How to size a distribution transformer (kVA) in five steps
Maximum demand, power factor, growth margin, loading and the next standard rating — a practical method buyers can check in five minutes.
By Xinya Power editorial team
Oversized transformers waste money on no-load loss; undersized ones overheat and age fast. Here is the method our engineers use for a first selection.
Step 1 — Find the maximum demand (kW)
Add up the connected loads and apply diversity: not every motor, heater and light runs at once. Utility bills or 15-minute interval data from an existing site are the best source.
Step 2 — Convert to apparent power (kVA)
kVA = kW ÷ power factor. Industrial sites typically run at 0.8–0.9; with power-factor correction you can design closer to 0.95.
Step 3 — Add a growth margin
Allow 15–30% for future loads unless the site is fixed. It is cheaper to buy the next rating now than to replace a transformer in five years.
Step 4 — Keep loading below about 90%
Divide by 0.9 so the transformer does not run at full load continuously. For heavy harmonic loads (VFDs, rectifiers, data centres) consider a K-rated design or a further margin.
Step 5 — Pick the next standard rating
Standard ratings run 30, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1,000, 1,250, 1,600, 2,000 and 2,500 kVA, then 3,150 kVA and up for power transformers.
Worked example
800 kW maximum demand at PF 0.85 with 20% growth: 800 ÷ 0.85 × 1.2 ÷ 0.9 ≈ 1,255 kVA → choose 1,600 kVA, or two 800 kVA units if redundancy matters.
Then check the details
Impedance (affects fault level and voltage drop), vector group (usually Dyn11), tapping range, cooling, and indoor or outdoor installation. Our online sizing tool does the arithmetic instantly and suggests matching products; an engineer then confirms the selection.
