3 Phase Transformer Chart: Standard kVA Sizes, Full Load Amps, and Sizing Guide

For the most common commercial electrical installation—a 200-amp, 120/208V 3-phase panelboard fed from a 480V utility service—the exact pick is a 75 kVA dry-type transformer. This unit provides 208 amps of secondary capacity, perfectly matching the panel's main breaker while allowing for continuous load margins. If you are sizing for a 400-amp panel, you need a 150 kVA unit.

This reference guide provides the definitive 3 phase transformer chart for standard ANSI preferred ratings, mapping kVA sizes to full load amps (FLA) and overcurrent protective device (OCPD) sizing based on NEC Article 450.

How to Read This 3 Phase Transformer Chart

Before pulling wire, you must understand which column applies to your specific installation scope. This chart assumes the most ubiquitous North American commercial voltage configuration: a 480V Delta primary stepping down to a 208Y/120V Wye secondary.

• Primary FLA (Full Load Amps): Use this column to size the upstream feeder conductors from the main switchgear to the transformer. Sizing is based on the 75°C column of NEC Table 310.16.
• Secondary FLA: Use this column to size the downstream feeder from the transformer secondary terminals to the panelboard main breaker.
• Primary & Secondary OCPD: Calculated per NEC 450.3(B). For transformers over 9 amps, the primary breaker is sized at 125% of the primary FLA, rounded up to the next standard breaker size. The secondary breaker is sized at 125% of the secondary FLA.
• Temperature Rating: All standard dry-type transformers in this chart assume a 150°C temperature rise with a 220°C insulation system, designed for a maximum 40°C ambient environment.

Standard 3 Phase Transformer kVA and Full Load Current Chart

The following data table is sourced from Eaton's Dry-Type Transformer Application Guide and aligns with ANSI C57.12.00 preferred kVA ratings and NFPA 70 (NEC) Article 450 overcurrent rules.

Ambient and Altitude Derating Modifiers

The FLA and kVA values in the table above represent the transformer's base capacity under standard IEEE C57.12.01 conditions: a maximum ambient temperature of 40°C (104°F) and an altitude at or below 3,300 feet (1,000 meters). When your installation falls outside these parameters, derating rows modify the base value, forcing you to step up to the next physical kVA frame size.

Temperature Derating

Dry-type transformers rely on convective air cooling. If the electrical room lacks adequate ventilation or is located near heat-generating equipment, the ambient temperature will exceed 40°C.

• 41°C to 50°C Ambient: Derate transformer capacity by approximately 5% to 10%. A 75 kVA transformer effectively becomes a ~67 kVA unit. Action: Upsize to a 112.5 kVA frame.
• Above 50°C Ambient: Standard 150°C rise transformers are no longer viable. You must specify a custom 115°C rise transformer or implement active forced-air cooling in the room.

Altitude Derating

Air density decreases at higher elevations, reducing its ability to carry heat away from the transformer coils.

• Above 3,300 ft (1,000 m): Derate the transformer kVA by 0.3% for every 330 feet (100 meters) above the 3,300 ft baseline.
• Example: At 5,000 ft elevation (1,700 ft above baseline), the derating factor is (1700 / 330) * 0.3% = 1.54%. While small, on a heavily loaded 500 kVA unit operating at 48°C ambient, this combined penalty will push the unit into thermal failure. Always specify "high altitude" winding designs when ordering above 3,300 ft.

Transformer Sizing Decision Path

Use this decision-tree-table to move from a known panelboard load to a concrete transformer part specification. This path assumes a standard 480V to 208Y/120V step-down application.

What This Chart Cannot Tell You

While this 3 phase transformer chart provides the baseline thermal and overcurrent parameters, it omits three critical edge cases that will cause nuisance tripping or equipment failure if ignored during the design phase.

1. Transformer Inrush (Magnetizing) Current

When a transformer is first energized, the magnetic core saturates, drawing an inrush current that can be 10 to 15 times the primary FLA for several cycles. If you size the primary breaker exactly to the NEC 125% rule using a standard thermal-magnetic breaker with a low magnetic trip setting, it will trip instantly upon energization.
The Fix: Specify primary breakers with high-magnetic trip settings (e.g., Square D PowerPact H-frame with a 500% to 1000% magnetic pickup) or use time-delay fuses to ride through the inrush spike.

2. Harmonic Loading and K-Factor

The FLA values in the chart assume a purely linear, 60Hz sinusoidal load. Modern commercial buildings are dominated by non-linear loads (switch-mode power supplies, VFDs) that generate 3rd, 5th, and 7th harmonics. These harmonics cause severe eddy current losses in the transformer core, leading to catastrophic overheating even if the total RMS current is below the secondary FLA.
The Fix: If your facility has high harmonic content, do not just buy a standard 150°C rise transformer and oversize it. Order a IEEE C57.110 compliant K-4, K-9, or K-13 rated transformer, which features heavier gauge windings and specialized core construction to dissipate harmonic heat.

3. Secondary Voltage Drop on Long Feeders

The chart tells you the transformer outputs 208V at the secondary lugs. It does not account for voltage drop across the secondary feeder conductors. If your panelboard is located 250 feet away from the transformer, a 400A load on 600 kcmil copper will drop nearly 4 volts, pushing the panel voltage down to 204V, which can cause HVAC compressors to overheat and trip on internal overload.
The Fix: For secondary runs exceeding 100 feet, calculate voltage drop per NEC Chapter 9, Table 8. If the drop exceeds 2%, you must either upsize the secondary conductors or order a transformer with secondary voltage taps (e.g., a +2.5% FCAN tap) to boost the output voltage at the source.