Transformer KVA Sizing Calculator
Determine the minimum KVA rating required for a transformer given your load characteristics, supply voltage, and power factor. Results include a recommended standard KVA size with a safety margin.
kW
0–1
V
V
%
Formulas Used
Required KVA:
KVA = kW ÷ Power Factor (PF)
KVA with Safety Margin:
KVAdesign = KVA × (1 + Margin% ÷ 100)
Secondary Full-Load Current:
Single-Phase: I = (KVA × 1000) ÷ Vsecondary
Three-Phase: I = (KVA × 1000) ÷ (√3 × Vsecondary)
Turns Ratio:
a = Vprimary ÷ Vsecondary
Reactive Power:
kVAR = KVA × √(1 − PF²)
Assumptions & References
- Power factor defaults to 0.85, typical for mixed commercial/industrial loads (IEEE Std 141).
- A 25% safety margin is recommended per NEC 450.3 and general engineering practice to account for load growth and inrush currents.
- Standard KVA sizes follow ANSI C57.12.91 and common utility/distribution transformer ratings.
- Three-phase calculations use the line-to-line voltage and the √3 factor.
- Full-load current calculations assume balanced loads and rated voltage at the transformer terminals.
- Transformer efficiency losses are not included; add 2–5% for more precise sizing in high-efficiency applications.
- For motor-heavy loads, consider starting kVA (typically 6–8× FLA) when sizing the transformer.
- Reference: NEC Article 450, ANSI/IEEE C57 series, NEMA ST-20.