What is the formula for calculating 3-phase transformer full load amps?

FLA = (kVA × 1000) / (Line-to-Line Voltage × √3). For example, a 75 kVA 480V 3-phase transformer has a primary FLA of approximately 90.2 Amps.

NEC Commercial Electrical Engineering

Three-Phase Transformer KVA & Ampacity Sizer

Calculate transformer standard KVA ratings (15, 30, 45, 75, 112.5, 150, 225, 300, 500 kVA), 480V primary Full Load Amps (FLA), and 208Y/120V secondary distribution breaker sizing.

Electrical Load & Voltage Configuration

Total Connected Secondary Load (Watts or VA)

Primary to Secondary Voltage Configuration

Continuous Load Expansion Margin

KVA Sizing & Ampacity Takeoff

Standard Transformer Size

NEMA standard dry-type KVA rating

112.5 KVA

Primary Full Load Amps (480V)

FLA = (KVA * 1000) / (1.732 * 480)

135.3 Amps

Secondary Full Load Amps (208V)

FLA = (KVA * 1000) / (1.732 * 208)

312.3 Amps

NEC Article 450 requires overcurrent protection for transformers. A 112.5 KVA 480V unit typically uses a 150A primary breaker (125% rating) and 400A secondary main breaker.