Transformer Sizing Troubleshooting

Transformer Sizing Problems Quick Answer

Use this when the selected kVA runs hot, trips protection, sags voltage under load, or the nameplate tap doesn't deliver expected secondary voltage.

1. Undersizing & Continuous Overload Trips

What it looks like: Transformer runs above nameplate temperature, insulation smell, breaker or fuse trips under normal production load, or voltage sags when multiple loads energize.

Likely causes: Connected load summed without diversity, future expansion ignored, or harmonic loads heating conductors without increasing apparent kVA on the bill. Dry-type units have strict continuous rating — no overload margin like some oil-filled designs.

Fixes: Re-sum loads with realistic diversity in the Transformer Sizing Calculator, add 15–25% growth margin where code allows, and verify with measured demand. Upgrade kVA or split loads across a second transformer.

2. Inrush Current Not Accounted For

What it looks like: Transformer and primary breaker tolerate steady-state load but trip when large motors or transformers downstream energize. Momentary voltage dip visible on scope or dimming lights.

Likely causes: Transformer inrush (first cycles after energizing) and motor inrush stacked in time. Breaker instantaneous trip curve intersects inrush. Sizing on RMS steady current only ignores peak magnetizing inrush — often 6–12× FLA for motors, several × for transformers.

Fixes: Use soft-start or reduced-voltage starting, time-delay fuses, or higher interrupting/overload coordination. The guide covers inrush duration; don't upsize kVA alone if the breaker is the limiter.

3. Tap Setting Errors — Wrong Secondary Voltage

What it looks like: Secondary reads 5–10% high or low vs expectation, equipment undervoltage trips, or boost/buck taps set for the wrong primary voltage after utility change.

Likely causes: Tap changer left on factory default while primary sits at upper or lower band of utility tolerance. Confusion between ±2×2.5% tap positions. Measuring secondary under no-load vs loaded conditions without accounting for regulation.

Fixes: Measure primary voltage at peak load, select tap to center secondary within equipment tolerance. Document tap position on the one-line. Re-run voltage drop checks after tap change.

4. Ambient Temperature Derating Overlooked

What it looks like: Transformer rated fine on paper but overheats in a hot mechanical room, rooftop pad, or crowded vault. Capacity seems adequate until summer.

Likely causes: Nameplate kVA is at 30 °C ambient (typical ANSI). Higher ambient or poor ventilation reduces allowable continuous load — often 4% per 5 °C above reference for dry-type units per manufacturer tables.

Fixes: Apply ambient derating from the Transformer Sizing Guide, improve ventilation, or specify a higher kVA / lower temperature rise unit. Measure ambient at the worst season, not commissioning day.