Transformer KVA Calculator
Calculate transformer kVA rating, primary and secondary full-load current (FLA), and overcurrent protection sizing for 1-phase and 3-phase systems.
Recommended Tools & Equipment
Tested hardware and components for high reliability
Transformer Full Load Current Formulas
Calculating transformer capacity and full load amperes (FLA) is essential for sizing primary and secondary feeder conductors, conduit sizes, and circuit breaker trip ratings according to NEC Article 450.
Three-Phase (3Φ) FLA
FLA = (kVA × 1000) / (V × √3)
Single-Phase (1Φ) FLA
FLA = (kVA × 1000) / V
Standard Commercial 3-Phase kVA Sizes
Common commercial dry-type distribution transformer ratings include: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA.
Frequently Asked Questions
Why do three-phase transformer calculations divide by sqrt(3)?
In a balanced three-phase system, line-to-line voltage is sqrt(3) (approx 1.732) times line-to-neutral voltage. Apparent power in VA is equal to V_line * I_line * sqrt(3).
How do you size the primary circuit breaker for a transformer?
NEC 450.3(B) allows primary-only overcurrent protection up to 125% of rated primary FLA (or rounded up to the next standard breaker size if under 9A). If primary and secondary protection is used, higher primary breaker ratings (up to 250%) are permitted.
What is transformer inrush current?
When an iron core transformer is energized, a momentary magnetizing inrush current occurs that can peak at 8 to 12 times rated full-load current for several cycles. Breakers must have suitable time-delay curves.