Transformer Sizing Calculator
Determine the required KVA rating for a transformer based on connected load, voltage levels, load factor, and future growth. Use this when specifying or replacing a transformer for commercial or industrial installations.
Last updated: September 2026
Formula below · 2 sources (ieee.org, Wikipedia) · Updated Sep 2026
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About this calculator
Transformer sizing ensures the unit can handle both present and anticipated future demand without overheating. Transformers are rated in kVA (apparent power), so a real-power load in kW must be divided by the load’s power factor: kVA = kW ÷ PF. The formula used here is: Required kVA = (loadPower × growthFactor) / (powerFactor / 100), where loadPower is the peak (demand) load in kW, growthFactor allows for future load, and the power factor is entered as a percentage (80 for 0.80). For example, 80 kW with 20% growth (1.2) at a power factor of 80% needs 80 × 1.2 / 0.8 = 120 kVA. Round the result up to the next standard transformer rating (e.g., 75, 112.5, 150, 225, 300 kVA three-phase, or 167 kVA single-phase). Enter the demand load, not the sum of every nameplate, if you have a demand figure; using the connected load is the conservative choice.
How to use
Suppose you have a peak load of 100 kW, a power factor of 80%, and a future growth factor of 1.25. Step 1 — Enter 100 kW as the Connected Load. Step 2 — Enter 80 as the Load Power Factor (%). Step 3 — Select 25% Growth (1.25). Step 4 — The calculator applies: Required kVA = (100 × 1.25) / (80 / 100) = 125 / 0.8 = 156.25 kVA. Step 5 — Round up to the next standard size: a 225 kVA three-phase transformer (the standard three-phase sizes are 150 and 225 kVA, so 150 kVA would be too small).
Frequently asked questions
What is load factor and why does it matter when sizing a transformer?
Power factor is the ratio of real power (kW) to apparent power (kVA). A transformer must carry the full apparent power, so a load with a power factor of 0.80 needs 1 / 0.80 = 1.25 kVA of capacity for every kW. Motor-heavy loads often run at 0.80–0.90, while resistive loads are close to 1.0. If you do not know it, 80% is a conservative assumption for mixed commercial loads.
How does a future growth factor affect transformer KVA requirements?
A future growth factor accounts for expected increases in electrical load over the transformer's service life, typically 15–25 years. It is applied as a multiplier (e.g., 1.25 for 25% anticipated growth) to the current connected load before calculating KVA. Failing to include this factor often leads to premature transformer replacement when new equipment is added. Engineers commonly select growth factors between 1.1 and 1.5 depending on the industry.
What is the difference between KW and KVA when sizing a transformer?
Kilowatts (kW) measure real power — the actual work performed by electrical equipment. Kilovolt-amperes (kVA) measure apparent power, which includes both real power and reactive power drawn by inductive or capacitive loads. Transformers are rated in KVA because they must handle the full apparent power, not just the real power. To convert kW to kVA, divide by the power factor (e.g., 100 kW ÷ 0.85 pf = 117.6 kVA).