kVA equals amps times volts times the square root of three, divided by 1000. A service rating is the breaker size, not the actual load, so sizing straight off it usually oversizes the transformer. Size to measured or calculated demand where you have it, and use this as the upper bound.
Standard ratings, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA. Specifying a non standard number adds weeks of lead time and costs more than the size above it.
Common service sizes
What each service rating works out to, and the standard transformer rating that covers it. These are full rated figures, not derated to 80 percent.
| Service | At 208Y/120 | Rating | At 480Y/277 | Rating |
|---|---|---|---|---|
| 100 A | 36 kVA | 45 kVA | 83 kVA | 112.5 kVA |
| 200 A | 72 kVA | 75 kVA | 166 kVA | 225 kVA |
| 400 A | 144 kVA | 150 kVA | 333 kVA | 500 kVA |
| 600 A | 216 kVA | 225 kVA | 499 kVA | 500 kVA |
| 800 A | 288 kVA | 300 kVA | 665 kVA | 750 kVA |
| 1000 A | 360 kVA | 500 kVA | 831 kVA | 1000 kVA |
| 1200 A | 432 kVA | 500 kVA | 998 kVA | 1000 kVA |
| 1600 A | 576 kVA | 750 kVA | 1330 kVA | 1500 kVA |
| 2000 A | 721 kVA | 750 kVA | 1663 kVA | 2000 kVA |
| 3000 A | 1081 kVA | 1500 kVA | 2494 kVA | 2500 kVA |
| 4000 A | 1441 kVA | 1500 kVA | 3326 kVA | over 2500 |
Standard ratings, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA. Free to cite with a link to this page. Size to real demand where you have it. A service rating is what the building could draw, not what it does.
Common questions
What size transformer is needed for a 200 amp service
At 208Y/120 volts a 200 amp service is about 72 kVA, so the smallest standard rating that covers it is 75 kVA. At 480Y/277 volts it is about 166 kVA, which takes a 225 kVA unit. The voltage matters as much as the amps, so a service described only in amps is not enough information to size a transformer.
What is the 80 percent rule for transformers
It is the continuous load allowance. Overcurrent devices are generally limited to 80 percent of their rating for loads running three hours or more, so a 400 amp service is treated as 320 amps of continuous capacity. Applied to transformer sizing it gives a smaller and usually more realistic number than the full service rating.
How do you calculate transformer kVA from amps
kVA equals amps times volts times the square root of three, divided by 1000, for a three phase system. For single phase leave out the square root of three. A 400 amp service at 480 volts three phase is 400 times 480 times 1.732 divided by 1000, which is about 333 kVA.
What are the standard kVA transformer sizes
For three phase liquid filled pad mounted units in the US, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000 and 2500 kVA. Specifying a number between those means a special build with a longer lead time and a worse price than the standard size above it.
Should I size a transformer to the service rating
It is an upper bound rather than an answer. A service rating is the breaker size, which is what the building could draw, not what it does draw. Sizing straight off it commonly oversizes the transformer, and an oversized unit costs more to buy and carries higher no load loss for its whole life.
Need the current rather than the rating. Use the kVA to amps calculator, or ask for a quote.