The correct kVA rating for a 3-phase transformer is determined by calculating your total continuous and non-continuous load in amps, multiplying by the line-to-line voltage and 1.732 (the square root of 3), and dividing by 1000. You must then round up to the next standard size dictated by the ANSI/IEEE C57.12.00 standard. For a 120A balanced load at 208V, the math yields 43.2 kVA, meaning your concrete pick is a 45 kVA transformer.
This guide provides the definitive 3 phase transformer sizing chart for commercial and industrial dry-type units. We assume standard 60Hz operation, copper windings, and a 150°C temperature rise (220°C insulation system), which covers 95% of modern indoor installations.
The Standard 3 Phase Transformer Sizing Chart (ANSI/IEEE C57)
Before reading the table, understand how the columns apply to your installation. The kVA column is your nameplate rating. The 480V Primary FLA (Full Load Amps) column applies if you are feeding the transformer from a 480V distribution panel. The 208V Secondary FLA column applies to your downstream load side if you are stepping down to 208Y/120V for standard commercial receptacles and lighting. If your secondary is 480V (stepping down from 4160V medium voltage, for instance), you would use the 480V FLA math for the secondary side instead. Source standard for preferred ratings: ANSI/IEEE C57.12.00.
| kVA Rating | 480V Primary FLA | 208V Secondary FLA | Typical Primary Breaker (NEC 450) | Typical Secondary Breaker |
|---|---|---|---|---|
| 15 | 18.0 A | 41.6 A | 25 A | 50 A |
| 30 | 36.1 A | 83.2 A | 45 A | 100 A |
| 45 | 54.1 A | 124.8 A | 70 A | 150 A |
| 75 | 90.2 A | 208.0 A | 110 A | 225 A |
| 112.5 | 135.3 A | 312.0 A | 175 A | 350 A |
| 150 | 180.4 A | 416.0 A | 225 A | 450 A |
| 225 | 270.6 A | 624.0 A | 350 A | 700 A |
| 300 | 360.8 A | 832.0 A | 450 A | 1000 A |
| 500 | 601.4 A | 1387.0 A | 700 A | 1600 A |
| 750 | 902.1 A | 2080.0 A | 1000 A | 2500 A |
| 1000 | 1202.8 A | 2774.0 A | 1500 A | 3000 A |
Decision Path: Picking Your Exact kVA Rating
Do not guess your transformer size based on square footage. Use this decision tree to terminate on a concrete part specification. For deeper load calculation methodologies, refer to the Schneider Electric Transformer Sizing Guidelines.
| If Your Load Profile Is... | Then Calculate Base kVA... | And Specify This Transformer Type |
|---|---|---|
| Purely resistive (heating, incandescent) | Sum of all branch amps × V × 1.732 / 1000 | Standard Dry-Type (K-1) |
| Mixed commercial (lighting, HVAC, receptacles) | Continuous load × 1.25 + Non-continuous load | Standard Dry-Type (K-1) or DOE 2016 High Efficiency |
| Heavy non-linear (>25% VFDs, LED drivers, UPS) | Base kVA × 1.15 (harmonic heating buffer) | K-4 or K-13 Rated Transformer |
| Large motor starting (across-the-line) | Motor FLA + Inrush (6× FLA) voltage dip check | Standard Dry-Type with low impedance (<5%) |
Worked Example: You are sizing a transformer for a small manufacturing bay. You have 80A of continuous lighting/HVAC load and a 30A non-continuous 3-phase lathe at 208V.
1. Continuous: 80A × 1.25 = 100A.
2. Non-continuous: 30A × 1.0 = 30A.
3. Total calculated amps = 130A.
4. kVA = (130 × 208 × 1.732) / 1000 = 46.8 kVA.
5. Concrete Pick: You must round up to the next ANSI standard size, which is 75 kVA. Do not use a 45 kVA unit, as it will run at 104% capacity and trigger thermal degradation.
Derating Modifiers: When the Base Chart Fails
The chart above assumes standard ambient conditions: 30°C (86°F) maximum ambient temperature and an altitude below 3,300 feet (1,000 meters). If your installation violates these assumptions, the base kVA value is invalid. You must apply derating multipliers to the nameplate rating to find your usable kVA.
- Ambient Temperature > 40°C (104°F): Multiply the base kVA by 0.85. A 75 kVA transformer in a hot boiler room only provides 63.7 kVA of usable capacity. You must step up to a 112.5 kVA unit.
- Ambient Temperature > 50°C (122°F): Multiply the base kVA by 0.70.
- Altitude > 3,300 ft: Air density drops, reducing convective cooling. Derate by 0.3% for every 330 feet above 3,300 ft. At 6,600 ft, you lose roughly 10% of your capacity.
- Enclosed Cabinets: If mounting a dry-type inside a NEMA 3R or NEMA 12 enclosure without forced ventilation, apply a 0.80 multiplier unless the enclosure is explicitly rated for the transformer's BTU dissipation.
What the Sizing Chart Cannot Tell You
A standard kVA lookup table only addresses steady-state thermal limits. It completely ignores three critical real-world failure modes:
- Harmonic Derating: Standard transformers are rated for K-1 (pure sine wave). Modern facilities are flooded with switch-mode power supplies and VFDs that generate 3rd and 5th harmonics. These harmonics cause severe eddy current losses in the transformer core. If your facility has >25% non-linear loads, a standard 75 kVA transformer will overheat and fail at 60% of its rated load. You must specify a K-4 or K-13 transformer, which features an electrostatic shield and derated core flux density.
- Inrush Current and Voltage Dip: If you are starting a 50HP motor across-the-line on a 75 kVA transformer, the motor will draw 600% of its FLA for the first few cycles. The transformer's internal impedance (typically 5.5%) will cause a massive voltage dip. If the dip exceeds 15%, contactors will chatter and PLCs will brownout. You must calculate the voltage dip using the transformer's exact impedance percentage from the manufacturer's cut sheet.
- Future Expansion: The chart calculates for today's load. In commercial real estate, tenant turnover frequently increases plug-load density. Standard industry practice is to add a 20% spare capacity buffer to your final calculated kVA before rounding to the nearest ANSI size.
NEC Article 450 Overcurrent Protection Defaults
Once your 3 phase transformer sizing chart lookup is complete, you must protect it per NEC Article 450.3. The code allows several methods, but the most common and practical approach for dry-type transformers under 600V is the 'Primary and Secondary Protection' method (Table 450.3(B)).
The Rule: Size the primary overcurrent protective device (OCPD) at no more than 125% of the primary FLA. Size the secondary OCPD at no more than 125% of the secondary FLA. If the 125% calculation does not land on a standard breaker size (NEC 240.6), you are permitted to round up to the next standard breaker size.
Concrete Application: For our 45 kVA transformer (480V primary, 208V secondary):
- Primary FLA = 54.1A. 54.1 × 1.25 = 67.6A. Next standard breaker up: 70A.
- Secondary FLA = 124.8A. 124.8 × 1.25 = 156A. Next standard breaker up: 175A (or 150A if you prefer to round down for tighter protection, though 175A is code-compliant and prevents nuisance tripping during magnetizing inrush).
Safety Caveat: Always de-energize, lock/tag out, and verify dead with a tested CAT III/IV meter before terminating transformer lugs. Torque all copper-to-copper or copper-to-aluminum connections to the manufacturer's exact inch-pound specification using a calibrated torque wrench; loose secondary terminations are the leading cause of 208V panel fires. NEC-style guidance provided here is for educational sizing; your local AHJ (Authority Having Jurisdiction) has final authority on all permitted installations.
