Intermediate
Transformer Formulas
Transformer formulas — kVA to amps, turns ratio, impedance, fault current, and primary/secondary current calculations.
Formulas
I = kVA × 1000 / (V × √3)
Three-phase current from kVA (line current)
I = kVA × 1000 / V
Single-phase current from kVA
V₁/V₂ = N₁/N₂ = I₂/I₁
Turns ratio — voltage/turns/current relationship
Isc = FLA / (%Z / 100)
Secondary fault current from impedance
Variable Definitions
kVA=Transformer rating(kilovolt-amperes)
V₁/V₂=Primary/Secondary voltage(volts (V))
N₁/N₂=Primary/Secondary turns(count)
I₁/I₂=Primary/Secondary current(amperes (A))
%Z=Percent impedance(%)
Isc=Short circuit current(amperes (A))
Worked Example
500 kVA, 480V/208V 3Φ transformer, 5.75%Z. Find currents and fault current.
- Primary I = 500,000 / (480 × 1.732) = 601.4A
- Secondary I = 500,000 / (208 × 1.732) = 1,388.5A
- Isc = 1,388.5 / (5.75/100) = 24,148A
Answer: 601A primary, 1,389A secondary, 24,148A fault current
When to Use
- •Sizing transformer overcurrent protection (NEC 450.3)
- •Calculating secondary fault current for AIC ratings
- •Determining primary/secondary conductor sizes
- •Selecting transformers for new installations
Standards & References
- NEC 450.3 — Transformer Overcurrent Protection
- NEC 450.9 — Ventilation
- NEC 240.21(C) — Secondary Conductor Tap Rules