When sizing conductors, overcurrent protection, and busbars for commercial or industrial power distribution, the three phase transformer chart is your primary reference tool. This chart maps standard ANSI/IEEE transformer kVA ratings to their exact Full Load Current (FLC) at common secondary line-to-line voltages. Rather than calculating $I = \frac{kVA \times 1000}{V \times \sqrt{3}}$ on the jobsite every time, use the verified data below to size your secondary feeders and breakers accurately.
The Master Three-Phase Transformer Chart (kVA to FLC)
How to read this table: The rows represent standard three-phase kVA sizes established by the ANSI/IEEE C57.12.00 standard for liquid-immersed and dry-type distribution transformers. The columns represent the secondary line-to-line voltage. The values inside the table are the secondary Full Load Current (FLC) in Amperes, assuming a balanced three-phase load at unity power factor. To use this chart, locate your transformer's kVA nameplate rating on the left, then read across to the column matching your secondary system voltage (e.g., 480V delta or 208Y/120V wye).
Bookmark quick-jump for the most queried commercial sizes:
- 75 kVA (Most common light commercial)
- 112.5 kVA (Standard mid-size office/retail)
- 150 kVA (Heavy commercial/HVAC loads)
- 300 kVA (Industrial/Multi-tenant)
| Transformer Size (kVA) | FLC @ 208V (Amps) | FLC @ 240V (Amps) | FLC @ 480V (Amps) |
|---|---|---|---|
| 15 | 41.6 | 36.1 | 18.0 |
| 30 | 83.3 | 72.2 | 36.1 |
| 45 | 124.9 | 108.3 | 54.1 |
| 75 | 208.2 | 180.4 | 90.2 |
| 112.5 | 312.3 | 270.6 | 135.3 |
| 150 | 416.4 | 360.8 | 180.4 |
| 225 | 624.6 | 541.3 | 270.6 |
| 300 | 832.7 | 721.7 | 360.8 |
| 500 | 1387.9 | 1202.8 | 601.4 |
| 750 | 2081.8 | 1804.2 | 902.1 |
| 1000 | 2775.8 | 2405.6 | 1202.8 |
Applying Derating and Installation Variables
While the three phase transformer chart provides the mathematical baseline for full load current, real-world installations require you to apply National Electrical Code (NEC) multipliers and environmental derating factors. According to Fluke's electrical transformer basics guide, ignoring these variables is the leading cause of nuisance tripping and conductor overheating in new commercial builds.
Which Column Applies to Your Installation?
Always use the column that matches your secondary line-to-line voltage, regardless of the primary voltage. If you are stepping down from 480V to a 208Y/120V wye secondary to feed a panelboard, you use the 208V column to size the secondary conductors and the secondary overcurrent protective device (OCPD). The primary side requires a separate calculation based on the primary voltage and the transformer's turns ratio.
How Derating Rows Modify the Base Value
The FLC values in the chart above represent 100% continuous thermal capacity. However, NEC Article 215.2(A)(1) and Article 450 dictate how we size the protection and wire:
- Continuous Loads (3 hours or more): You must multiply the FLC by 1.25. For a 75 kVA transformer at 480V (90.2A FLC), your secondary conductors and breaker must be sized for $90.2 \times 1.25 = 112.75A$. This pushes you from a standard 90A/100A breaker up to a 125A breaker, and requires wire rated for at least 112.75A (typically 1 AWG copper at the 75°C column).
- Ambient Temperature Derating: The transformer itself is rated for a specific ambient (usually 30°C or 40°C depending on the insulation class). If installed in a 50°C boiler room, the transformer's kVA capacity must be derated per the manufacturer's chart. More importantly, the conductors leaving the transformer must be derated per NEC Table 310.15(B)(1), which may force you to upsize the wire by 1 or 2 AWG sizes beyond what the base FLC suggests.
What the Table Cannot Tell You
This chart is strictly for steady-state thermal sizing. It does not account for:
- Inrush Current: When a transformer is first energized, magnetizing inrush current can spike to 10 to 15 times the FLC for 3 to 6 electrical cycles. A 150 kVA transformer (180.4A FLC) can momentarily draw over 2,000A. Your primary fuses or breakers must be time-delay or have adjustable instantaneous trip settings to avoid tripping on energization.
- Voltage Drop Under Load: To calculate actual voltage drop at the secondary terminals, you need the transformer's nameplate impedance (typically 3.0% to 5.75% for dry-type distribution units). A 5.75% impedance on a 112.5 kVA unit means the secondary voltage will drop by nearly 5.75% when loaded to its absolute maximum FLC.
- Harmonic Heating: If your load consists of variable frequency drives (VFDs) or heavy LED switching power supplies, the resulting triplen harmonics add heat to the transformer core and windings without increasing the fundamental FLC. In these cases, you must specify a K-rated transformer (e.g., K-4 or K-13) rather than relying solely on the standard kVA chart.
Frequently Asked Questions
How do I calculate primary breaker size using the three-phase transformer chart?
The chart only gives you secondary FLC. To find the primary breaker size, you must first calculate the primary FLC using the primary voltage. Under NEC Table 450.3(B), if the primary current is over 9A, you can size the primary overcurrent device at up to 250% of the primary FLC to accommodate magnetizing inrush. For example, a 45 kVA transformer with a 480V primary has a primary FLC of 54.1A. Multiplying by 2.5 yields 135.25A, allowing you to round up to the next standard breaker size of 150A, provided the secondary is protected at 125% of its secondary FLC.
What is the practical load difference between a 75 kVA and 112.5 kVA three-phase transformer?
At a standard 480V secondary, a 75 kVA transformer provides 90.2A of FLC, which typically allows the use of 3 AWG copper THHN wire (rated 100A at 75°C). Stepping up to a 112.5 kVA transformer yields 135.3A of FLC. This crosses the 100A threshold, forcing a jump to 1/0 AWG copper wire (rated 150A at 75°C) and a larger panelboard bus. The physical footprint and weight of the 112.5 kVA unit also increase significantly, often requiring upgraded unistrut bracing or a reinforced concrete pad for floor-mounted dry-type installations.
Does the three phase transformer chart account for power factor?
No. The chart is based on kVA (apparent power), which is the thermal limit of the transformer's copper windings and iron core. Power factor (PF) dictates the ratio of real working power (kW) to apparent power (kVA). A 150 kVA transformer will draw exactly 180.4A at 480V whether the load is purely resistive (PF = 1.0) or highly inductive (PF = 0.7). While a low power factor means you are getting less actual mechanical work (kW) out of the transformer, the windings still experience the exact same $I^2R$ heating losses. Always size the transformer based on the total kVA demand of the facility, not the kW demand.
