Transformer Sizing & Full Load Current Calculator
Commercial electrical engineering, dry-type step-down transformer installation. Computes primary and secondary full-load currents, recommended primary/secondary fuse or circuit breaker ratings per NEC 450.3.
Step-by-Step Derivation ▼ Show Steps
Calculations adhere to standard electrical engineering physics and National Electrical Code (NFPA 70) regulations.
Engineering Principles & Calculations
This calculator uses the exact mathematical relationships between Transformer Rating (kVA: 15, 30, 45, 75, 112.5, 150, 300, 500 kVA), Primary Voltage (e.g. 480V), Secondary Voltage (e.g. 208V), Phase (1Φ / 3Φ) to determine Primary Full Load Amps, Secondary Full Load Amps, Max Overcurrent Protection Rating. In practical installations, factors such as ambient raceway temperature, harmonic distortion, duty cycle continuous operation, and conductor impedance must be accounted for by a certified electrician.
Computes primary and secondary full-load currents, recommended primary/secondary fuse or circuit breaker ratings per NEC 450.3.
Typical Application & Industry Guidelines
Commercial electrical engineering, dry-type step-down transformer installation.
Transformer Sizing & Full Load Current Calculator Benchmark Lookup Table
| 3-Phase kVA | 480V Primary Amps | 208Y/120V Secondary Amps | Suggested Main Secondary Breaker |
|---|---|---|---|
| 15 kVA | 18.0 A | 41.6 A | 60 A Breaker |
| 30 kVA | 36.1 A | 83.3 A | 100 A Breaker |
| 45 kVA | 54.1 A | 124.9 A | 150 A Breaker |
| 75 kVA | 90.2 A | 208.2 A | 250 A Breaker |
| 112.5 kVA | 135.3 A | 312.3 A | 400 A Breaker |
| 150 kVA | 180.4 A | 416.4 A | 500 A Breaker |
| 300 kVA | 360.8 A | 832.7 A | 1000 A Breaker |