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 transformer kVA chart and the short-circuit current calculator to screen available fault current from transformer impedance and source/downstream impedance, the voltage drop calculator for feeder length checks; it can use the transformer/load kVA directly or the full-load current reported here. Use the conduit fill calculator for raceway area and the electrical box fill calculator for junction-box volume. If the same feeder route needs an obstruction offset, use the conduit bend offset calculator and conduit pull tension calculator. If the source is a standalone PV/battery system instead of a transformer, start with the off-grid solar calculator.
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
Use the transformer kVA chart as a worksheet: calculate load kVA, add the practical design margin, then round up to the next listed size for the selected phase.
| Worksheet step | Use this value | Result |
|---|---|---|
| Known volts + amps | Phase, load voltage, load current | Load kVA |
| Known kW + PF | Real power and power factor | Load kVA |
| Design margin | Capacity margin for future load, continuous load or motor starting | Design kVA |
| Transformer kVA chart | Single-phase or three-phase listed sizes | Next standard kVA size |
| Feeder follow-up | Full-load current, impedance and selected source size | Fault current, voltage drop, conduit fill and grounding context |
Source: Workflow maps this calculator's inputs and outputs; final transformer selection still depends on manufacturer catalogue data, system voltage, enclosure, impedance, temperature rise and protection rules.
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 do I use the transformer kVA chart?
Calculate load kVA and design kVA, then use the transformer kVA chart as a round-up reference to pick the next listed size. Verify the exact manufacturer line, voltage, enclosure, impedance, temperature rise and protection rules before buying or installing equipment.
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, temperature rise or enclosure; use the short-circuit current calculator for a separate impedance-based fault-current worksheet.
- Motor starting, nonlinear loads, harmonics, K-rated transformers and continuous-load rules need separate review.