Transformer kVA Sizing Calculator
Size a transformer based on secondary load current and voltage.
| Load kVA | 12.00 kVA |
|---|---|
| Required kVA (with safety factor) | 15.00 kVA |
| Recommended Standard Size | 15.0 kVA |
| Estimated Primary Amps | 31.3 A |
· Load kVA at secondary (120V): 12.00 kVA. With 1.25x safety factor: 15.00 kVA.
· Next standard size: 15 kVA.
· Primary amps are estimated. Actual transformer primary FLA depends on efficiency and losses. Verify with manufacturer data.
How It Works
A transformer must be sized to handle the total load connected to its secondary. The basic relationship is: kVA (1 phase) = secondary voltage x load amps / 1000 kVA (3 phase) = 1.732 x secondary voltage x load amps / 1000 A safety factor of 1.25 is commonly applied to allow for future growth and inrush from motor loads. The result is compared against standard transformer kVA ratings to find the next size up. Primary amps are estimated and should be verified with the actual transformer efficiency and manufacturer nameplate data.
Example
120 V x 100 A / 1000 = 12 kVA. With 1.25 safety factor = 15 kVA. Recommended: 15 kVA standard size.
Frequently Asked Questions
Why does the primary voltage matter?
It is used to estimate primary side amps, which matters for conductor and fuse sizing on the primary.
Can I use a transformer smaller than the calculated size?
Only if you are certain the load will never reach the transformer rating. Operating a transformer beyond its kVA rating causes overheating and shortens its life.
Does this account for harmonic loads?
No. Loads with significant harmonics (like computer power supplies or VFDs) can increase transformer losses beyond what this simple calculation shows. Use a K-rated or harmonic-tolerant transformer for those applications.