Methods: Transformer sizing and loading
calc.transformer.size · Version 0.1.0
Method
Apparent power is √3 × V line-to-line × A ÷ 1,000 for three-phase service and V × A ÷ 1,000 for single-phase, or kW ÷ power factor. Loading is kVA ÷ rating, and headroom is rating − kVA.
Assumptions
- Three-phase voltage is line-to-line, and the load is balanced across phases.
- A load given in kW uses the power factor entered.
Sources
- NIST Special Publication 811, 2008, Section 8: apparent power (volt ampere) and its relation to voltage and current
- IEEE C57.91, Guide for Loading Mineral-Oil-Immersed Transformers, and IEEE C57.96 for dry-type transformers, named only; not quoted or applied, Overload duration, hot-spot temperature, and loss of life
Worked examples
| Example | Independent source | Tolerance | Last result |
|---|---|---|---|
| 600 A at 480 V three-phase on a 500 kVA transformer | calc/transformer-sizing spec, scenario "Three-phase from amps": √3 × 480 × 600 ÷ 1,000 | ±0.05, ±0.05 | Passed on Oct 31, 2026 |
| A 412 kW peak at 0.95 on a 500 kVA transformer | calc/transformer-sizing spec, scenario "Peak loading" | ±0.05, ±0.05, ±0.05 | Passed on Oct 31, 2026 |
| 200 A at 240 V single-phase | Hand computation: 240 × 200 ÷ 1,000 | ±0.05 | Passed on Oct 31, 2026 |
Known differences from reference engines
None recorded.
Cite this tool
Transformer sizing and loading, version 0.1.0. gridprims. https://gridprims.com/tools/calc/transformer/size/