How to Specify Transformer Capacity (kVA)

Undersized transformers overheat and fail early; oversized transformers waste money on no-load loss. Specifying capacity correctly balances cost, reliability and efficiency.

Key Points

• 1. Sum connected load (kW) of all equipment the transformer will feed
• 2. Apply demand factor (0.6-0.8) — not everything runs at full load simultaneously
• 3. Convert kW to kVA using power factor (typical PF 0.8-0.9): kVA = kW / PF
• 4. Add future expansion margin (typically 20-25%)
• 5. Apply derating for ambient >40C and altitude >1000m per IEC 60076-2
• 6. Round up to standard rating (e.g. 100, 200, 400, 630, 800, 1000, 1600, 2500 kVA)
• 7. Verify against motor starting inrush (largest motor LRA / 6 rule of thumb)
• 8. Check harmonic derating (K-factor) if feeding non-linear loads (VFD, UPS, PCS)

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