How to use this calculator
- Choose phase and input basis. Select single-phase or three-phase, then choose volts plus amps or kW plus power factor.
- Enter voltage and load. Use line-to-line voltage for three-phase loads and the expected load current or real power.
- Set power factor and margin. Use the load power factor and any practical capacity margin before standard-size rounding.
- Read recommended kVA. Compare load kVA, design kVA, standard size, full-load current and utilization.
How it works
Transformers are sized by apparent power. For a single-phase load:
kVA = V x I / 1000
For a balanced three-phase load, use line-to-line voltage and line current:
kVA = sqrt(3) x VLL x I / 1000
If the known load is real power, apparent power is kVA = kW / PF.
The calculator applies your sizing margin, then rounds up to the next listed standard size. After selecting transformer capacity, use the voltage drop calculator for feeder length checks and the conduit fill calculator for raceway area.
Worked example
Verified against the live calculator
A three-phase 480 V load at 60 A is
sqrt(3) x 480 x 60 / 1000 = 49.9 kVA. With a 25%
margin, design kVA is 62.4 kVA. The next listed three-phase size is
75 kVA, with full-load current of about 90 A at
480 V.
Reference data
Common dry-type transformer size families used for the round-up result.
| Rating family | Listed kVA |
|---|---|
| Single phase | 1 |
| Single phase | 1.5 |
| Single phase | 2 |
| Single phase | 3 |
| Single phase | 5 |
| Single phase | 7.5 |
| Single phase | 10 |
| Single phase | 15 |
| Single phase | 25 |
| Single phase | 37.5 |
| Single phase | 50 |
| Single phase | 75 |
| Three phase | 3 |
| Three phase | 6 |
| Three phase | 9 |
| Three phase | 15 |
| Three phase | 30 |
| Three phase | 45 |
| Three phase | 75 |
| Three phase | 112.5 |
| Three phase | 150 |
| Three phase | 225 |
Source: Standard dry-type transformer catalogue rating families; verify the exact manufacturer line, voltage, enclosure, temperature rise and impedance.
Frequently asked questions
How do I calculate transformer kVA?
For single phase, kVA = volts * amps / 1000. For three phase, kVA = sqrt(3) * line-to-line volts * line current / 1000. If you know real power instead, use kVA = kW / power factor.
What voltage should I enter for three phase?
Enter line-to-line voltage, such as 208 V, 240 V, 480 V or 600 V. The three-phase formula already includes the sqrt(3) multiplier.
Why does power factor matter?
Transformers are rated in apparent power, kVA. A low power factor means the same kW load draws more kVA and more current, so kW alone can undersize the transformer.
Does this size transformer protection?
No. It estimates apparent power and rounds to a standard kVA size. Primary/secondary overcurrent protection, conductor sizing, grounding, inrush and local code rules are separate checks.
How much margin should I use?
Use the margin for continuous loading, expected future load, motor starting and design practice. The default 25% is a screening allowance, not a code rule.
Method & assumptions
- Single-phase kVA uses volts times amps; three-phase kVA uses sqrt(3) times line-to-line volts times line current.
- Power-factor mode assumes balanced steady-state load and uses kVA = kW / PF.
- Does not size primary/secondary protection, conductor ampacity, grounding, short-circuit duty, impedance, temperature rise or enclosure.
- Motor starting, nonlinear loads, harmonics, K-rated transformers and continuous-load rules need separate review.