How it works
Single-phase: A = kVA × 1000 / V. Three-phase: A = kVA × 1000 / (√3 × V). Reverse: kVA = A × V / 1000 (1φ) or A × √3 × V / 1000 (3φ).
V is the line-to-line (phase-to-phase) voltage from the nameplate — always use line-to-line for three-phase full-load current. The √3 (≈1.732) factor appears because in a balanced three-phase system the three line currents are 120° apart, so the same kVA at the same line-to-line voltage is delivered at a lower per-line current than single-phase. Power factor does not appear anywhere: kVA is apparent power and already includes the reactive component. If your number is in kW instead, divide kW by the power factor to get kVA first, then convert. The downstream OCPD size uses 125% of the full-load current (NEC 450.3(B) primary-only protection), rounded up to the next standard rating in NEC 240.6(A).
Code references
- Transformer overcurrent protection NEC 2023, 450.3(B) Table — primary/secondary OCPD as a percent of full-load current
- Standard OCPD ratings NEC 2023, 240.6(A) — standard ampere ratings for fuses and breakers
- Continuous-load feeder sizing NEC 2023, 215.2(A)(1) — 125% of continuous load for feeder conductors
- Continuous-load service sizing NEC 2023, 230.42(A) — same 125% rule for service conductors
FAQ
Do I need the power factor to convert kVA to amps?
No. kVA is apparent power, which already accounts for power factor — it is the magnitude of the volt-amperes the equipment draws regardless of how much is real (kW) versus reactive. Power factor only matters when you convert between kW and kVA: divide kW by the power factor to get kVA, then convert kVA to amps here.
Why is my three-phase current about one-third of single-phase at the same kVA and voltage?
Three-phase delivers power over three line conductors at once, with the currents 120° out of phase, so the same kVA at the same line-to-line voltage spreads across the lines. The math: single-phase divides by V, three-phase divides by √3 × V. Since √3 ≈ 1.732, a three-phase line current is roughly 1/√3 (about 58%) of the single-phase current at the same kVA — not exactly one third, but much lower.
What is the difference between kVA and kW?
kVA is apparent power (volts × amps), kW is real power (the part that does work). They are related by the power factor: kW = kVA × PF. A purely resistive load (heater) has PF ≈ 1, so kW ≈ kVA. Motors, UPS units and HVAC draw reactive current, so their kVA is larger than their kW. For amps, always start from kVA — if you only have kW, convert with the power factor first.
How do I size the breaker for a 45 kVA transformer?
At 208 V three-phase, 45 kVA draws 124.9 A. For primary-only protection of a transformer over 9 A, NEC 450.3(B) allows the overcurrent device at up to 125% of full-load current: 124.9 × 1.25 = 156.1 A. Round up to the next standard rating in NEC 240.6(A), which is 175 A — so a 175 A breaker is the maximum. Secondary conductors and any secondary protection are sized separately. Always confirm with the transformer’s listing and a licensed electrician.
Should I use line-to-line or line-to-neutral voltage for three-phase?
Always line-to-line (phase-to-phase) for full-load current. Using the line-to-neutral voltage (line-to-line ÷ √3, e.g. 120 V on a 208 V system) makes the denominator smaller and overstates the current by about 73% — a common and costly mistake. The nameplate voltage of a three-phase transformer or motor is the line-to-line value.
This calculator is provided for estimation purposes. Transformer and feeder sizing depends on the equipment listing, conductor type, secondary protection and local amendments. Always verify against the current NEC edition with a licensed electrician or electrical engineer before sizing overcurrent protection or conductors.