A transformer kVA-to-amps calculation converts a transformer’s apparent-power rating into the current carried at a specified voltage. The result is used when reviewing feeder and secondary-conductor loading, selecting overcurrent protection, checking equipment ratings, estimating raceway space, and comparing current on the primary and secondary sides of a transformer.
The calculator uses Transformer kVA, Voltage, and Phase to calculate transformer current. Enter the transformer nameplate kVA rating, the voltage for the side being evaluated, and whether the system is single phase or balanced three phase.
A transformer carries substantially more current on its lower-voltage side. The kVA rating remains the same across the transformer in an ideal calculation, but voltage and current move in opposite directions.
Transformer Current Formula
Transformer current is calculated from apparent power:
\(I = \frac{kVA \times 1000}{m \times V}\)
Where:
- (I) = transformer current in amperes
- (kVA) = transformer rating in kilovolt-amperes
- (1000) converts kVA to volt-amperes
- (V) = voltage entered for the transformer side being calculated
- (m) = phase multiplier
For single-phase transformers:
\(I = \frac{kVA \times 1000}{V}\)
For balanced three-phase transformers:
\(I = \frac{kVA \times 1000}{\sqrt{3} \times V}\)
The three-phase multiplier, \(\sqrt{3}\), applies when the entered voltage is the line-to-line system voltage. The result is line current.
Primary and Secondary Current
Use the same transformer kVA rating with the primary-side voltage to calculate primary current. Then use the secondary-side voltage to calculate secondary current.
For example, a 75 kVA transformer may be supplied from a 480 V three-phase system and serve a 208 V three-phase secondary system. The transformer rating does not change, but the secondary conductors carry more current because the secondary voltage is lower.
| Transformer Side | Transformer kVA | Voltage | Phase | Calculated Current |
|---|---|---|---|---|
| Primary | 75 kVA | 480 V | Three Phase | 90.2 A |
| Secondary | 75 kVA | 208 V | Three Phase | 208.2 A |
The lower-voltage secondary current is more than twice the primary current. This is why primary feeders, secondary feeders, disconnects, and overcurrent protection cannot be selected from transformer kVA alone without identifying the voltage and phase on the side under review.
Calculation Example
Calculate the full-load current for a 75 kVA, 480 V, Three Phase transformer primary.
\(\displaystyle I = \frac{75 \times 1000}{\sqrt{3} \times 480}\)
\(\displaystyle I = \frac{75{,}000}{831.38}\)
\(\displaystyle I = 90.2\text{ A}\)
The calculated primary current is 90.2 A.
Calculate the same 75 kVA transformer at a 208 V, Three Phase secondary:
\(\displaystyle I = \frac{75 \times 1000}{\sqrt{3} \times 208}\)
\(\displaystyle I = \frac{75{,}000}{360.27}\)
\(\displaystyle I = 208.2\text{ A}\)
The calculated secondary current is 208.2 A.
For a single-phase example, a 25 kVA transformer with a 240 V secondary produces:
\(\displaystyle I = \frac{25 \times 1000}{240} = 104.2\text{ A}\)
The calculated single-phase secondary current is 104.2 A.
Using Transformer Amps in Design
The calculated amps establish a starting current value for several electrical design and installation checks:
- Feeder and secondary conductor review: Compare calculated current with conductor ampacity after applying the applicable conductor conditions, terminal rating, insulation temperature rating, correction factor, and adjustment factor.
- Overcurrent protection review: Compare transformer current with the protection approach for the transformer primary and secondary. Protection sizing requires a separate review of the installation, equipment listing, transformer type, and applicable electrical requirements.
- Raceway fill and layout: Higher secondary current can require larger AWG or kcmil conductors, which affects raceway fill, conductor pulling conditions, bend radius, pull-box layout, and conduit routing.
- Voltage-drop review: Use the calculated current with conductor length, conductor impedance, and circuit configuration to evaluate voltage drop. Long low-voltage secondary runs often require closer attention because they carry higher current.
- Load and equipment review: Transformer current can be compared with panelboard bus ratings, disconnect ratings, switchboard ratings, breaker frame sizes, and downstream load calculations.
The kVA-to-amps result represents rated transformer current at the entered voltage. It does not determine final conductor size, breaker size, fuse size, or raceway size by itself.
Field Verification
Verify the transformer nameplate before using the calculated current for an installation decision. Confirm:
- The transformer kVA rating and winding voltages
- Whether the winding and load are single phase or balanced three phase
- Whether the entered three-phase voltage is line-to-line voltage
- The actual primary and secondary configuration
- Nameplate taps, impedance, temperature-rise information, and equipment ratings where applicable
- Connected-load characteristics, including motors, nonlinear loads, and continuous loading conditions
The formula does not include transformer losses, magnetizing current, inrush current, harmonic effects, voltage regulation, conductor ampacity, or protection requirements. Those items require separate engineering, manufacturer, installation, and AHJ review.
Related Transformer Calculators
- Transformer Current Calculator
- Transformer kVA Calculator
- Transformer Primary Current Calculator
- Transformer Secondary Current Calculator