Transformer Sizing Formula
The rating of a transformer is mathematically measured in kVA (kilo-volt-amperes). The calculation depends on whether the system is single-phase or three-phase.
A safety factor (typically 1.2 or 1.25) is multiplied to the final result to guarantee that the transformer is not operated at 100% full capacity constantly, preventing overheating and allowing room for future minor expansions.
Frequently Asked Questions
For three-phase systems, calculate the kVA by multiplying the line-to-line voltage (V) by the line current (I) and by the square root of 3 (1.732), then divide by 1000. Recommend adding a 20-25% safety margin.
Single-phase: kVA = (Load kW / Power Factor). Three-phase: kVA = (Load kW / Power Factor), where Load kW is the total active load. For three-phase from line voltage and current: kVA = √3 × V_line × I_line / 1000. Always add 20–25% safety margin and select the next standard IEC or ANSI kVA rating above your calculated requirement.
kW (active power) is the actual useful work done by the load. kVA (apparent power) is the total electrical demand the transformer must supply, including reactive power from inductive loads. kVA = kW / Power Factor. Transformers are rated in kVA because they must handle apparent power — a 100kVA transformer at PF=0.8 delivers only 80kW of useful power.
IEC standard single/three-phase distribution transformer ratings (kVA): 25, 50, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150 kVA. ANSI/IEEE (North America) ratings: 37.5, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA. Always select the next rating above your calculated kVA requirement.
Percentage impedance (%Z) is a transformer nameplate value (typically 4–6% for distribution transformers) that determines fault current at the secondary terminals. Maximum secondary fault current: Isc = Irated / (%Z/100). A 1000kVA, 400V, 5%Z transformer: Irated = 1443A; Isc = 1443 / 0.05 = 28,860A. Lower %Z = higher fault current but better voltage regulation under load.
Transformer efficiency peaks at 50–75% of rated load (when copper losses equal iron losses). Loading above 100% causes: rapid temperature rise (transformer life halves for every 10°C above rated temperature); accelerated insulation degradation; potential failure. IEEE C57.91 provides transformer thermal modelling. Most utilities design for 70–80% average load factor to maximise transformer service life (target >25 years).