Transformer Sizing Calculator

Size a 3-phase transformer for a given continuous and non-continuous load following NEC Article 450 and NEC 215.2.

Related Calculators

How to Size a 3-Phase Transformer

What Is Transformer Sizing?

Transformer sizing determines the appropriate standard kVA rating for a 3-phase transformer based on the connected loads. NEC Article 450 requires that transformers be sized to handle the total connected load, with continuous loads (those operating 3 hours or more) derated by 125% per NEC 215.2(A)(1). Proper sizing prevents overheating, nuisance tripping, and premature transformer failure.

NEC 450 and 215.2 — The Rules

Derated Load: (Continuous Load × 1.25) + Non-Continuous Load

With Future Expansion: Derated Load × (1 + Expansion Factor)

Recommended kVA: Smallest standard size ≥ Total Load

Full-Load Current: kVA × 1000 / (√3 × Secondary Voltage)

Standard 3-phase transformer sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA.

Worked Example: 50 kVA Continuous + 30 kVA Non-Continuous

Derated load: (50 × 1.25) + 30 = 92.5 kVA
With 15% future expansion: 92.5 × 1.15 = 106.4 kVA
Next standard size: 112.5 kVA
Full-load current at 480V: 112.5 × 1000 / (√3 × 480) = 135.3 A

Practical Tips

Always round UP to the next standard kVA size — never size below the calculated load.
Continuous loads are defined as those operating for 3 hours or more per NEC 100.
Common secondary voltages: 208V (commercial), 480V (industrial), 240V (multi-family residential).
A 15-25% future expansion factor is typical for new installations.
For single-phase transformers, use kVA × 1000 / V (no √3 factor).

Code References

NEC 450, NEC 215.2(A)(1), NEC 100 (Definitions)

Frequently Asked Questions

Why do continuous loads get multiplied by 1.25?

NEC 215.2(A)(1) requires that continuous loads be rated at 125% of the continuous load. This provides a safety margin to prevent overheating of transformers and conductors that operate for 3 hours or more continuously.

What are standard 3-phase transformer sizes?

Standard 3-phase transformer kVA ratings are: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, and 2500 kVA. Always round up to the next standard size.

How do I calculate full-load current for a 3-phase transformer?

Full-load current (FLA) = kVA × 1000 / (√3 × Voltage). For example, a 112.5 kVA transformer at 480V: FLA = 112.5 × 1000 / (1.732 × 480) = 135.3 A.

Should I include a future expansion factor?

Yes, for most new installations. A 10-25% expansion factor accounts for future load growth and prevents having to replace the transformer prematurely. Industrial installations often use 25%, while residential uses 10-15%.