A 15 kVA transformer’s secondary full-load current is determined by its secondary nameplate voltage and whether the secondary is single-phase or three-phase. Because the secondary is commonly the lower-voltage winding, secondary current is usually much higher than primary current.
For a 15 kVA transformer, enter the secondary voltage and phase arrangement in the Transformer Secondary Current Calculator. The result is the transformer’s calculated secondary full-load current, expressed in amperes.
That current becomes an input for feeder ampacity review, secondary overcurrent-protection planning, disconnect and panelboard selection, raceway layout, voltage-drop calculations, and termination verification. It is not, by itself, a final conductor size or breaker size.
Secondary Current Formula
Use the transformer kVA rating on the secondary-voltage basis.
For a single-phase transformer:
\(\displaystyle I_\text{secondary}=\frac{15{,}000}{V_\text{secondary}}\)
For a three-phase transformer using line-to-line secondary voltage:
\(\displaystyle I_\text{secondary}=\frac{15{,}000}{\sqrt{3}\times V_\text{secondary}}\)
Where:
- \(I_\text{secondary}\) = secondary line current in amperes
- 15,000 = 15 kVA converted to VA
- \(V_\text{secondary}\) = actual secondary nameplate voltage
- \(\sqrt{3}\) = 1.732 for a balanced three-phase system
The voltage basis must match the phase connection. A 208 V three-phase secondary normally uses 208 V line-to-line, while a 240/120 V single-phase secondary normally uses 240 V for the full secondary winding calculation. Using 120 V for a 240/120 V transformer would calculate the current of one 120 V leg under a particular loading arrangement, not the full 240 V secondary rating.
15 kVA Secondary Current Examples
| Transformer secondary | Calculation | Secondary current |
|---|---|---|
| 120 V single-phase | (15{,}000 \div 120) | 125 A |
| 208 V single-phase | (15{,}000 \div 208) | 72.1 A |
| 240 V single-phase | (15{,}000 \div 240) | 62.5 A |
| 480 V single-phase | (15{,}000 \div 480) | 31.3 A |
| 208 V three-phase | (15{,}000 \div \(1.732 \times 208\)) | 41.6 A |
| 240 V three-phase | (15{,}000 \div \(1.732 \times 240\)) | 36.1 A |
| 480 V three-phase | (15{,}000 \div \(1.732 \times 480\)) | 18.0 A |
A 15 kVA transformer with a 208Y/120 V three-phase secondary therefore has a calculated full-load secondary line current of approximately 41.6 A. A 15 kVA single-phase transformer with a 240/120 V secondary has a full-load current of 62.5 A at 240 V.
The difference is caused by both voltage and system configuration. Three-phase power distributes the same apparent power across three line conductors, using the \(\sqrt{3}\) relationship between line voltage and line current.
Single-Phase and Three-Phase Voltage Basis
Transformer labels can show several voltage values. Select the value that represents the circuit basis being calculated.
| Nameplate example | Correct voltage input | Calculation use |
|---|---|---|
| 240/120 V, 1-phase | 240 V | Full secondary winding or 240 V feeder current |
| 120/240 V, 1-phase, 3-wire | 240 V | Total transformer secondary kVA basis |
| 208Y/120 V, 3-phase, 4-wire | 208 V | Three-phase line-to-line secondary current |
| 480Y/277 V, 3-phase, 4-wire | 480 V | Three-phase line-to-line secondary current |
| 240 V delta, 3-phase | 240 V | Three-phase line-to-line secondary current |
For a 208Y/120 V secondary, 120 V is the line-to-neutral voltage available to loads, but the 15 kVA three-phase current calculation uses 208 V line-to-line. For a 480Y/277 V secondary, use 480 V for three-phase secondary line current, not 277 V.
A separately calculated line-to-neutral load can still be necessary when reviewing phase balance, neutral loading, branch circuits, or panelboard loading. That is a load-distribution calculation, separate from the transformer’s three-phase full-load secondary current.
Using Secondary Current for Feeder Review
Calculated secondary current establishes the electrical loading basis for the conductors leaving the transformer. A 15 kVA transformer at 240 V single-phase produces 62.5 A, while the same transformer at 208 V three-phase produces 41.6 A. The equipment and installation review follows from those different current levels.
Typical follow-on checks include:
- Secondary feeder conductor ampacity, expressed in AWG or kcmil, based on the applicable conductor insulation temperature rating and terminal rating
- Adjustment factor for the number of current-carrying conductors in a raceway or cable
- Ambient-temperature correction factor where installation conditions require it
- Secondary overcurrent-protection arrangement and disconnect rating
- Raceway fill and physical conductor routing
- Transformer secondary termination limitations
- Available fault current and interrupting-rating coordination
- Grounding and bonding of the derived system where applicable
- Voltage-drop review for long secondary feeders
- Load characteristics, including continuous loads, nonlinear loads, motor loads, and unbalanced line-to-neutral loads
For example, a calculated 62.5 A secondary current does not automatically mean that any conductor with a nominal 62.5 A ampacity is acceptable. The final ampacity can change with conductor material, insulation temperature rating, terminal limitations, ambient conditions, conductor count, installation method, and the adopted electrical code.
Motor loads require a further review because transformer loading and feeder design may need to account for motor full-load current, starting conditions, simultaneous operation, and the protective-device arrangement. A transformer kVA rating does not independently establish the motor branch-circuit conductor or overcurrent-protection requirements.
Voltage Drop and Raceway Layout
Secondary current is directly used in voltage-drop calculations. For a fixed conductor size and length, higher current produces greater voltage drop. A 120 V secondary carrying 125 A can require substantially more attention to conductor impedance and feeder length than a 480 V secondary carrying 18.0 A for the same 15 kVA transformer rating.
Voltage drop is normally evaluated using the actual:
- Secondary conductor length
- Conductor material
- AWG or kcmil size
- Raceway or cable configuration
- System voltage
- Calculated load current
- Power factor and AC impedance where applicable
The same current also affects practical raceway work. Larger conductors can increase raceway fill, bending space, pulling tension, box dimensions, lug compatibility, and termination clearance. Confirm the transformer’s secondary lugs accept the selected conductor type and size; a conductor that meets ampacity requirements may still be incompatible with the installed terminal.
Calculator Inputs and Related Tools
Use the Transformer Secondary Current Calculator when the known values are transformer kVA, secondary voltage, and phase configuration.
Use the Transformer Current Calculator to evaluate transformer current from the corresponding kVA, voltage, and phase inputs.
Use the Transformer kVA Calculator when voltage and current are known and the transformer apparent-power requirement must be calculated.
Field Verification
Verify the transformer nameplate before using any calculated value. Confirm the kVA rating, primary and secondary voltage designation, phase configuration, winding connection, and whether the selected voltage is line-to-line or line-to-neutral.
The secondary current calculation provides the electrical full-load current from transformer kVA and secondary voltage. Conductor ampacity, overcurrent protection, terminal ratings, available fault current, grounding and bonding, and the adopted code requirements require a separate design and field review by the responsible electrical professional and AHJ.