What Is the Three-Phase Transformer kVA & FLA Sizing Calculator?
A three-phase transformer uses electromagnetic induction to step voltage levels up or down between three-phase electrical circuits while preserving power frequency.
How Does the Three-Phase Transformer kVA & FLA Sizing Calculator Work?
Solves secondary Full-Load Amps: I = (kVA × 1,000) ÷ (1.732 × V_secondary).
Solves primary Full-Load Amps: I = (kVA × 1,000) ÷ (1.732 × V_primary).
Rounds up overcurrent protection device (OCPD) ratings using the 125% rule specified in NEC Table 450.3(B).
Three-Phase Transformer kVA & FLA Sizing Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Standard three-phase apparent power formula where current is inversely proportional to square root of 3 (1.732) times line-to-line voltage.
How to Use the Three-Phase Transformer kVA & FLA Sizing Calculator
- Select transformer kVA rating.
- Enter primary and secondary line-to-line voltage.
- Read full-load amps and recommended breaker ratings for both sides.
Step-by-Step Example Calculation
75 kVA 480V Primary to 208V Secondary Transformer
Input Values:
Understanding Your Result
Provides primary and secondary FLA for sizing feed conductors, lugs, and main disconnect breakers.
Factors That Affect the Result
- Inrush current (typically 8x to 12x FLA during initial coil magnetization).
When Should You Use This Calculator?
- Commercial electrical build-outs, manufacturing plant feeder design, and utility service entrance planning.
Assumptions & Limitations
- Three-phase balanced load operation.
Frequently Asked Questions
Calculation Accuracy & Reference Note
National Electrical Code (NEC 450.3 / NFPA 70) compliant calculation.