Standard 3 phase transformer sizes follow a strict geometric progression defined by industry standards: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA. When sizing a unit for a commercial or industrial panel, you must calculate the total connected load in kilovolt-amperes (kVA) and round up to the next standard size. For a 480V primary to 208Y/120V secondary system, a 75 kVA transformer yields roughly 208 amps of secondary full-load current, making it the most common workhorse for light commercial subpanels.
How to Read the 3 Phase Transformer Sizing Chart
The table below provides the baseline Full Load Amps (FLA) for standard 3 phase transformer sizes. How to read this table: The primary FLA column assumes a standard 480V delta primary feed. The secondary columns provide the line current for the two most common wye-configured secondaries: 208Y/120V (for mixed lighting/receptacle loads) and 480Y/277V (for HVAC and heavy lighting). The impedance column reflects standard values per IEEE C57.12.01 for dry-type transformers. These baseline values assume copper windings, a 60Hz frequency, and a standard 30°C (86°F) ambient temperature. Always use the secondary FLA column that matches your actual downstream panel voltage to size your secondary overcurrent protection per NEC Article 450.
| kVA Rating | Primary FLA (480V) | Secondary FLA (208Y/120V) | Secondary FLA (480Y/277V) | Standard Impedance |
|---|---|---|---|---|
| 15 kVA | 18.0 A | 41.6 A | 18.0 A | 4.0% |
| 30 kVA | 36.1 A | 83.3 A | 36.1 A | 4.0% |
| 45 kVA | 54.1 A | 124.9 A | 54.1 A | 4.0% |
| 75 kVA | 90.2 A | 208.2 A | 90.2 A | 4.0% |
| 112.5 kVA | 135.3 A | 312.3 A | 135.3 A | 5.7% |
| 150 kVA | 180.4 A | 416.4 A | 180.4 A | 5.7% |
| 225 kVA | 270.6 A | 624.6 A | 270.6 A | 5.7% |
| 300 kVA | 360.8 A | 832.7 A | 360.8 A | 5.7% |
| 500 kVA | 601.4 A | 1387.9 A | 601.4 A | 5.7% |
| 750 kVA | 902.1 A | 2081.9 A | 902.1 A | 5.7% |
| 1000 kVA | 1202.8 A | 2775.8 A | 1202.8 A | 5.7% |
Applying Derating Factors to Your Base kVA
The chart above assumes ideal conditions. In the real world, environmental and electrical factors force you to derate the transformer's capacity. If you ignore these, the transformer's insulation system will degrade prematurely, leading to catastrophic failure.
Ambient Temperature Derating: Standard dry-type transformers are designed for a 55°C or 65°C temperature rise above a 30°C ambient environment. If you install a transformer in a hot mechanical room or an unventilated enclosure, you must apply a multiplier to the base kVA. At 40°C ambient, multiply the base kVA by 0.93. At 50°C ambient, multiply by 0.85. For example, a 150 kVA transformer installed in a 50°C boiler room effectively becomes a 127.5 kVA unit. If your calculated load is 140 kVA, you must step up to a 225 kVA transformer.
Altitude Derating: Air density decreases at higher elevations, reducing its ability to carry heat away from the windings. Per IEEE standards, if your installation is above 3,300 feet (1,000 meters), you must derate the transformer capacity by 0.3% for every additional 330 feet of elevation.
Harmonic Derating (K-Factor): Modern facilities are packed with non-linear loads like VFDs, LED drivers, and server switch-mode power supplies. These loads generate triplen harmonics that circulate in the transformer windings, causing severe eddy current heating. If your facility has a high harmonic profile, a standard transformer will overheat even if the RMS current is below the FLA. In these cases, specify a K-4 or K-13 rated transformer, which features an electrostatic shield and heavier gauge windings to handle the heat.
What the Sizing Chart Cannot Tell You
A reference chart gives you steady-state thermal limits, but it omits critical dynamic and physical parameters required for a complete installation design.
- Inrush Magnetizing Current: When a 3 phase transformer is first energized, it draws a massive asymmetrical inrush current—often 10 to 15 times the primary FLA—for the first few electrical cycles. The chart only lists steady-state FLA. If you size your primary fuse or breaker too close to the FLA without accounting for inrush, the transformer will trip the breaker every time you close the disconnect. NEC 450.3 allows primary overcurrent devices to be sized up to 250% of the primary FLA specifically to accommodate this inrush.
- Voltage Drop Under Load: The chart does not show what the secondary voltage will be when fully loaded. A transformer with 5.7% impedance will experience a proportional voltage drop at full load, heavily dependent on the load's power factor. If you are feeding sensitive CNC machinery at the end of a long feeder, you may need to adjust the primary tap settings (e.g., to the -5% tap) to boost the secondary voltage under load.
- Physical Footprint and Weight: A 150 kVA dry-type transformer weighs roughly 1,200 lbs and requires specific seismic bracing in earthquake zones. A 500 kVA unit pushes 3,000 lbs. The electrical chart won't tell you if your mezzanine floor can support the dead load or if the unit will fit through the facility's standard double doors.
3 Phase Transformer Sizing FAQ
How do I calculate the exact kVA for a 3 phase motor load?
Do not use the motor's horsepower rating directly. Look at the motor nameplate for the Full Load Amps (FLA) and the rated voltage. Use the formula: kVA = (Volts × Amps × √3) / 1000. For a 480V motor drawing 52A, the calculation is (480 × 52 × 1.732) / 1000 = 43.2 kVA. Furthermore, NEC Article 430 requires you to size the transformer to handle the motor's locked-rotor current (LRA) if it is the only load, or apply a 125% multiplier to the continuous portion of the load to prevent excessive voltage drop during motor starting.
Should I size my 3 phase transformer for future expansion?
Generally, yes, but with a caveat. Sizing up one standard increment (e.g., choosing a 112.5 kVA unit instead of a 75 kVA unit for an 80 kVA calculated load) is standard practice to accommodate future panel additions. However, massively oversizing a transformer (e.g., installing a 300 kVA unit for a 60 kVA load) is inefficient. Transformers suffer from core losses (no-load losses) that occur 24/7 simply by being energized. An oversized transformer will operate at a very low percentage of its capacity, resulting in a poor power factor and wasted energy over its 30-year lifespan.
What is the difference between a 3 phase delta and wye transformer secondary?
A wye (Y) secondary provides a neutral point, giving you two voltages: line-to-line (e.g., 208V) and line-to-neutral (e.g., 120V). This is mandatory for commercial buildings with mixed single-phase 120V receptacle loads. A delta (Δ) secondary has no true neutral and only provides line-to-line voltage (e.g., 240V or 480V). Delta secondaries are typically used for dedicated 3-phase motor loads or industrial HVAC systems where 120V single-phase loads are not required on that specific panel.
Can I parallel two smaller 3 phase transformers to get a larger kVA?
Yes, but it is highly complex and rarely recommended for standard commercial jobs. To parallel transformers safely, they must have identical voltage ratios, identical tap settings, identical polarity, and nearly identical impedance (within 0.5% of each other). If the impedances do not match perfectly, the transformer with the lower impedance will hog the load and overheat while the other sits underutilized. For 95% of applications, buying a single larger kVA unit is cheaper, safer, and requires less physical footprint than paralleling two smaller units.
