Transformer kVA Sizing Calculator
Transformer size, primary and secondary current, NEC 450.3(B) protection and the available fault current that decides every rating downstream.
The primary breaker is allowed to be much bigger than you expect
NEC 450.3(B) permits the primary device to reach 250% of primary full-load current when there is secondary protection as well, against 125% when there is not. That looks reckless and is not. Magnetising inrush on energisation runs eight to twelve times full-load current for a few cycles, so a device sized tightly at 125% trips every time the transformer is switched on. The overcurrent device is protecting the conductors; the transformer is protected by the secondary device and by its own thermal mass.
- Rated in kVA, not kW, because the transformer has no idea what power factor you will present it with. Its limit is winding current and core heat, and both follow apparent power. Sizing from kW at 0.8 power factor undersizes by a quarter.
- Lower impedance means MORE fault current, not less. A stiffer transformer holds its voltage better under load and delivers a harder short. Available fault current at the secondary is roughly full-load amps ÷ per-unit impedance — a 75 kVA unit at 5% can push over 4,000 A into a 208 V panel that draws 208 A in service.
- Do not oversize much. Core losses are essentially constant and run 24 hours a day whether the unit is loaded or not, so a transformer bought "for the future" wastes energy continuously for its entire life.
- Non-linear loads need a K-rated unit. Switching supplies, VFDs and LED drivers heat a transformer far more than the same RMS current of a clean sine wave, because eddy losses climb with the square of frequency. Using a standard transformer there is an overheating risk, not an efficiency question.
A planning aid, not a design. Secondary conductor protection under the 240.21(C) tap rules, grounding and bonding of the separately derived system, and the equipment listing all matter and are outside what any calculator settles.
How to use
- Enter the connected load in kVA, or in kW with a power factor.
- Set the primary and secondary voltages and the nameplate impedance.
- Read the next standard size, then the primary and secondary currents.
- Check the available fault current — it decides interrupting ratings downstream.
Frequently asked questions
Why can the primary breaker be so much larger than the transformer?
Because magnetising inrush on energisation runs eight to twelve times full-load current for a few cycles, so a device sized tightly at 125 percent trips every time the transformer is switched on. NEC 450.3(B) permits up to 250 percent of primary current where there is secondary protection as well. The key point is what the device is for: the primary breaker protects the conductors, while the transformer is protected by the secondary device and by its own thermal mass.
Why is a transformer rated in kVA rather than kW?
Because it has no idea what power factor you will present it with. Its limits are winding current and core heating, and both follow apparent power rather than real power. A transformer feeding a load at 0.8 power factor carries 25 percent more current than the kW figure suggests, so sizing from kilowatts undersizes it by a quarter. The plate says kVA for exactly this reason.
What is available fault current and why does it matter?
It is how much current a bolted short at the secondary terminals would draw, and it is roughly full-load amps divided by the per-unit impedance. A 75 kVA unit at 5 percent impedance can deliver over 4,000 amps into a 208 volt panel that only draws 208 amps in normal service. Every breaker and panel downstream must have an interrupting rating above that figure — a device rated below it is not protection, it is a hazard.
Does lower impedance mean less fault current?
No, the opposite, and it catches people out. Impedance is what limits current under fault conditions, so a low-impedance transformer is stiffer, holds its voltage better under load, and delivers a harder short circuit. Halving the impedance doubles the available fault current. If you replace a transformer with a more efficient low-impedance one, the downstream equipment ratings need rechecking.
Should I oversize for future growth?
A little, not a lot. Core losses are essentially constant and run 24 hours a day whether the transformer is loaded or not, so a unit bought at twice the size it needs wastes energy continuously for its entire service life. A 25 percent allowance is sensible; running at 20 percent of rating for twenty years is an expensive way to plan ahead.
What happens if I run it near its rating?
It works, and its insulation ages faster. Transformers tolerate short overloads well because of their thermal mass, but sustained operation near the nameplate raises winding temperature, and the usual rule of thumb is that every 8 to 10 degrees Celsius of extra winding temperature halves insulation life. Somewhere between 50 and 80 percent loading is the comfortable range.
Do I need a K-rated transformer?
If the load is substantially electronic, yes. Switching power supplies, variable frequency drives and LED drivers draw non-sinusoidal current, and the harmonics heat a transformer far more than the same RMS current of a clean sine wave because eddy losses climb with the square of frequency. Feeding a heavily electronic load from a standard transformer is a genuine overheating risk rather than an efficiency question.
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