current if it is listed. Large loads that never run together can be counted once, but ask an electrician before you assume that. Add planned growth, like a new compressor or a second welder.

To see the current a transformer will carry, use the kVA to amps calculator.

What the utility and inspector need

A transformer does not stand alone. The primary side connects to the utility, and the secondary feeds a panel or switchgear sized for the load. The utility may supply and own the unit, or you may buy a dry-type transformer for inside a building. Either way, wiring, grounding, overcurrent protection and clearance around the unit follow electrical code and the manufacturer’s instructions. Permits and inspection are usually required.

Bring your load list, the voltage and phase on each side, and the power factor to the electrician. Then the kVA from this tool is a starting point for a conversation, and the electrician can confirm it. Ask about future loads too, since a transformer that is a little larger now can save a replacement later.

Worked check on headroom

The headroom factor changes the answer directly. For the 24,000 W load, a factor of 1.0 gives 24 kVA, 1.25 gives 30 kVA and 1.5 gives 36 kVA. The first would probably be rounded up to 25 kVA, the second is exactly 30, and the third lands between standard sizes and goes to 37.5 kVA. Pick the factor with an electrician, then round.

Common mistakes

• Using no headroom.
• Ignoring power factor and motor starting current.
• Picking the size below the result.
• Mixing up single-phase and three-phase ratings.
• Treating this as a final design.

Because transformers connect to high voltage, work with a licensed electrician and the utility company.

Related pages

For a backup source, see the whole house generator cost calculator. For protective devices, use the breaker size calculator.