Three-Phase Transformer Calculator
Enter kVA, voltages and connections to get primary and secondary amps, turns ratio and neutral size.
Step-by-step
- Enter valid values to begin.
Infinite-bus fault current at the secondary terminals. Neutral multipliers are design allowances for triplen harmonics (NEC 220.61(C)(2) forbids reducing the neutral for nonlinear loads), not code-mandated numbers; verify with measured harmonic currents.
Three-Phase Transformer Sizing: FLA, Turns Ratio and the Neutral
Core Engineering Principles
A three-phase transformer is three single-phase windings sharing one core. What you see on the nameplate, kVA and line-to-line voltages, tells you almost everything, but the connection decides how those voltages translate into winding turns. A delta winding sees the full line voltage across each coil. A wye winding sees the line voltage divided by 1.732. So a 13.8 kV delta to 480Y/277 V transformer has a line-voltage ratio of 28.75 to 1, but the turns ratio on the winding is 49.8 to 1. Mix those two up and the numbers on your drawings will never match the nameplate.
Full-load amps are what you size breakers, cables and bus from: kVA times 1000 divided by 1.732 times the line voltage, on each side. The secondary current is large and the primary current is small, because the same power flows through both. And the neutral deserves its own paragraph. In a four-wire wye secondary, the neutral carries the unbalance of single-phase loads, and on a floor of computers and LED drivers it also carries the triplen harmonics, which add up in the neutral instead of canceling. I’ve seen neutrals running hotter than the phases in a transformer that was nominally half loaded.
Neutral ≥ 100% of the phase for linear loads, up to 173–200% for nonlinear loads
NEC & Standard References
NEC 450.3(B) sets primary overcurrent protection at 125% of rated primary current (next standard size permitted) for a transformer of 1000 V or less with primary-only protection, and up to 250% with secondary protection also in place. NEC 220.61 sizes the feeder neutral on the maximum unbalance and says the neutral of nonlinear loads cannot be reduced, and 310.15(E) counts that neutral as a current-carrying conductor for derating. IEEE C57.12.00 covers standard ratings and the 30° shift for delta-wye banks, and C57.110 describes derating for harmonic currents. For a separately derived system, NEC 250.30 requires a bonding jumper and grounding electrode conductor at the transformer or first disconnect.1. Primary FLA = 500,000 / (1.732 × 13,800) = 20.9 A.
2. Secondary FLA = 500,000 / (1.732 × 480) = 601.4 A.
3. Line ratio = 13,800 / 480 = 28.75 : 1. Winding turns ratio = 13,800 / 277 = 49.8 : 1.
4. Fault at secondary terminals = 601.4 / 0.0575 = 10.5 kA (infinite bus).
5. Neutral: for nonlinear loads size the neutral at 200% of the phase, which is 1,203 A, usually done with a double-size or parallel neutral.
- Inrush on energizing. A transformer can pull 8 to 12 times full-load current for a few cycles. Set the primary protection to ride through it.
- Heat from harmonics. Nonlinear loads cause eddy-current heating. A K-rated unit or an oversized standard unit is the cure.
- Check the vector group. Paralleling two transformers with a different phase shift, such as Dyn1 and Dyn11, will wreck both.
- Don’t lose the neutral. A broken neutral on a wye secondary lets phase voltages float, and the lightly loaded phase can go to 150% of nominal.
- Leave clearances. Ventilation spacing in the manual isn’t optional; a boxed-in dry-type runs hot.