Transformer Sizing Calculator
Convert load volts and amps into the kVA rating a transformer needs.
Transformer selection must account for impedance, inrush, overcurrent protection per NEC Article 450 and grounding. This is an estimating tool only — use a licensed electrical engineer or electrician.
How this calculator works
kVA = volts × amps × phase multiplier ÷ 1000
Standard kVA ratings are 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 — round up to the next one. Transformers tolerate brief overload but run hot and lose life expectancy when continuously loaded past their rating.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- NFPA 70 (National Electrical Code) 2023, Article 450 — transformer overcurrent protection and the ratings the result is rounded up to
Check the edition. Codes are amended every few years, and the one your inspector works from may not be the one named above.
Worked example
With secondary voltage 480 , load current 100 , phase multiplier 1.732 , spare capacity factor 1.25 , this calculator returns 103.92 .
Transformer size for common services
Every row here opens the calculator already filled in. Copy that link and the numbers travel with it.
| Job | Size | Transformer size needed |
|---|---|---|
| Small shop, single phase | 240 × 100 × 1 × 1.25 | 30.00 |
| Light industrial, three phase | 480 × 100 × 1.732 × 1.25 | 103.9 |
| Larger three-phase service | 480 × 200 × 1.732 × 1.25 | 207.8 |
| Sized with no headroom | 480 × 200 × 1.732 × 1 | 166.3 |
| Small commercial 208 V | 208 × 150 × 1.732 × 1.25 | 67.55 |
Round up to a standard rating: 15, 30, 45, 75, 112.5, 150, 225, 300, 500 kVA.
Frequently asked questions
How do I calculate transformer kVA?
Multiply secondary voltage by load current, then by 1.732 for three-phase, and divide by 1,000. A 480 V three-phase load drawing 100 amps needs 83 kVA before any allowance for growth.
How much spare capacity should a transformer have?
Twenty to 25% is typical, which also puts the unit nearer its efficiency sweet spot. Oversizing beyond that wastes money and increases no-load losses that run continuously.
What are standard transformer sizes?
Dry-type units come in 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 kVA. Always round up — running a transformer above its nameplate shortens insulation life sharply.
Why is three-phase multiplied by 1.732?
It is the square root of three, which comes from the 120 degree phase relationship between the three legs. Three-phase delivers 1.732 times the power of single-phase at the same line voltage and current.