A 3 phase transformer's amperage depends entirely on its kVA rating and secondary voltage. For example, a standard 45 kVA transformer at 208V yields 124.9A, while at 480V it yields 54.1A. Use the chart below to find your exact Full Load Current (FLC) for conductor sizing and overcurrent protection.

How to Read the 3 Phase Transformer Amperage Chart

The chart below provides the baseline Full Load Current (FLC) for standard 3 phase transformer sizes. To use it correctly, you must match your transformer's secondary voltage to the corresponding column.

  • Which column applies to your installation? Look at the transformer nameplate. If you are stepping down 480V to a 120/208V wye system, use the 208V Secondary column to size your secondary conductors and breaker. If you are feeding a 240V delta motor load, use the 240V Secondary column.
  • Primary vs. Secondary: This chart focuses on secondary output current, which dictates your branch circuit or feeder sizing. To find primary current (for sizing the feed to the transformer), use the column that matches your primary supply voltage.
Benchmark Quick-Jump: The most frequently installed commercial sizes are 45 kVA (standard office lighting/receptacles) and 112.5 kVA (mixed commercial loads). Bookmark the 208V and 480V columns, as these cover 90% of US commercial step-down applications.

Complete 3 Phase Transformer Full Load Current (FLC) Chart

Source Standard: Calculations derived from the standard 3-phase power formula ($I = \frac{kVA \times 1000}{V \times \sqrt{3}}$) and aligned with baseline FLC definitions in NFPA 70 (NEC) Article 450 and IEEE Std C57.12.00 for liquid-immersed and dry-type transformers.

kVA Rating 208V Secondary 240V Secondary 480V Secondary 600V Secondary
15 kVA 41.6 A 36.1 A 18.0 A 14.4 A
30 kVA 83.2 A 72.2 A 36.1 A 28.9 A
45 kVA 124.9 A 108.3 A 54.1 A 43.3 A
75 kVA 208.2 A 180.4 A 90.2 A 72.2 A
112.5 kVA 312.3 A 270.6 A 135.3 A 108.3 A
150 kVA 416.4 A 360.8 A 180.4 A 144.3 A
225 kVA 624.5 A 541.3 A 270.6 A 216.5 A
300 kVA 832.7 A 721.7 A 360.8 A 288.7 A
500 kVA 1387.9 A 1202.8 A 601.4 A 481.1 A

Derating, Inrush, and What the Table Cannot Tell You

While the table above gives you the baseline FLC, real-world installations require adjustments. Here is how environmental and electrical factors modify these base values.

How Derating Modifies the Base Value

The FLC values above assume a standard 30°C (86°F) ambient temperature. If you install a dry-type transformer in a hot mechanical room or a confined enclosure, the transformer's ability to dissipate heat drops. Per IEEE C57.96 loading guides, a transformer in a 40°C ambient may need to be derated by roughly 10-15%, and at 50°C, it may only safely handle 75-80% of its nameplate kVA.

Furthermore, NEC 310.15(B)(1) ampacity correction factors apply to the conductors connected to the transformer. If your secondary conductors are routed through a 45°C rooftop environment, you must apply the 0.80 correction factor to your wire's base ampacity, often forcing you to upsize the wire by one or two AWG steps beyond what the transformer FLC alone would dictate.

What the Table Cannot Tell You

  • Inrush (Magnetizing) Current: When a transformer is first energized, it draws a massive surge of current to establish the magnetic field in the core. This inrush can be 8 to 12 times the FLC for several cycles. The table does not reflect this; you must account for it by selecting breakers with appropriate magnetic trip curves or time-delay fuses to prevent nuisance tripping on startup.
  • Harmonic Loads (K-Factor): If your load consists of VFDs, LED drivers, or server racks, non-linear loads generate triplen harmonics. These harmonics cause excess heat in the transformer windings and add up on the neutral conductor. For these loads, standard FLC charts are insufficient; you must specify a K-rated transformer (e.g., K-13 or K-20) and upsize the neutral conductor to 200% of the phase conductor ampacity.
Safety Warning: Always de-energize, lock out/tag out, and verify dead with a tested CAT III or CAT IV multimeter before terminating transformer lugs. Transformer secondary terminals can backfeed lethal voltages to the primary side if not properly isolated.

Frequently Asked Questions

How do I calculate 3 phase transformer amperage if my voltage isn't on the chart?

Use the standard 3-phase power formula: I = (kVA × 1000) / (V × 1.732). For example, if you have a 75 kVA transformer with a 380V secondary (common in international or specific marine applications), the calculation is: (75,000) / (380 × 1.732) = 113.9A. Always round up to the next standard breaker size per NEC 240.6 when sizing overcurrent protection.

Does a 3 phase transformer amperage chart account for inrush current?

No. The chart only provides steady-state Full Load Current (FLC). Inrush current is handled during the overcurrent protection selection phase. Per NEC 450.3(B), for transformers under 600V, the primary overcurrent device can be sized up to 125% of the primary FLC if the secondary is also protected at 125%. If secondary protection is not provided, the primary breaker can be sized up to 250% of the primary FLC to accommodate the magnetizing inrush without nuisance tripping.

What size breaker do I need for a 45 kVA 3 phase transformer?

Assuming a standard 480V primary to 208V secondary step-down:
Secondary Breaker: The FLC at 208V is 124.9A. NEC 450.3(B) allows secondary protection up to 125% of FLC. 124.9A × 1.25 = 156.1A. The next standard breaker size up per NEC 240.6 is 175A.
Primary Breaker: The FLC at 480V is 54.1A. 54.1A × 1.25 = 67.6A. The next standard size up is a 70A breaker.
Note: Your secondary conductors must be sized to handle the 175A breaker (e.g., 2/0 AWG copper THHN at 75°C), not just the 124.9A FLC.