A 1500 kVA transformer can deliver several thousand amperes on its low-voltage secondary. At 480 V, a three-phase 1500 kVA transformer produces approximately 1,804 A of full-load secondary current:
\(\displaystyle I_\text{secondary}=\frac{1{,}500{,}000}{\sqrt{3}\times480}=1{,}804\text{ A}\)
That current is the starting value for reviewing the secondary bus, feeder conductors, raceway arrangement, disconnecting means, overcurrent protection, and available fault current. It is not a conductor size, breaker size, or busway rating by itself.
Use the Transformer Secondary Current Calculator when the secondary nameplate voltage and transformer phase configuration are known.
Secondary Current Formula
Transformer secondary current is calculated from apparent power, phase multiplier, and secondary voltage:
\(\displaystyle I_\text{secondary}=\frac{\text{kVA}\times1{,}000}{m\times V_\text{secondary}}\)
For a 1500 kVA transformer:
\(\displaystyle I_\text{secondary}=\frac{1{,}500{,}000}{m\times V_\text{secondary}}\)
Where:
- \(I_\text{secondary}\) is the transformer low-voltage full-load current in amperes
- 1,500,000 is the transformer rating in volt-amperes
- \(V_\text{secondary}\) is the actual secondary nameplate voltage
- (m) is the phase multiplier used by the calculation
For a three-phase transformer, the current calculation uses \(\sqrt{3}\). For a single-phase transformer, the calculation uses 1.
The kVA rating remains the same on both sides of the transformer, while voltage and current move in opposite directions. Reducing voltage on the secondary increases current substantially.
Three-Phase Current Examples
The secondary voltage must come from the transformer nameplate or the actual transformer design data. A transformer described only as “1500 kVA” does not have one universal secondary-current value.
| Secondary voltage | Calculation | Secondary current |
|---|---|---|
| 208 V, 3-phase | (1{,}500{,}000 \div \(\sqrt{3}\times208\)) | 4,163 A |
| 240 V, 3-phase | (1{,}500{,}000 \div \(\sqrt{3}\times240\)) | 3,608 A |
| 400 V, 3-phase | (1{,}500{,}000 \div \(\sqrt{3}\times400\)) | 2,165 A |
| 415 V, 3-phase | (1{,}500{,}000 \div \(\sqrt{3}\times415\)) | 2,087 A |
| 480 V, 3-phase | (1{,}500{,}000 \div \(\sqrt{3}\times480\)) | 1,804 A |
| 600 V, 3-phase | (1{,}500{,}000 \div \(\sqrt{3}\times600\)) | 1,443 A |
A 480 V secondary may therefore require equipment intended for roughly 1,800 A of transformer full-load current before load characteristics, conductor ampacity, terminal limitations, and protection requirements are evaluated.
Use the Transformer Current Calculator to compare primary and secondary full-load current. Use the Transformer kVA Calculator when voltage and current are known and the transformer kVA must be determined.
480 V Calculation Example
For a 1500 kVA, 480 V, three-phase transformer:
\(\displaystyle I_\text{secondary}=\frac{1{,}500{,}000}{\sqrt{3}\times480}\)
\(\displaystyle I_\text{secondary}=\frac{1{,}500{,}000}{831.38} = I_\text{secondary}\approx1{,}804\text{ A}\)
The calculated result is the transformer’s nominal full-load secondary current. It can be used to frame the downstream distribution review:
- Secondary bus and busway ratings must accommodate the calculated transformer output and the installed system conditions.
- Secondary feeder conductors may require parallel runs, large kcmil conductors, or a busway arrangement rather than a single conventional feeder.
- Raceway fill, conductor pulling tension, bending space, lug range, and termination access become practical design constraints at this current level.
- The load review must account for the actual connected load, demand characteristics, continuous loading, and motor-related current where applicable.
- Voltage-drop calculations should use the actual secondary feeder length, conductor arrangement, impedance, and expected load current.
- Protective-device selection must account for the transformer and system design, including the available short-circuit current at the secondary equipment.
Secondary Feeder and Bus Review
A calculated 1,804 A secondary current does not mean every component is selected at exactly 1,804 A. The installation may use a higher-rated secondary main, busway, switchboard section, or multiple parallel conductor sets based on the electrical design.
Conductor ampacity depends on the installed conductor material, AWG or kcmil size, insulation temperature rating, terminal rating, ambient conditions, and the number of current-carrying conductors. Where conductors share a raceway or cable assembly, applicable adjustment factors and correction factors may change the usable ampacity.
Large transformer secondaries also affect physical layout. Parallel feeders require a consistent arrangement of phase conductors, properly sized raceways, sufficient bending space, and terminations listed for the installed conductor sizes and quantities. A layout that works electrically on a one-line diagram can still fail in the field because of inadequate gutter space, lug limitations, pull-box dimensions, or raceway routing.
For motor loads, the transformer full-load current is not a substitute for motor feeder or branch-circuit calculations. Motor starting current, motor load characteristics, and coordination of downstream protective devices require a separate review.
Protection and Fault Current
The transformer secondary current calculation describes normal full-load current. It does not calculate available fault current.
A 1500 kVA transformer with a low-voltage secondary can produce substantial short-circuit current at terminals close to the transformer. The actual available fault current depends on transformer impedance and the upstream source, then decreases through secondary conductors, busway, and other distribution impedance.
Verify the following separately:
- Transformer impedance and available short-circuit current at the secondary terminals
- Short-circuit current rating of switchgear, switchboards, panelboards, busway, disconnects, and protective devices
- Interrupting rating and coordination of breakers or fuses
- Secondary conductor and bus configuration
- Equipment grounding and bonding design
- Requirements imposed by the applicable electrical code, project specifications, utility conditions, manufacturer instructions, and the AHJ
Do not use calculated secondary current alone to select busway, conductors, breakers, fuses, or transformer secondary connections.