A 15 kVA transformer’s primary current is calculated from the transformer’s 15,000 VA rating and its actual primary nameplate voltage. Secondary voltage is not part of the primary-current calculation.
For a 15 kVA transformer supplied at 480 V, the calculated primary current is:
- 18.0 A for a balanced three-phase transformer
- 31.25 A for a single-phase transformer
That current is a starting value for reviewing the primary-side feeder or branch-circuit load, conductor ampacity, primary overcurrent protection, raceway fill, voltage drop, disconnect ratings, and upstream equipment loading. It does not by itself select a conductor, fuse, breaker, or utility connection.
Primary Current Formula
Use the transformer kVA rating as volt-amperes:
\(\displaystyle 15\text{ kVA} = 15{,}000\text{ VA}\)
For a single-phase transformer:
\(\displaystyle I_{primary} = \frac{15{,}000}{V_{primary}}\)
For a balanced three-phase transformer:
\(\displaystyle I_{primary} = \frac{15{,}000}{\sqrt{3} \times V_{primary}}\)
Where:
- \(I_{primary}\) = primary current in amperes
- \(V_{primary}\) = actual primary line voltage from the transformer nameplate
- \(\sqrt{3}\) = 1.732 for balanced three-phase calculations
The phase arrangement changes the denominator. A 15 kVA rating at the same voltage produces a different primary current for single-phase and three-phase equipment.
15 kVA Primary Current Examples
| Transformer configuration | Primary voltage | Calculation | Primary current |
|---|---|---|---|
| Single phase | 120 V | (15{,}000 \div 120) | 125 A |
| Single phase | 240 V | (15{,}000 \div 240) | 62.5 A |
| Single phase | 480 V | (15{,}000 \div 480) | 31.25 A |
| Balanced three phase | 208 V | (15{,}000 \div \(1.732 \times 208\)) | 41.6 A |
| Balanced three phase | 240 V | (15{,}000 \div \(1.732 \times 240\)) | 36.1 A |
| Balanced three phase | 480 V | (15{,}000 \div \(1.732 \times 480\)) | 18.0 A |
A three-phase 15 kVA transformer with a 480 V primary therefore places approximately 18 A of full-load current on the primary line conductors. A single-phase 15 kVA transformer at 480 V places 31.25 A on its primary conductors.
Calculation Example
Assume a transformer nameplate shows:
- Transformer rating: 15 kVA
- Primary: 480 V
- Configuration: three phase
Convert the rating to VA:
\(\displaystyle 15\text{ kVA} \times 1{,}000 = 15{,}000\text{ VA}\)
Apply the balanced three-phase primary-current formula:
\(\displaystyle I_{primary} = \frac{15{,}000}{1.732 \times 480}\)
\(\displaystyle I_{primary} = 18.04\text{ A}\)
The calculated result is 18.0 A primary current.
For the same 15 kVA transformer rating on a 480 V single-phase primary:
\(\displaystyle I_{primary} = \frac{15{,}000}{480} = 31.25\text{ A}\)
Do not substitute the secondary voltage into either calculation. A transformer with a 480 V primary and 208Y/120 V secondary may supply substantially higher secondary current, but its primary current is still based on 480 V.
Primary-Side Field Review
Enter the actual primary voltage shown on the transformer nameplate and identify whether the transformer is single phase or balanced three phase. Confirm the calculated amperes against transformer documentation before reviewing the primary-side installation.
The calculated primary current can be carried into the field review for:
- Primary feeder or branch-circuit load calculations
- Preliminary conductor AWG or kcmil evaluation
- Ampacity review after applicable terminal rating, insulation temperature rating, correction factor, and adjustment factor considerations
- Primary disconnect and overcurrent-protection review
- Raceway fill and conduit layout planning
- Voltage-drop review for a long primary feeder
- Upstream panel, switchboard, generator, or utility service load review
A transformer’s calculated full-load current does not establish final conductor ampacity. Final conductor selection can depend on the installation configuration, current-carrying conductors, conductor insulation, terminal limitations, ambient temperature, bundling, raceway conditions, and requirements enforced by the AHJ. Primary protection also requires a separate review of the transformer documentation and applicable electrical requirements.
Use the Calculator
Use the Transformer Primary Current Calculator to enter the transformer kVA rating, the actual primary nameplate voltage, and the phase arrangement.
Related calculation tools: