A transformer nameplate says 500 kVA. The building loads add up to 480 kW. Looks like 20 kW of headroom. Breakers trip anyway.
The conversion
kW = kVA Ć Power Factor
A 500 kVA transformer at different power factors:
Power Factor Available kW Typical loads
āāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāā
1.00 500 kW resistive heaters only
0.95 475 kW LED lighting, electronics
0.90 450 kW mixed commercial
0.85 425 kW motors + mixed loads
0.80 400 kW motor-heavy industrial
At PF 0.85 (typical commercial building):
Available = 500 Ć 0.85 = 425 kW
Demand = 480 kW
Deficit = 55 kW ā overcurrent trip
Why this catches people
The nameplate shows one number: 500 kVA. Most people read it as "500 kW capacity." That is only true if every load is purely resistive with PF = 1.0.
Real buildings have motors (compressors, pumps, fans, elevators), VFDs, and electronic equipment. Combined PF typically runs 0.80ā0.90.
Building type Typical PF 500 kVA delivers
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Office (modern) 0.90ā0.95 450ā475 kW
Retail 0.85ā0.90 425ā450 kW
Hospital 0.80ā0.85 400ā425 kW
Industrial 0.75ā0.85 375ā425 kW
Data center 0.95ā0.99 475ā495 kW
Data centers are high PF because server power supplies include PFC circuits. Everything else is lower.
Sizing the transformer correctly
If building demand is 480 kW at PF 0.85:
Required kVA = kW / PF
Required kVA = 480 / 0.85
Required kVA = 565 kVA
Select a 750 kVA transformer (next standard size above 565). Not 500.
Standard transformer sizes (kVA):
75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500
The derating stack
Transformers lose additional capacity from:
Factor Typical loss
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Power factor < 1.0 5ā25%
Ambient temp > 40Ā°C reduce per ANSI
Harmonic loads (VFDs) 10ā15% K-factor
Altitude > 1000m derate per IEEE
A 500 kVA transformer in a hot mechanical room with VFD loads and PF 0.85 might effectively deliver 350 kW. That is 30% less than the nameplate suggests.
The quick check
1. Sum all load kW
2. Estimate combined PF (0.85 default for mixed commercial)
3. Divide kW by PF to get required kVA
4. Select transformer ā„ required kVA (next standard size up)
5. Check for harmonic, temperature, and altitude derating
For quick kVA to kW conversion at any power factor, use the kVA to kW calculator on CalcEngineer.
