Sizing a three-phase transformer comes down to one core formula: multiply your line-to-line voltage by your full-load amperage by 1.732 (the square root of 3), then divide by 1,000 to get your required kVA rating. That number is your starting point, but the actual transformer you select will be larger once you account for motor inrush, harmonic loads, altitude, ambient temperature, and future growth.
The Basic kVA Formula
Three-phase power differs from single-phase because three separate AC lines operate 120 degrees out of phase from each other. They don’t all deliver peak power at the same instant, which is why the constant 1.732 (√3) appears in every three-phase calculation. The formula is straightforward:
Volts × Amps × 1.732 ÷ 1,000 = kVA
Use line-to-line voltage (not line-to-neutral) and the maximum expected load current. For example, if you have a 480V system drawing 300 amps at full load: 480 × 300 × 1.732 ÷ 1,000 = 249.4 kVA. You’d then select the next standard size up, which is typically 250 kVA.
If you already know your total load in kilowatts, you can convert to kVA by dividing by your power factor. A facility running at 0.85 power factor with 200 kW of load needs roughly 235 kVA (200 ÷ 0.85).
Accounting for Motor Loads and Inrush
Motors are the trickiest loads to size for because they draw far more current during startup than during normal operation. A motor starting across the line can pull six to eight times its full-load current for several seconds. If you have multiple large motors that may start simultaneously, the transformer needs enough capacity to handle those surges without excessive voltage drop.
The general best practice for overcurrent protection devices on the transformer side is to rate them at 125% to 150% of the transformer’s full-load current. But the transformer itself should be sized so that combined motor inrush doesn’t sag voltage below acceptable limits for other connected equipment. For facilities with several large motors, oversizing the transformer absorbs those transient surges while keeping voltage and frequency stable across the system. If your load is predominantly motors, adding 25% to 30% above your calculated kVA is a reasonable starting point, though a detailed motor schedule with starting sequences gives you a more precise answer.
Harmonic Loads and K-Factor Ratings
Non-linear loads like variable frequency drives, UPS systems, LED lighting, and computers generate harmonic currents that cause extra heating inside a transformer. A standard transformer rated for purely resistive loads will overheat and fail prematurely if it’s feeding a building full of servers or electronic equipment. This is where K-factor ratings come in.
K-factor is a multiplier that tells you how much additional harmonic heating a transformer can handle. The right rating depends on what you’re powering:
- K-4: Mixed loads with some computers, light use of variable speed drives, and electronically controlled lighting. Typical for general office spaces.
- K-9 or K-13: Heavier electronic content. Schools, hospitals, and commercial buildings with significant drive or UPS installations.
- K-20: Data centers, critical care facilities, and mission-critical UPS environments with sustained high harmonic content.
- K-30 to K-50: Severe harmonic environments, engineered case by case based on actual power quality measurements.
If you’re unsure of your harmonic profile, a portable power quality analyzer can measure distortion over a representative operating period. That data lets you calculate a site-specific K-factor rather than guessing. Choosing too low a K-factor means premature transformer failure. Choosing too high wastes money on a more expensive unit you don’t need.
Percent Impedance and Voltage Drop
Every transformer has an internal impedance, expressed as a percentage (%Z), that determines how much voltage drops between no-load and full-load conditions. A transformer with lower impedance delivers more consistent voltage to your equipment but allows higher fault currents. Higher impedance limits fault current (which can simplify your protective device ratings) but causes more voltage sag under heavy loads.
For most commercial and industrial applications, standard impedance values work fine. But if you’re feeding voltage-sensitive equipment like CNC machines, medical imaging, or precision manufacturing tools, you’ll want to pay attention to this spec. A transformer with unnecessarily high impedance on a long feeder run can push voltage at the load below acceptable limits, especially during motor starting events. If voltage regulation is critical, select a transformer with lower impedance or size up to reduce the percentage of full-load current you’re actually drawing.
Derating for Altitude and Temperature
Transformers are rated for operation at or below 3,300 feet (1,000 meters) above sea level and at a standard ambient temperature of 40°C (104°F). If your installation exceeds either threshold, the transformer can’t dissipate heat as effectively and you need to derate its capacity.
For altitude, reduce the nameplate kVA by 0.3% for every 330 feet above 3,300 feet. A facility at 6,600 feet, for instance, sits 3,300 feet above the baseline. That’s 10 increments of 330 feet, so you’d derate by 3%. A transformer rated at 500 kVA effectively becomes a 485 kVA unit at that elevation. This sounds small, but it compounds with temperature derating if your transformer sits in a hot mechanical room or an unconditioned desert enclosure.
If your ambient temperature regularly exceeds 40°C, you’ll need to either upsize the transformer or specify one with a higher temperature rise rating. Manufacturer data sheets typically include derating curves for both factors.
Planning for Future Load Growth
Installing a transformer that exactly matches today’s load is a common and expensive mistake. Adding capacity later means replacing the transformer entirely, along with potentially upsizing switchgear, conduit, and cabling. Industry practice varies, but engineering studies that optimize for total lifecycle cost, including both no-load and load-dependent losses, commonly apply a 40% margin above projected maximum load to accommodate future demand.
For most commercial and light industrial projects, a 20% to 25% margin is a practical minimum. If you know specific expansions are planned (additional production lines, building additions, EV charging infrastructure), size for those loads now. The incremental cost of a slightly larger transformer at installation is a fraction of the cost of a full replacement in five years.
Overcurrent Protection and Code Requirements
NEC Article 450 governs transformer overcurrent protection. Table 450.3(A) covers transformers with primary voltages over 1,000 volts. On the primary side, protection can be set up to 300% of the transformer’s rated current for certain configurations. When your calculation lands on a non-standard device size, the code allows you to round up to the next standard rating for devices at 1,000 volts and below, or to the next commercially available rating for devices above 1,000 volts.
Standard overcurrent device sizes are listed in NEC Section 240.6, but only for devices rated 1,000 volts and below. Above that threshold, “standard” sizes aren’t codified, so you use whatever is commercially available from your manufacturer. This distinction matters when you’re specifying medium-voltage primary protection: your protective device selection depends on what’s actually on the market, not a fixed table of sizes.
Putting It All Together
A practical sizing process follows these steps in order. First, calculate your base kVA from voltage, amperage, and the 1.732 constant. Second, adjust upward for motor inrush if you have significant motor loads. Third, determine your K-factor requirement based on harmonic content. Fourth, apply derating factors for altitude or high ambient temperature. Fifth, add your growth margin. The final number points you to the correct standard transformer rating.
As a quick example: you calculate a base load of 400 kVA on a 480V system. Your facility runs moderate variable frequency drives, so you specify K-13. You’re at 5,000 feet elevation, so you derate by about 1.5%, meaning you need roughly 406 kVA of effective capacity. Adding a 25% growth margin brings you to approximately 508 kVA. You’d select a 500 kVA unit if the derating and growth math is conservative, or step up to 750 kVA (the next common standard size) if you expect meaningful load additions. Each decision in the chain stacks on the previous one, which is why starting with an accurate load inventory matters more than any single formula.