LoadCairn / Power conversion guide
How to convert kW, kVA and amps
Equipment specs come in watts, panels are sized in amps, and UPS and generators are rated in kVA. Converting between them is simple once you know which voltage to use and where power factor fits. This guide covers the formulas for single-phase and three-phase power, with worked examples from real data center circuits.
01 / Power factor
kW vs kVA: power factor
kVA (apparent power) is voltage times current. kW (real power) is the part that does work. Power factor (PF) links them:
kW = kVA × PF
PF is between 0 and 1, so kW is never more than kVA. Modern IT power supplies with power-factor correction typically run around 0.95 or higher at normal load. Motors and older equipment are often lower. When you don't know the PF, use the equipment spec sheet rather than guessing.
02 / Single phase
Single-phase formulas
kVA = V × A ÷ 1,000
kW = V × A × PF ÷ 1,000
Amps = kW × 1,000 ÷ (V × PF)
03 / Three phase
Three-phase formulas
Use line-to-line voltage (e.g. 208 V or 480 V):
kVA = √3 × V × A ÷ 1,000
kW = √3 × V × A × PF ÷ 1,000
Amps = kW × 1,000 ÷ (√3 × V × PF)
(√3 ≈ 1.732, which comes from the 120° offset between phases.)
04 / Worked examples
Worked examples
1. A 100 kW load on a 480 V three-phase feed at 0.9 PF:
kVA = 100 ÷ 0.9 = 111.1 kVA
Amps = 100,000 ÷ (1.732 × 480 × 0.9) = 133.6 A per phase2. A 5 kW rack on a 208 V single-phase circuit at 0.95 PF:
kVA = 5 ÷ 0.95 = 5.26 kVA
Amps = 5,000 ÷ (208 × 0.95) = 25.3 A3. Capacity of a 208 V, 30 A three-phase PDU:
kVA = 1.732 × 208 × 30 ÷ 1,000 = 10.8 kVA
At 0.9 PF that's 9.7 kW, or about 7.8 kW if you plan to 80% of the rating.
05 / Review checklist
Common mistakes
- Using √3 on a single-phase circuit. In example 2, that would give 14.6 A instead of 25.3 A, undersizing the circuit by more than 40%.
- Using line-to-neutral voltage with √3. For 100 kW at 0.9 PF on a 208Y/120 V system, plugging 120 V into the three-phase formula gives 534.6 A, nearly double the correct 308.6 A. Either use line-to-line voltage with √3, or use 3 × line-to-neutral voltage without it.
- Treating kW and kVA as the same. A 100 kVA UPS is not a 100 kW UPS unless its output power factor is 1.0. Check the kW rating on the spec sheet.
- Sizing to 100% of the breaker. The NEC generally requires circuits serving continuous loads to be sized at 125% of the load, which is where the common "plan to 80%" rule comes from. The 25.3 A rack in example 2 needs at least 31.6 A of capacity, so a 30 A circuit is too small. Confirm requirements with the code edition your jurisdiction has adopted and your AHJ.
06 / Design boundary
When to get a detailed design
These conversions are for planning and sanity checks. Conductor sizing, overcurrent protection, voltage drop and selective coordination need a licensed electrical engineer and the applicable code.
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