Transformer Sizing Calculator
Size a single- or three-phase transformer from load in kVA, kW, or secondary amps. Get the required kVA, the next standard size, primary and secondary full-load amps, and the NEC 450.3(B) primary OCPD. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Wire Size & Ampacity Calculator
Find the minimum copper or aluminum wire size for a load using NEC Table 310.16, with 310.15 ambient and conductor-count derating and the 110.14(C) terminal limit. Free, no sign-up.
Voltage Drop Calculator
Calculate voltage drop % and the minimum copper or aluminum wire size to stay within the NEC 3% / 5% recommendation, single- or three-phase. Free, no sign-up.
Explanation
Sizing a transformer means matching its kVA rating to the load it will serve and then rounding up to the nearest standard size you can actually buy. This calculator takes the load as kVA, as kW with a power factor, or as a secondary current, picks the next standard ANSI/IEEE size, and reports the primary and secondary full-load amps plus the NEC 450.3(B) primary overcurrent device — the numbers you need to specify the unit and its feeder.
How transformer sizing works
Apparent power (kVA) is what a transformer is rated for, so any load given in kW is first converted with its power factor. Full-load amps then follow from the kVA rating and the line-to-line voltage, with a √3 factor for three-phase.
| Symbol | Meaning |
|---|---|
| kVA | Apparent power — the transformer rating |
| PF | Power factor (0–1); about 0.8 for motor-heavy loads |
| V | Line-to-line voltage on the side being calculated |
| FLA | Full-load amps at the rated kVA |
Once you know the secondary full-load amps, size the conductors and overcurrent device for that feeder with the wire size & ampacity calculator, then check the run length against the 3% guideline with the voltage drop calculator.
Standard kVA sizes
Transformers are manufactured in fixed steps, so the calculated load is rounded up to the next standard rating. A 40 kW, 0.9 PF load is about 44.4 kVA, which rounds up to a standard 45 kVA three-phase unit. Single-phase and three-phase units use different size ladders.
| Phase | Common standard kVA ratings |
|---|---|
| 1Ø | 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 |
| 3Ø | 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 |
A common rule of thumb is to load a transformer to no more than about 80% of its rating, leaving roughly 20–25% headroom for future growth and to keep it running cool. Enter that as the spare capacity and the calculator sizes from the adjusted load.
Full-load amps and overcurrent protection
NEC 450.3(B) sets the maximum overcurrent protection for transformers rated 1000 V or less. With protection on the primary side only, the device is sized as a percentage of the primary full-load amps. When 125% does not land on a standard breaker or fuse size, the next higher standard rating is permitted.
| Primary current | Max primary OCPD |
|---|---|
| 9 A and over | 125% (next standard size up permitted) |
| 2 A to under 9 A | 167% |
| under 2 A | 300% |
If the installation also has overcurrent protection on the secondary, the primary device may be allowed up to 250% with a corresponding secondary limit. This calculator reports the primary-only case; verify any secondary-protected design against the full Table 450.3(B).
Notes and limitations
This tool sizes a transformer from a single connected load figure. It does not perform a full NEC Article 220 load study — adding up lighting, receptacle, motor, and HVAC loads with their demand factors — so for a service or panel calculation, total the demand load first and enter that result here. Always confirm the final selection, conductor sizes, and overcurrent protection against the current NEC and the authority having jurisdiction.