Transformer Bank Reconnection for a Voltage Complaint
A customer on a 240/120 V single-phase service complains that motors run hot and lights are dim. The service is fed from a 50 kVA overhead transformer connected to a 12.47 kV primary. You measure 118 V on one leg, 122 V on the other, and 240 V line-to-line. The customer has added a large 240 V air compressor and a 120 V space heater on the same leg as the compressor's control circuit. The transformer nameplate shows dual 120 V secondaries. The primary cutout is a 15 K. Ambient is 35°C.
1. Evaluate whether the voltage readings indicate a transformer problem or a load/connection problem, and explain your reasoning.
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The readings - 118 V and 122 V leg-to-neutral, 240 V line-to-line - are within normal tolerance for a 120/240 V service (roughly ±5%). They do not indicate a failed transformer or an open neutral; an open neutral would typically show a much larger split and line-to-line would still be 240 V but the legs would diverge sharply under load.
The complaint is more likely load-related:
- A large 240 V compressor draws balanced current, but its 120 V control circuit and the space heater on the same leg create an unbalanced 120 V load, which shifts the neutral point slightly and increases voltage drop on the loaded leg.
- Motor heating and dim lights can result from voltage drop in the service conductors under starting current, not from the transformer itself.
So the diagnosis is a load/connection issue: check service conductor size, terminations, and the customer's panel for loose connections and unbalanced loading, rather than replacing the transformer.
2. The customer insists the transformer is undersized. How would you assess transformer loading and what would you recommend?
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Assess loading:
- Measure actual load current on each leg with a clamp meter under normal and peak conditions; convert to kVA.
- Compare against the 50 kVA rating; a 50 kVA transformer at 240 V single-phase delivers about 208 A total (roughly 104 A per leg at balanced load).
- Check ambient derating: at 35°C the transformer is near its rated ambient; sustained loading above nameplate in high ambient accelerates insulation aging.
- Check for motor starting voltage dip; a large compressor can cause a dip that dims lights even if average loading is fine.
Recommendation: if measured peak kVA approaches or exceeds 50 kVA, or voltage dip on starting is excessive, upsize the transformer or require the customer to reduce/redistribute load. Also verify the 15 K cutout is appropriate for the transformer's inrush and continuous rating; do not simply upsize the fuse to mask a loading problem. Correct the unbalanced 120 V loading by redistributing circuits.