Transformer kVA Sizing Calculator
Size a single- or three-phase transformer from load kW and power factor, amps or kVA: required kVA with demand factor, growth and altitude derating, the next standard ANSI or IEC rating, full-load currents, NEC 450.3 overcurrent limits, fault current and motor-starting voltage dip.
How it works
kVA = kW ÷ PF, or (√3 ×) V × I ÷ 1000 · required = kVA × demand × (1 + growth) ÷ (1 − altitude derate) · size = next standard rating ≥ required · I_FL = kVA × 1000 ÷ ((√3 ×) V) · I_fault ≈ I_FL ÷ Z · NEC 450.3 max OCPD = % × I_FL
Transformers are rated in kVA, not kW, because what heats them is current, whatever the power factor of the load — a motor load at 0.85 power factor draws 18% more current than its kilowatts suggest. Add up the load in kVA (or convert it from kW or amps), apply a demand factor if not everything runs together, add margin for growth, and round up to the next standard rating: North American three-phase units come in 75, 112.5, 150, 225, 300, 500 kVA and so on, while IEC countries use 100, 160, 200, 250, 315, 400, 630 kVA and similar. Running at 50–80% of nameplate gives a good balance of efficiency and headroom. Above 1,000 m the thinner air cools the windings less, so the usable rating falls by about 0.3–0.4% per 100 m. The nameplate kVA then fixes the full-load currents on both windings, which set conductor sizes and the maximum fuse or breaker ratings in NEC Table 450.3 — Table 450.3(A) when the primary is over 1,000 V, Table 450.3(B) at 1,000 V and below. The impedance on the nameplate limits the fault current the secondary can deliver and sets how far the voltage dips when a large motor starts.
Worked example
120 kW at 0.85 power factor is 141.2 kVA; with 25% growth that is 176.5 kVA, so on an 11 kV / 415 V three-phase system the next IEC rating is 200 kVA (an ANSI list would give 225 kVA). Full-load currents are 200,000 ÷ (1.732 × 415) = 278 A on the secondary and 10.5 A on the primary, and the unit runs at 71% today. Because the primary is over 1,000 V, NEC Table 450.3(A) allows a primary breaker up to 600% (63 A → 70 A) or fuses up to 300% (31.5 A → 35 A) for a 5% impedance unit, with secondary protection up to 125% of 278 A = 348 A → 350 A. With 5% impedance the secondary can deliver about 5.6 kA of fault current.
Sources: NFPA 70 (NEC) — Article 450.3, overcurrent protection of transformers
Frequently asked questions
What about motor starting?
A motor started across the line draws about 6 times its running current — roughly 6 kVA per horsepower — for several seconds. Enter the largest motor under More options to see the voltage dip: under about 10% is usually fine, more can drop out contactors and flicker lights. A soft starter or VFD cuts the inrush to 2–3 times running current.
Can I run a transformer above its rating?
Briefly, within its thermal time constant, and continuously by a few percent if the ambient is cool (liquid-filled units have overload curves in IEEE C57.91). Sustained overload shortens insulation life quickly — roughly every 6–8 °C of extra hot-spot temperature halves it.
How big a fuse or breaker can protect a transformer?
In the US, NEC Table 450.3(B) (1,000 V and below) allows 125% of primary full-load current if the primary device is the only protection, or up to 250% on the primary when the secondary is also protected at 125%. Over 1,000 V, Table 450.3(A) allows larger primary devices — up to 600% for a breaker or 300% for fuses on a unit of 6% impedance or less. The calculator works out both and picks standard device sizes from NEC 240.6(A); conductors still need their own protection.
Why is a transformer rated in kVA instead of kW?
Because heating depends on the current, and the current depends on apparent power (kVA), not only on real power (kW). The same kW at a lower power factor needs more kVA, so the transformer must be bigger.
Should I pick an ANSI or an IEC size?
Use the series sold where you are. North American manufacturers build to ANSI/NEMA ratings (45, 75, 112.5, 150, 225, 300, 500 kVA…), while most other countries use IEC ratings (100, 160, 200, 250, 315, 400, 500, 630 kVA…). The difference matters near a boundary: 176 kVA of load gets a 200 kVA IEC unit but a 225 kVA ANSI unit.
Related calculators
Educational estimate. Electrical, plumbing and HVAC work must comply with the local code (NEC/CEC/IEC, IPC/UPC) and be checked by a licensed professional; safety limits (NIOSH, ACGIH, OSHA) are guidance, not a substitute for a workplace assessment.