ELECTRICAL TOPIC
Transformer ratings, full-load current and measured loading
A transformer nameplate, a calculated full-load current and a measured loading percentage describe different things. Follow them in that order.
Rating and operating load are different
100 kVA at 415 V → 139.12 A rated; 415 V × 100 A × √3 ÷ 1000 → 71.88 kVA operating
The second number is 71.88% of a 100 kVA nameplate in a balanced sinusoidal example. It is not a thermal loading allowance or a forecast of how long the transformer may run there.
1. Read kVA and the voltage side together
Nameplate kVA describes apparent-power duty at stated conditions. Load power factor changes the associated kW, which is why kVA cannot be replaced by a fixed kW value. Note the primary and secondary voltages and whether each is a single-phase terminal voltage or a three-phase line-to-line voltage.
2. Use measurements from one side and one moment
To estimate operating kVA, use voltage and current from the same transformer side and operating condition. Dividing by nameplate kVA gives a percentage for that snapshot. It does not measure winding temperature, losses, permissible overload duration or spare design capacity.
3. Keep fault and inrush questions outside the rating conversion
An apparent-power rating does not provide prospective fault current by itself. A preliminary fault estimate also needs appropriate impedance and system assumptions; energisation inrush is a different transient. Equipment selection and protection require manufacturer information and a separate engineering review.