kVA formulas
Single phase: kVA = V × I ÷ 1000 · I = kVA × 1000 ÷ V
Three phase: kVA = 1.732 × V × I ÷ 1000 · I = kVA × 1000 ÷ (1.732 × V)
kVA (kilovolt-amperes) is apparent power, voltage times current with no power factor. Transformers, generators and UPS units are rated in kVA because the heat in their windings depends on current, whatever the load's power factor. For three phase, V is the line-to-line voltage.
Worked examples
1. 75 kVA three-phase transformer, 480 V to 208Y/120 V
Primary: 75,000 ÷ (1.732 × 480) = 90.2 A
Secondary: 75,000 ÷ (1.732 × 208) = 208.2 A
This is the most common building transformer. Under NEC Table 450.3(B), primary-only protection at 125% would be 112.8 A, rounded up to the next standard size, 125 A. The secondary conductors must still be protected per NEC 240.21(C), usually by a 225 A or 250 A main in the panel they feed.
2. 25 kVA single-phase pole transformer
I = 25,000 ÷ 240 = 104.2 A
A 25 kVA utility transformer often serves several homes. Because residential loads are diverse and intermittent, utilities routinely load these well beyond nameplate for short periods.
3. Amps to kVA for a service
A 200 A, 120/240 V single-phase service: kVA = 240 × 200 ÷ 1000 = 48 kVA. A 400 A, 120/208 V three-phase service: kVA = 1.732 × 208 × 400 ÷ 1000 = 144 kVA.
4. Generator sizing
A building's measured peak is 160 A per phase at 480 V. Load: 1.732 × 480 × 160 ÷ 1000 = 133 kVA. At a 0.8 power factor that is 106 kW, so a 150 kW / 188 kVA standby generator leaves room for motor starting and growth.
Three-phase transformer full-load amps
| kVA | 208 V | 480 V | 600 V |
|---|---|---|---|
| 15 | 41.6 A | 18.0 A | 14.4 A |
| 30 | 83.3 A | 36.1 A | 28.9 A |
| 45 | 124.9 A | 54.1 A | 43.3 A |
| 75 | 208.2 A | 90.2 A | 72.2 A |
| 112.5 | 312.3 A | 135.3 A | 108.3 A |
| 150 | 416.4 A | 180.4 A | 144.3 A |
| 225 | 624.5 A | 270.6 A | 216.5 A |
| 300 | 832.7 A | 360.8 A | 288.7 A |
Full-load current at nameplate kVA, computed from the formula above.
kVA vs kW
kW = kVA × power factor. A 100 kVA load at a 0.85 power factor delivers 85 kW of real power, but the conductors and transformer carry the full current for 100 kVA. Size equipment in kVA and estimate energy use in kW. Converting between them needs the power factor; see the 3 phase power calculator to work with both.
Sizing tips
- Use NEC Article 220 load calculations, not connected load, to estimate demand.
- Allow for future growth and for non-linear loads such as computers and LED drivers, which may need K-rated transformers.
- Check voltage drop on long secondary feeders with the 3 phase voltage drop calculator.
- Size the conductors from the overcurrent device and full-load current with the wire size calculator.