A 3 phase transformer's size refers to its apparent power capacity, measured in kilovolt-amperes (kVA), which dictates the maximum continuous load it can safely step up or step down across three alternating current phases without exceeding its thermal limits.
In a real installation, selecting the correct kVA size determines your primary overcurrent protection settings, the physical footprint and weight of the vault or pad, and the maximum motor starting inrush current your system can handle without excessive voltage drop. Undersizing leads to premature insulation failure and nuisance tripping, while grossly oversizing increases core losses and wastes capital on copper you will never use.
Standard 3 Phase Transformer Sizes and Ratings
Manufacturers build dry-type and liquid-filled 3 phase transformers to standard kVA increments defined by NEMA and IEEE standards. You cannot order a 62 kVA transformer off the shelf; you must round up to the next standard size. The table below outlines the most common low-voltage distribution sizes, their full-load secondary amperage at 208Y/120V and 480V, and typical physical weights for indoor dry-type units.
| kVA Rating | Secondary Voltage | Full Load Amps (FLA) | Typical Dry-Type Weight | Common Application |
|---|---|---|---|---|
| 15 kVA | 208Y/120V | 41.6 A | 280 lbs | Small commercial lighting panels |
| 30 kVA | 208Y/120V | 83.2 A | 410 lbs | Retail storefronts, small HVAC |
| 45 kVA | 208Y/120V | 124.7 A | 550 lbs | Office buildings, machine shops |
| 75 kVA | 480V Delta | 90.2 A | 750 lbs | Industrial motor control centers |
| 112.5 kVA | 208Y/120V | 312.1 A | 1,050 lbs | Mid-size data centers, schools |
| 150 kVA | 480Y/277V | 180.4 A | 1,300 lbs | Large commercial lighting, EV chargers |
| 300 kVA | 480Y/277V | 360.8 A | 2,200 lbs | Manufacturing plants, hospitals |
| 500 kVA | 480Y/277V | 601.4 A | 3,400 lbs | Heavy industrial, utility substations |
Calculating Load: A Worked Numeric Example
Let us size a 480V Delta primary to 208Y/120V Wye secondary dry-type transformer for a small commercial tenant space. We need to calculate the total apparent power (kVA) and apply National Electrical Code (NEC) multipliers for continuous loads.
The Loads:
- Load A: One 20 HP, 3-phase, 208V HVAC blower motor. Per NEC Table 430.250, the Full Load Amps (FLA) is 59.4A. We will use 60A for design margin.
- Load B: 15 kW of 120V single-phase LED lighting and receptacles, balanced evenly across the three phases. Assuming a power factor of 1.0, this equals 15 kVA.
Step 1: Calculate Motor kVA
For 3-phase loads, the formula is: kVA = (Volts × Amps × √3) / 1000
Motor kVA = (208V × 60A × 1.732) / 1000 = 21.6 kVA
Step 2: Add Single-Phase kVA
Total Base Load = 21.6 kVA (motor) + 15 kVA (lighting) = 36.6 kVA
Step 3: Apply NEC Continuous Load Multipliers
Under NEC Article 215.2 and 450.3, continuous loads (those expected to run for 3 hours or more) require the transformer and overcurrent devices to be sized at 125% of the continuous load. Both the HVAC blower and the commercial lighting qualify as continuous.
Step 4: Select the Standard Size
Our calculated requirement is 45.75 kVA. Looking at the standard sizes table above, the 45 kVA transformer is 0.75 kVA too small. You cannot overload a transformer beyond its nameplate rating without voiding warranties and risking thermal damage. Therefore, we must step up to the next standard size: 75 kVA.
While jumping from 45.75 kVA to 75 kVA feels like oversizing, it is standard industry practice. The extra capacity also provides headroom for motor starting inrush currents (which can be 6x the FLA for a few seconds) and allows for future tenant expansion without replacing the transformer.
Where You Meet This in Practice
On the jobsite, transformer sizing extends far beyond the math on a load calculation sheet. The physical and electrical realities of the kVA rating dictate your installation methodology.
Physical Footprint and Structural Support
A 75 kVA dry-type transformer weighs roughly 750 lbs. If you are mounting it on a wall in an electrical room, you need structural backing and heavy-duty unistrut framing. If it is floor-mounted, the concrete slab must be rated for the point-load. For sizes 150 kVA and above, floor mounting is mandatory, and you must account for seismic bracing in regions like California or the Pacific Northwest.
Impedance and Available Fault Current
Every transformer has an internal impedance, typically stamped on the nameplate (e.g., 5.7% for a 75 kVA unit). This impedance limits the maximum short-circuit current the transformer can deliver to the secondary bus. If you install a 300 kVA transformer with a low 2% impedance, the available fault current on the secondary side might exceed 40,000 Amps. If your downstream panelboards are only rated for 10,000 AIC (Ampere Interrupting Capacity), they will violently fail during a short circuit. Sizing a transformer always requires verifying that the secondary fault current aligns with the interrupting ratings of your downstream breakers.
Harmonic Loads and K-Factor
If your 3 phase transformer size calculation includes heavy concentrations of Variable Frequency Drives (VFDs), LED drivers, or IT server power supplies, standard sizing is not enough. These non-linear loads generate triplen harmonics that circulate in the transformer's neutral and cause excessive core heating. In these environments, you must specify a K-4 or K-13 rated transformer, which features a heavier neutral busbar and specialized winding geometry to dissipate harmonic heat.
Common Confusions and Sizing Mistakes
What is the difference between kW and kVA when sizing?
Kilowatts (kW) measure real power—the actual work being done. Kilovolt-amperes (kVA) measure apparent power, which includes both real power and reactive power (kVAR). Transformers are sized in kVA because their windings must carry the total current regardless of whether that current is doing useful work or just sustaining magnetic fields. If a load has a power factor of 0.80, a 80 kW load will actually draw 100 kVA from the transformer. Always size based on kVA, never kW.
Can I just multiply Volts × Amps for a 3-phase transformer?
No. Multiplying Volts × Amps only works for single-phase circuits. For 3-phase circuits, you must multiply by the square root of 3 (approximately 1.732). Forgetting this multiplier is the most common mistake apprentices make, resulting in a calculated load that is nearly half of what it actually is, leading to a severely undersized transformer.
Do I need to size the primary breaker based on the secondary load?
The primary overcurrent protection is sized based on the transformer's primary full-load amps and the rules in NEC Article 450.3. However, the primary breaker must also be large enough to allow the transformer to energize without tripping on magnetizing inrush current, which can briefly spike to 10 to 12 times the primary FLA. This is why primary breakers are often sized up to 250% of the primary FLA, provided the secondary protection is correctly sized at 125%.
For deeper design criteria, referencing the Schneider Electric Dry-Type Transformer catalog provides excellent derating curves for high-ambient temperatures and altitude adjustments, which can significantly alter your final kVA selection if your installation sits above 3,300 feet or in a room exceeding 30°C (86°F).
