Transformer kVA Chart by Voltage
This reference chart shows the full-load current (in amperes) for standard transformer kVA ratings at common single-phase and three-phase voltages. Use these tables to quickly verify transformer ampacity, size overcurrent protection, or select the right transformer for your application.
How to Read These Charts
Each table lists standard kVA ratings in the left column and the corresponding full-load current (FLC) in amperes for a specific voltage and phase configuration. The FLC is calculated using the standard transformer formulas:
- Single-phase: I = (kVA × 1,000) / V
- Three-phase: I = (kVA × 1,000) / (V × √3)
These are continuous-duty full-load currents. Per NEC 450.3, the primary overcurrent protection should be sized at 125% of FLC for fuses or 150% for breakers, and secondary protection (if installed) at 125% of FLC.
Single-Phase Transformer kVA to Amps
120V Single-Phase
Common in residential and light commercial applications. Used for small dedicated circuits and low-power equipment.
| kVA | Full-Load Current (A) |
|---|---|
| 1 | 8.3 |
| 1.5 | 12.5 |
| 2 | 16.7 |
| 3 | 25.0 |
| 5 | 41.7 |
| 7.5 | 62.5 |
| 10 | 83.3 |
| 15 | 125.0 |
| 25 | 208.3 |
240V Single-Phase
The standard residential service voltage in North America. Also used for single-phase motors, welders, and ovens.
| kVA | Full-Load Current (A) |
|---|---|
| 1 | 4.2 |
| 1.5 | 6.3 |
| 2 | 8.3 |
| 3 | 12.5 |
| 5 | 20.8 |
| 7.5 | 31.3 |
| 10 | 41.7 |
| 15 | 62.5 |
| 25 | 104.2 |
| 37.5 | 156.3 |
| 50 | 208.3 |
| 75 | 312.5 |
| 100 | 416.7 |
| 167 | 695.8 |
| 250 | 1,041.7 |
277V Single-Phase
Line-to-neutral voltage of a 480Y/277V system. Used for lighting circuits in commercial buildings.
| kVA | Full-Load Current (A) |
|---|---|
| 1 | 3.6 |
| 3 | 10.8 |
| 5 | 18.1 |
| 7.5 | 27.1 |
| 10 | 36.1 |
| 15 | 54.2 |
| 25 | 90.3 |
| 37.5 | 135.4 |
| 50 | 180.5 |
Three-Phase Transformer kVA to Amps
208V Three-Phase (Delta or Wye)
The most common three-phase voltage for small and medium commercial buildings. The 208Y/120V configuration provides both 208V (line-to-line) and 120V (line-to-neutral) from a single transformer.
| kVA | Full-Load Current (A) |
|---|---|
| 3 | 8.3 |
| 6 | 16.6 |
| 9 | 25.0 |
| 15 | 41.6 |
| 30 | 83.3 |
| 45 | 124.9 |
| 75 | 208.2 |
| 112.5 | 312.3 |
| 150 | 416.4 |
| 225 | 624.5 |
| 300 | 832.7 |
| 500 | 1,387.9 |
240V Three-Phase (Delta)
Common in industrial facilities, older commercial buildings, and motor-heavy applications. Often a corner-grounded or ungrounded delta system.
| kVA | Full-Load Current (A) |
|---|---|
| 3 | 7.2 |
| 6 | 14.4 |
| 9 | 21.7 |
| 15 | 36.1 |
| 30 | 72.2 |
| 45 | 108.3 |
| 75 | 180.4 |
| 112.5 | 270.6 |
| 150 | 360.8 |
| 225 | 541.3 |
| 300 | 721.7 |
| 500 | 1,202.8 |
480V Three-Phase (Delta or Wye)
The standard voltage for large commercial and industrial distribution. The 480Y/277V wye configuration is used in most modern commercial buildings for HVAC, motors, and 277V lighting.
| kVA | Full-Load Current (A) |
|---|---|
| 3 | 3.6 |
| 6 | 7.2 |
| 9 | 10.8 |
| 15 | 18.0 |
| 30 | 36.1 |
| 45 | 54.1 |
| 75 | 90.2 |
| 112.5 | 135.3 |
| 150 | 180.4 |
| 225 | 270.6 |
| 300 | 360.8 |
| 500 | 601.4 |
| 750 | 902.1 |
| 1000 | 1,202.8 |
| 1500 | 1,804.2 |
| 2000 | 2,405.6 |
| 2500 | 3,007.0 |
600V Three-Phase
Common in Canadian industrial systems and some large US industrial installations. Used for heavy motor loads and primary distribution.
| kVA | Full-Load Current (A) |
|---|---|
| 15 | 14.4 |
| 30 | 28.9 |
| 45 | 43.3 |
| 75 | 72.2 |
| 112.5 | 108.3 |
| 150 | 144.3 |
| 225 | 216.5 |
| 300 | 288.7 |
| 500 | 481.1 |
| 750 | 721.7 |
| 1000 | 962.3 |
Quick Reference Formula Card
kVA to Amps:
Single-phase: A = (kVA × 1,000) / V
Three-phase: A = (kVA × 1,000) / (V × 1.732)
Amps to kVA:
Single-phase: kVA = (V × A) / 1,000
Three-phase: kVA = (V × A × 1.732) / 1,000
Common Applications by Voltage
| Voltage | Configuration | Typical Use |
|---|---|---|
| 120V 1φ | — | Residential receptacles, small equipment |
| 120/240V 1φ | Center-tap | Residential service entrance |
| 208Y/120V 3φ | Wye | Small commercial, office buildings |
| 240V 3φ | Delta | Industrial motors, older buildings |
| 277V 1φ | Line-to-neutral (480Y) | Commercial lighting circuits |
| 480Y/277V 3φ | Wye | Commercial/industrial main distribution |
| 480V 3φ | Delta | Industrial motor loads |
| 600V 3φ | Delta or Wye | Heavy industrial, Canadian systems |
Tips for Using These Charts
- Overcurrent protection: Size primary fuses at 125% and primary breakers at 150% of the FLC values shown, per NEC 450.3.
- Wire sizing: The FLC values here help you select conductor sizes per NEC 310.16. Always add the 125% continuous load factor per NEC 215.2 when sizing feeder conductors.
- Voltage drop: For long feeder runs from the transformer to the load, verify that voltage drop does not exceed 3% (NEC recommended maximum for feeders). Use our Voltage Drop Calculator.
- Motor loads: Motors draw significantly more current at startup (typically 6–8× FLC for across-the-line starting). Size the transformer so that voltage dip during motor starting does not exceed 10–15% to avoid nuisance tripping of other loads.
- Unbalanced loads: These charts assume balanced three-phase loads. If loads are significantly unbalanced, calculate each phase individually and use the highest phase current for sizing.
Frequently Asked Questions
How do I convert kVA to amps for a three-phase transformer?
Use the formula: Amps = (kVA × 1,000) / (V × √3). For example, a 45 kVA transformer at 480V three-phase: Amps = (45 × 1,000) / (480 × 1.732) = 54.1A.
What is the difference between single-phase and three-phase kVA ratings?
A single-phase transformer uses the formula kVA = (V × A) / 1,000. A three-phase transformer uses kVA = (V × A × √3) / 1,000. For the same kVA rating and voltage, a three-phase transformer delivers approximately 73% more current than a single-phase unit because the √3 factor is already included in the three-phase calculation.
What are the most common transformer voltages in the US?
The most common transformer voltages in the US are 120/240V single-phase (residential), 208Y/120V three-phase (commercial), 240V three-phase (industrial delta), 277/480V three-phase (commercial/industrial), and 480V delta (industrial). Three-phase systems may be delta or wye (star) configuration.
How do I find the full-load current on a transformer nameplate?
Transformer nameplates list the full-load current (FLC) for both primary and secondary windings. If the nameplate is missing or unreadable, calculate FLC using the kVA and voltage: Single-phase FLC = (kVA × 1,000) / V. Three-phase FLC = (kVA × 1,000) / (V × √3).
Can I use a 480V transformer on a 240V system?
No. A transformer rated for 480V primary must be connected to a 480V supply. Applying 240V to a 480V primary will result in half the expected secondary voltage and potentially improper operation. Always match the transformer voltage rating to your supply system.