Why a transformer is rated in kVA
The losses inside a transformer come from two sources: resistive losses in the windings, which depend on current, and core losses, which depend on voltage. Neither depends on the phase relationship between them. That means the heating is set by the product of volts and amps — apparent power — rather than by the real power delivered.
The practical consequence is that a 75 kVA transformer can deliver 75 kW to a resistive load, but only 60 kW to a load at 0.8 power factor, because 60 ÷ 0.8 = 75 kVA. Sizing from a load schedule that lists kW without a power factor is a common way to end up with an undersized transformer.
Standard kVA ratings
| Rating (kVA) | Typical use | 208 V three-phase FLA | 480 V three-phase FLA |
|---|---|---|---|
| 15 | Small commercial, one suite | 42 A | 18 A |
| 30 | Small office or retail | 83 A | 36 A |
| 45 | 125 A service at 208 V | 125 A | 54 A |
| 75 | Small industrial, light process | 208 A | 90 A |
| 112.5 | 200 A service at 208 V | 312 A | 135 A |
| 150 | Light industrial | 416 A | 180 A |
| 225 | Medium industrial | 625 A | 271 A |
| 300 | Larger industrial | 833 A | 361 A |
| 500 | Heavy industrial | 1,388 A | 602 A |
Secondary current is what sets your panel bus rating, so 45 kVA at 208 V producing exactly 125 A is why that pair appears so often in commercial fit-outs.
Two worked examples
1. A light industrial load of 65 kW at 0.85 power factor
Apparent power is 65 ÷ 0.85 = 76.5 kVA. With a 25% growth margin the design figure is 95.6 kVA, so the standard size chosen is 112.5 kVA. Its 208 V secondary full-load current is 312 A, which sets the minimum main bus rating of the downstream panel. Primary protection at 125% of the 480 V primary current of 135 A gives about 169 A.
2. A 40 kW resistive heating load at 0.98
Apparent power is 40 ÷ 0.98 = 40.8 kVA. With a 20% margin that is 49 kVA, so a 50 kVA unit would be marginal and 75 kVA is the sensible standard choice. This example shows how much the power factor changes the outcome: at 0.7 the same 40 kW would need 57 kVA at unity growth, and 68 kVA with the same 20% margin.
What the calculation does not cover
- Motor starting inrush. A squirrel cage motor draws five to eight times its running current at start. A transformer sized only on running load may see a voltage dip during starting that trips other equipment. Where large motors are present, check the starting kVA.
- Harmonics. A standard transformer feeding variable frequency drives or switch-mode loads can overheat from harmonic losses even though its kVA rating is not exceeded. K-rated transformers and derating factors exist for this case.
- Ambient temperature and altitude. Standard ratings assume 30°C ambient and altitudes below 3,300 feet. A transformer in an unconditioned mechanical room needs derating.
- Impedance. A low-impedance transformer gives better voltage regulation but higher fault current, which raises the interrupting rating needed on the downstream equipment.
How this calculator is verified
Standard kVA ratings follow the commonly manufactured dry-type sizes. Full-load currents are derived directly from the kVA rating and voltage using the standard three-phase formula, and protection sizing follows NEC 450.3(B) at 125% of primary full-load current.
- NFPA 70, National Electrical Code — Article 450 (transformers), Article 430 (motors), Article 220 (load calculation).
- NEMA ST 20 — dry-type transformer standards, including the standard kVA and voltage ratings.
- UL — UL 1561 for dry-type general purpose transformers.
Standard ratings and worked examples last verified: 19 September 2026.