A 15 kVA transformer is rated at 15,000 VA, but it does not have one fixed amp value. The calculated current depends on the transformer winding voltage and whether the applicable circuit is single-phase or balanced three-phase.
That current is the starting value for reviewing transformer primary and secondary conductors, feeder ampacity, raceway fill, disconnects, overcurrent protection, panel loading, voltage drop, and equipment terminations. A lower-voltage secondary carries substantially more current than a higher-voltage primary for the same 15 kVA rating.
Transformer Current Formula
Enter the 15 kVA rating, select the applicable voltage, and select the phase arrangement in the transformer current calculator.
For a single-phase transformer:
\(\displaystyle I = \frac{15{,}000}{V}\)
For a balanced three-phase transformer:
\(\displaystyle I = \frac{15{,}000}{\sqrt{3} \times V_{LL}}\)
Where:
- (I) = transformer line current in amperes
- (15{,}000) = 15 kVA converted to VA
- (V) = circuit voltage for the single-phase calculation
- \(V_{LL}\) = line-to-line voltage for the three-phase calculation
- \(\sqrt{3}\) = approximately 1.732
For three-phase equipment, use the line-to-line nameplate voltage in the formula. The 1.732 multiplier is part of the three-phase relationship and must not be applied to a single-phase calculation.
15 kVA Amp Chart
| Transformer rating | Voltage | Phase | Calculation | Current |
|---|---|---|---|---|
| 15 kVA | 120V | Single-phase | (15{,}000 \div 120) | 125.0 A |
| 15 kVA | 208V | Single-phase | (15{,}000 \div 208) | 72.1 A |
| 15 kVA | 240V | Single-phase | (15{,}000 \div 240) | 62.5 A |
| 15 kVA | 480V | Single-phase | (15{,}000 \div 480) | 31.3 A |
| 15 kVA | 208V | Three-phase | (15{,}000 \div \(1.732 \times 208\)) | 41.6 A |
| 15 kVA | 240V | Three-phase | (15{,}000 \div \(1.732 \times 240\)) | 36.1 A |
| 15 kVA | 480V | Three-phase | (15{,}000 \div \(1.732 \times 480\)) | 18.0 A |
For example, a 15 kVA, 480V three-phase winding has a calculated full-load line current of approximately 18.0 amps. A 15 kVA, 208V three-phase winding has approximately 41.6 amps of line current. The voltage reduction increases current because the same apparent power is delivered at a lower voltage.
Primary and Secondary Current
The transformer kVA rating stays the same across the transformer, while the voltage and current change between windings.
For a typical 15 kVA, 480V-to-208V three-phase transformer:
\(\displaystyle I_{primary} = \frac{15{,}000}{1.732 \times 480} = 18.0\text{ A}\)
\(\displaystyle I_{secondary} = \frac{15{,}000}{1.732 \times 208} = 41.6\text{ A}\)
The primary-side calculation supports review of the supply feeder and primary disconnect. The secondary-side calculation supports review of the secondary feeder, panelboard rating, conductor ampacity, raceway size, and voltage-drop exposure. Secondary conductors generally require more cross-sectional area because their current is higher.
Use the dedicated Transformer Primary Current Calculator when evaluating the supply winding, and use the Transformer Secondary Current Calculator for the load-side winding.
Calculation Example
A 15 kVA transformer is connected to a 240V single-phase secondary.
1. Convert 15 kVA to VA:
\(\displaystyle 15 \text{ kVA} \times 1{,}000 = 15{,}000 \text{ VA}\)
2. Divide VA by secondary voltage:
\(\displaystyle 15{,}000 \div 240 = 62.5 \text{ A}\)
The calculated secondary current is 62.5 amps.
That 62.5A value is not automatically a conductor or breaker size. Use it to begin the installation review: identify the actual load characteristics, determine the required conductor ampacity, account for applicable correction and adjustment factors, confirm the terminal rating and insulation temperature rating, and verify equipment listings and AHJ requirements.
Field Verification
Use the Transformer Current Calculator with the transformer’s actual nameplate voltage and phase configuration. A 15 kVA value alone does not establish the correct primary or secondary current.
Confirm the final installation against the transformer nameplate, winding connection diagram, available fault current, conductor termination limits, overcurrent protection arrangement, and the applicable electrical code requirements. For transformers supplying motor loads, also evaluate the motor branch-circuit and feeder calculation separately; transformer full-load current does not replace motor circuit calculations.
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