Transformer Sizing Calculator
Convert load volts and amps into the kVA rating a transformer needs.
Transformer selection must account for impedance, inrush, overcurrent protection per NEC Article 450 and grounding. This is an estimating tool only — use a licensed electrical engineer or electrician.
How this calculator works
kVA = volts × amps × phase multiplier ÷ 1000
Standard kVA ratings are 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 — round up to the next one. Transformers tolerate brief overload but run hot and lose life expectancy when continuously loaded past their rating.
Worked example
With secondary voltage 480 , load current 100 , phase multiplier 1.732 , spare capacity factor 1.25 , this calculator returns 103.92 .
Frequently asked questions
How do I calculate transformer kVA?
Multiply secondary voltage by load current, then by 1.732 for three-phase, and divide by 1,000. A 480 V three-phase load drawing 100 amps needs 83 kVA before any allowance for growth.
How much spare capacity should a transformer have?
Twenty to 25% is typical, which also puts the unit nearer its efficiency sweet spot. Oversizing beyond that wastes money and increases no-load losses that run continuously.
What are standard transformer sizes?
Dry-type units come in 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 kVA. Always round up — running a transformer above its nameplate shortens insulation life sharply.
Why is three-phase multiplied by 1.732?
It is the square root of three, which comes from the 120 degree phase relationship between the three legs. Three-phase delivers 1.732 times the power of single-phase at the same line voltage and current.