Transformer Sizing Calculator helps determine the appropriate transformer rating based on load current, active power, or apparent power. It applies demand factors and design margins, and estimates kVA, kW, and load current in the early stage of electrical design.
About transformer sizing. Transformers are rated in apparent power (kVA), which represents the total power they can deliver regardless of load power factor. Proper sizing ensures the transformer can handle both the current and heat generated by the load without overloading. Undersized transformers can overheat and fail prematurely, while oversized units increase cost and losses.
About power factor. Power factor (PF) is the ratio between real power (kW) and apparent power (kVA). Low power factor indicates more current for the same useful power, which affects transformer current loading. When sizing from active power (kW), you must divide by the power factor to get apparent power (kVA). When sizing from current, PF is only used to estimate the equivalent real power (kW).
About demand and design margins. Demand (or diversity) factor accounts for the fact that not all connected loads operate at full power simultaneously. It is applied as a percentage reduction to the input load. A design margin is then added to the resulting base transformer size to allow for future load growth, temperature rise, and safety buffer.
About single-phase and three-phase systems. In single-phase systems, apparent power is calculated from voltage and current using S = V × I. In three-phase systems, the formula becomes S = √3 × V × I for line-to-line voltage and line current. The calculator handles both systems and shows estimated current if voltage is known.
Calculation. The calculator uses the following formulas depending on the selected workflow:
By Current: Base kVA = (K × V × I × DF) / 1000; Estimated kW = Base kVA × PF
By Power: Base kVA = (kW × DF) / PF; Estimated kW = kW × DF
By Apparent: Base kVA = kVA × DF; Estimated kW = Base kVA × PF
Recommended kVA = Base kVA × (1 + Margin/100)
Where: K = √3 for 3-phase or 1 for 1-phase, DF = demand factor (fraction), PF = power factor (0–1).
- Example 1: 3φ, 400 V, 80 A, PF 0.9, DF 100%, Margin 15% → Base ≈ 49.9 kVA → Recommended ≈ 57.4 kVA
- Example 2: 3φ, 400 V, 50 kW, PF 0.85, DF 80%, Margin 20% → Base ≈ 58.8 kVA → Recommended ≈ 70.5 kVA
- Example 3: 1φ, 230 V, 63 kVA, DF 90%, Margin 15% → Base ≈ 56.7 kVA → Recommended ≈ 65.2 kVA
Corresponding tools. Use the Breaker Size Calculator to select protective devices, the Voltage Drop Calculator to verify conductor sizing, and the Motor Circuit Calculator for dedicated motor loads.
