Transformer Sizing Calculator (kVA & Amps)
Select the ideal kVA capacity and protection for any equipment load.
1System Specs
2Load Specs
3Protection Method
Recommended Transformer
Primary Protection (NEC 450.3)
Primary Current
Based on 45 kVA Transformer
Max Rating
125% of Primary (Next Size Up)
Popular transformer Configurations
Transformer kVA Formula
3Φ: kVA = (V × I × 1.732) / 1000
NEC 450.3 Overcurrent Protection
NEC Table 450.3(B) dictates the maximum rating for primary and secondary overcurrent protection (fuses/breakers) for transformers 1000V or less. Common values are 125% for primary only, or 250% primary / 125% secondary.
Frequently Asked Questions
How do I size the breaker for a transformer?
Standard practice is to size the primary breaker at 125% of the primary full load current (FLC). If that doesn't correspond to a standard size, you may go up to the next standard size (Note 1 to Table 450.3(B)).
What is the difference between kVA and kW?
kW is real power (working power). kVA is apparent power (total power). They are related by the Power Factor (PF): kW = kVA × PF.
Why uses 1.732 for three-phase?
1.732 is the square root of 3. In three-phase power, it accounts for the phase angle difference between the three legs.