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Evgenii Konkin
Evgenii Konkin

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Quick Tip: The 3 Multiplier That Undersizes Three-Phase Generators by 73%

#electrical #engineering #quicktip #power

Single-phase and three-phase kVA formulas differ by one multiplier: √3 (1.732). Use the wrong one and you undersize by 42% — or oversize by 73%.

The two formulas 

Single-phase:
  kVA = V × I / 1,000
  kW  = V × I × PF / 1,000

Three-phase:
  kVA = V × I × 1.732 / 1,000
  kW  = V × I × 1.732 × PF / 1,000
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The ONLY difference is × 1.732 for three-phase.

The mistake 

200A, 480V, three-phase panel:

Wrong (single-phase formula):
  kVA = 480 × 200 / 1,000 = 96 kVA
  kW  = 96 × 0.85 = 81.6 kW

Correct (three-phase formula):
  kVA = 480 × 200 × 1.732 / 1,000 = 166.3 kVA
  kW  = 166.3 × 0.85 = 141.4 kW

Error: 73% undersized
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A 100 kW generator ordered. Actual load: 141 kW. Overload shutdown on first test.

How to know which formula 

2 poles on breaker → single-phase
3 poles on breaker → three-phase (× 1.732)

Nameplate says "1PH" → single-phase
Nameplate says "3PH" → three-phase

Voltage 208Y/120, 480Y/277, 600Y/347 → three-phase
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Quick reference: 480V three-phase at PF 0.85 

Amps    kVA      kW
─────────────────────
100A     83 kVA    71 kW
200A    166 kVA   141 kW
400A    333 kVA   283 kW
600A    499 kVA   424 kW
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Quick reference: 208V three-phase at PF 0.85 

Amps    kVA      kW
─────────────────────
100A     36 kVA    31 kW
200A     72 kVA    61 kW
400A    144 kVA   123 kW
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The rule

Before any kVA or kW calculation, check the phase. One multiplier changes the answer by 73%. For generators, that is the difference between a system that runs and one that trips on overload.

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