Electrical
kVA / kW / Amps Reference
Single-phase and three-phase power formulas, typical power factor by load type, and a quick kVA-to-amps lookup at common voltages. Use alongside the kVA / kW / Amps Converter.
Last updated: June 2026
Terms
- UPSUninterruptible Power Supply
- A battery backup that keeps equipment running for a short time if mains power cuts out, giving generators time to start or equipment time to shut down safely.
- kVAKilovolt-Ampere
- A unit of electrical power, similar to kW but measured slightly differently — it includes power that's drawn but not fully put to use. Equipment ratings often use kVA because it's the more cautious, safer number to size for.
The Formulas
Three-phase: I = (kVA × 1000) ÷ (√3 × V) | kW = kVA × cos φ
Single-phase: I = (kVA × 1000) ÷ V
Where: I = current (A) | V = voltage — line-to-line for three-phase, line-to-neutral for single-phase | cos φ = power factor
kVA, kW and kVAR (reactive power) form the "power triangle": kVA² = kW² + kVAR².
Typical Power Factor by Load Type
| Load type | Typical cos φ | Notes |
|---|---|---|
| Purely resistive (heaters, incandescent lighting) | 1.0 | No reactive component — kVA = kW |
| Modern IT / server PSU loads | ~0.95 – 0.99 | Active PFC supplies are close to unity power factor |
| Motors (induction, general) | 0.8 – 0.9 | Lower at partial load; nameplate PF is at full load |
| UPS / generator nameplate rating | 0.8 – 0.9 | Often quoted at 0.8 PF (older designs) or 0.9 PF (modern double-conversion) |
| Fluorescent/LED lighting with ballast | 0.85 – 0.95 | Varies by ballast/driver design; check datasheet PF rating |
Use the nameplate power factor when known — these are planning-stage typical values, not substitutes for actual equipment data.
Three-Phase kVA → Amps Quick Lookup
At 400 V line-to-line, three-phase.
| kVA | Current (A) | kW at PF 0.9 |
|---|---|---|
| 10 kVA | 14.4 A | 9 kW |
| 25 kVA | 36.1 A | 22.5 kW |
| 50 kVA | 72.2 A | 45 kW |
| 100 kVA | 144.3 A | 90 kW |
| 250 kVA | 360.8 A | 225 kW |
| 500 kVA | 721.7 A | 450 kW |
Worked Example
Inputs: 100 kVA at power factor 0.9, 400 V three-phase.
Real power: kW = 100 × 0.9 = 90 kW
Current: I = (100 × 1000) ÷ (1.732 × 400) ≈ 144 A
Frequently Asked Questions
What's the difference between kVA, kW and kVAR?
kW is real power — the power that actually does useful work (heat, light, mechanical force). kVAR is reactive power — power that oscillates between source and load without doing useful work, caused by inductive/capacitive loads. kVA is apparent power — the vector sum (kVA² = kW² + kVAR²), and it's what generators, UPS and transformers are actually rated in, since their windings/conductors must carry the full apparent current regardless of power factor.
Why are UPS and generators rated in kVA, not kW?
A UPS or generator's output windings and conductors are limited by current, which is driven by apparent power (kVA), not real power (kW). A UPS rated 100 kVA at 0.9 PF delivers 90 kW of real power — running it at a load with worse power factor reduces the available real power even though the kVA capacity is unchanged.
Why does three-phase use √3?
In a three-phase system, the three line currents are 120° apart in phase. The √3 (≈1.732) factor accounts for this phase relationship between line-to-line voltage and the resulting current/power — it's a direct consequence of the vector geometry of a balanced three-phase system, not an arbitrary constant.
How does this relate to the Fuse & Breaker Sizing tool?
This converter gives you the design current (IB) from a known kVA/kW rating — that current is exactly the input the Fuse & Breaker Sizing tool needs to select an appropriately rated protective device.