3150 kVA Transformer Full Load Current: Calculation and What It Means
Full load current is the number everything downstream is sized from — cable, busbar, breaker and protection settings. Get it wrong and you either pay for copper you do not need or you undersize and run hot.
The formula
For a three-phase transformer: I = P ÷ (√3 × V), where P is the rating in volt-amperes and V is the line-to-line voltage. √3 is 1.732. Nothing else is needed — you can check any figure below on a calculator.
3150 kVA at common voltages
| Secondary / primary voltage | Full load current |
|---|---|
| 400 V | 4,547 A |
| 415 V | 4,382 A |
| 11 kV | 165 A |
| 22 kV | 82.7 A |
| 33 kV | 55.1 A |
The 400 V and 415 V figures are the secondary side — that is the number the LV cable and the main breaker are sized from, and it is large enough that cable selection usually goes to multiple runs or a busbar rather than a single large cable. The 11 kV, 22 kV and 33 kV figures are the primary side, and they are what the HV protection sees.
What the number drives
- LV cable or busbar rating — the secondary current at 400 V is over four thousand amps.
- Main breaker frame size and trip setting.
- Current transformer ratios for metering and protection.
- Busbar and termination ratings inside the panel.
- Voltage drop over the run to the main board.
Two things people get wrong
First, quoting the rating without stating the voltage. 3150 kVA is 165 A on the primary and 4,382 A on the secondary — the same transformer, two numbers that differ by a factor of twenty-six. Always say which side you mean.
Second, treating full load current as the design current for the cable. Cable sizing also takes the installation method, grouping, ambient temperature and permissible voltage drop into account. Full load current is the starting point, not the answer.
If you are sizing a 3150 kVA unit now, send the voltages on both sides and the fault level, and we will confirm the configuration and the drawings.
Send us the specification
Rating, primary and secondary voltage, frequency, quantity and the application. We answer within one working day.
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