A transformer’s kVA (kilovolt-ampere) rating defines its maximum apparent power capacity before thermal limits are exceeded. Use the transformer kVA chart below to match standard NEMA frame sizes to primary and secondary amperage for common 480V, 240V, and 208V distribution systems. For a quick benchmark: a 75 kVA three-phase transformer at 208V secondary delivers 208 amps, while a 50 kVA single-phase unit at 240V delivers 208 amps.
Standard Transformer kVA Chart & Ampacity Ratings
The following table lists standard catalog sizes based on NEMA TR 1 and IEEE C57.12.00 standards. Current values assume a unity power factor (1.0) and standard 60Hz operation.
| kVA Rating | 1-Ø 240V Amps | 1-Ø 480V Amps | 3-Ø 208V Amps | 3-Ø 480V Amps |
|---|---|---|---|---|
| 15 | 62.5 | 31.2 | 41.6 | 18.0 |
| 25 | 104.2 | 52.1 | 69.4 | 30.1 |
| 37.5 | 156.3 | 78.1 | 104.1 | 45.1 |
| 45 | 187.5 | 93.8 | 125.0 | 54.2 |
| 50 | 208.3 | 104.2 | 138.8 | 60.2 |
| 75 | 312.5 | 156.3 | 208.3 | 90.3 |
| 112.5 | 468.8 | 234.4 | 312.5 | 135.4 |
| 150 | 625.0 | 312.5 | 416.6 | 180.5 |
| 225 | 937.5 | 468.8 | 624.9 | 270.8 |
| 300 | 1250.0 | 625.0 | 833.2 | 361.0 |
| 500 | 2083.3 | 1041.7 | 1388.7 | 601.7 |
How to Read the Chart: Columns, Derating, and Installation Limits
Which column applies to your installation? First, identify your phase configuration (single-phase vs. three-phase) and your secondary voltage. If you are feeding a standard US commercial lighting panel, use the 3-Ø 208V Amps column. If you are stepping down to a residential split-phase service, use the 1-Ø 240V Amps column. Never size wire based on the primary (480V) column unless you are specifically sizing the primary feeder and overcurrent protection.
How derating rows modify the base value: The ampacities in the main chart assume a standard 40°C (104°F) maximum ambient temperature and an altitude below 1,000 meters (3,300 ft). If your installation violates these baselines, you must apply derating factors to the base kVA before calculating your final ampacity.
| Environmental Condition | Derating Factor | Impact on 75 kVA Base Rating |
|---|---|---|
| Ambient 40°C / Altitude < 3300ft | 1.00 (Base) | 75.0 kVA |
| Ambient 50°C (122°F) | 0.90 (-10%) | 67.5 kVA (Derates to ~187A at 208V) |
| Altitude 5,000 ft (1524m) | 0.95 (-5%) | 71.2 kVA |
| Ambient 50°C AND Altitude 5,000 ft | 0.85 (Cumulative) | 63.7 kVA |
Thinner air at high altitudes reduces the convective cooling of dry-type transformers. Similarly, high ambient temperatures in unventilated electrical rooms shrink the thermal headroom. Always check the manufacturer's specific derating curves, as copper vs. aluminum windings and insulation class (150°C rise vs. 115°C rise) slightly alter these multipliers.
What This Transformer kVA Chart Cannot Tell You
While this transformer kVA chart is essential for steady-state wire and breaker sizing, it omits three critical transient and power-quality variables that dictate real-world performance:
- Inrush Magnetizing Current: When a transformer is first energized, the core must be magnetized. This inrush current can spike to 12 to 15 times the full-load amperage (FLA) for a few cycles. A 75 kVA transformer pulling 90A on the primary might draw 1,200A momentarily. Your primary breaker must have a magnetic trip setting high enough to ignore this inrush, or it will nuisance-trip every time you close the switch.
- Impedance and Voltage Drop: The chart assumes ideal voltage delivery. In reality, transformer impedance (typically 5.75% for a 75 kVA unit) causes secondary voltage to sag under heavy motor starting loads. If you are starting a 50 HP motor across-the-line, you must calculate the voltage dip using the nameplate %Z to ensure contactors don't chatter and drop out.
- Harmonic Derating (K-Factor): Modern facilities are loaded with VFDs, LED drivers, and switched-mode power supplies. These non-linear loads generate triplen harmonics that cause severe eddy current heating in the transformer core and neutral bus. If your load profile exceeds 30% non-linear, a standard kVA chart is useless; you must specify a K-4 or K-13 rated transformer, or heavily derate a standard unit.
Transformer kVA Chart FAQ
How do I calculate the exact kVA needed for my load?
Sum the total continuous and non-continuous wattage of your load, divide by the expected power factor (usually 0.85 to 0.95 for mixed commercial loads), and add a 20% to 25% growth margin. For three-phase systems, use the formula: kVA = (Volts × Amps × 1.732) / 1000. For single-phase, drop the 1.732 multiplier. If your calculated load requires 62 kVA, step up to the next standard NEMA catalog size, which is 75 kVA.
Why is my transformer kVA rating in kVA and not kW?
Transformers are rated in kVA (apparent power) because their thermal limits are dictated by current (Amps) and voltage, regardless of the phase angle between them. kW (real power) factors in the power factor of the load. A transformer supplying 100 Amps at 480V generates the exact same amount of internal copper and core heat whether the load is purely resistive (1.0 PF) or highly inductive (0.6 PF). Therefore, the manufacturer rates the physical thermal capacity in kVA, leaving the kW calculation to the specific load profile.
What size breaker do I need for a 45 kVA three-phase transformer?
Per NEC Article 450.3, transformer overcurrent protection depends on whether you are protecting the primary or secondary side. For a 45 kVA, 480V primary / 208V secondary unit:
Secondary Protection: The 45 kVA chart shows 125A at 208V. NEC 450.3(B) allows sizing the secondary breaker at 125% of FLA (125A × 1.25 = 156.2A). Per NEC 240.6, you round up to the next standard breaker size: 175A.
Primary Protection: The primary draws 54.2A. If the secondary is protected at 125%, NEC allows the primary breaker to be sized up to 250% of primary FLA to accommodate inrush, though 125% is standard if inrush isn't an issue. At 125% (54.2A × 1.25 = 67.7A), you would use a 70A breaker. If nuisance tripping occurs due to inrush, you can step up to the 250% maximum limit (135A -> 150A breaker).
