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.
About this calculator
Transformer sizing ensures the unit can handle both present and anticipated future electrical demand without overheating or failing. The required KVA is derived from the connected load in kilowatts, adjusted upward by a future growth factor and divided by the load factor (the ratio of average load to peak load). The core formula is: Required KVA = (loadPower × growthFactor) / (loadFactor / 100). For example, if your connected load is 80 kW with a 20% growth factor (1.2) and a load factor of 80%, the formula corrects for the fact that the transformer must handle peak demand, not just average demand. The result is then compared against standard transformer ratings (e.g., 75, 100, 150 kVA) and rounded up to the next standard size. Primary and secondary voltages determine the transformer's turns ratio, which affects winding selection but not the KVA sizing formula directly.
How to use
Suppose you have a connected load of 100 kW, a load 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 Factor (%). Step 3 — Enter 1.25 as the Future Growth Factor. 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: choose a 167 KVA or 200 KVA transformer. Enter your primary and secondary voltages (e.g., 480V / 208V) to confirm current ratings on each winding.
Frequently asked questions
What is load factor and why does it matter when sizing a transformer?
Load factor is the ratio of the average load over a period to the peak load during that same period, expressed as a percentage. A low load factor means the system occasionally spikes to much higher demand than average, so the transformer must be sized for those peaks rather than the average. Ignoring load factor can result in an undersized transformer that overheats during peak demand. Most commercial installations use a load factor between 70% and 90%.
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).