Transformer Sizing Calculator

Transformer sizing means choosing a transformer whose kVA rating covers your load with the right safety margin. The core math is apparent power: kVA = (kW ÷ power factor) × (1 + margin) ÷ demand factor. This calculator does that math, snaps the result up to the nearest standard size (15 to 500 kVA), and also computes full-load amps, turns ratio, impedance voltage effects, and inrush estimates.

Use I = S ÷ (√3 × V), where S is the kVA rating in VA and V is the line-to-line voltage. Example: a 75 kVA, 3-phase, 480 V transformer gives 75,000 ÷ (1.732 × 480) = 90.21 A per phase. Forgetting the √3 factor is the most common 3-phase mistake — it makes the answer 73% too high.

Use I = S ÷ V. Example: a 25 kVA single-phase 240 V transformer draws 25,000 ÷ 240 = 104.17 A. At 120 V the same 25 kVA would draw 208.33 A — halving the voltage doubles the current, which is why low-voltage runs need heavier conductors and bigger breakers.

Yes — the formulas work in both directions. Single-phase: kVA = amps × volts ÷ 1000. Three-phase: kVA = √3 × amps × volts ÷ 1000. Use the “Amps → kVA” mode in the calculator above; it automatically picks the right formula for 1-phase or 3-phase.

The common dry-type standard sizes are 15, 30, 45, 75, 112.5, 150, 225, 300, and 500 kVA. After computing the required kVA from your load, always round UP to the next standard size — a 93.75 kVA requirement becomes a 112.5 kVA transformer, never a 75 kVA one.

First convert real power to apparent power: kVA = kW ÷ power factor (use 0.8 for mixed motor loads, 1.0 for pure resistive heating). Then apply a 25% margin per common NEC practice for continuous loads and divide by your demand factor. Example: 60 kW at PF 0.8 with demand factor 1.0 → (60 ÷ 0.8) × 1.25 = 93.75 kVA → next standard size 112.5 kVA.

Demand factor is the fraction of connected load expected to run at once. Use 1.0 if everything can run simultaneously, 0.8 for typical commercial panels, and lower (0.5–0.7) for diversified loads like lighting plus receptacles. A realistic demand factor avoids buying a transformer twice as big as you need.

Impedance voltage (%Z) is the percentage of rated voltage needed to drive full-load current through a shorted secondary — typically 2–6% for distribution transformers. It sets the available short-circuit current: Isc = FLA ÷ (%Z ÷ 100). Example: 75 kVA 480 V 3-phase has 90.21 A FLA; with %Z = 3.5, the bolted-fault current is about 2,578 A. Use this for breaker interrupting-rating checks.

When a transformer is first energized, magnetizing inrush can reach 8–12 times full-load current for roughly 0.1 seconds as the core saturates. A 75 kVA 480 V unit (90.21 A FLA) can briefly pull 720–1,080 A. This is why transformer primary protection uses time-delay fuses or breakers with inrush-tolerant trip curves — an instantaneous-only breaker sized at FLA would nuisance-trip on every energization.

Turns ratio links voltages, turns, and currents: Vp ÷ Vs = Np ÷ Ns = Is ÷ Ip. A 480 V to 120 V transformer has a 4:1 ratio. Current transforms inversely: if the secondary carries 100 A at 120 V, the primary carries 100 × 120 ÷ 480 = 25 A at 480 V. Power in equals power out (minus small losses).

Approximate load voltage drop as %Z × load fraction. A transformer with 3.5% impedance running at 50% load drops about 1.75% of rated voltage (8.4 V on a 480 V system). At full load the drop is the full %Z. Keep total system drop within your code limit — typically 5% from service to the farthest outlet.

Current is inversely proportional to voltage for the same kVA. A 112.5 kVA 3-phase transformer draws 312.5 A at 208 V but only 135.3 A at 480 V. Higher distribution voltage means smaller conductors, smaller conduits, and lower I²R losses — the reason industrial plants distribute at 480 V and step down near the load.

Moderate oversizing (one standard size up) is normal and harmless, but gross oversizing wastes money and energy: core (no-load) losses run 24/7 whether loaded or not, and efficiency peaks around 50–80% loading. Size for the real load plus margin, not for the biggest unit you can afford.

This calculator gives engineering estimates from standard formulas, but real installations must follow your local electrical code (NEC Article 450 in the US, or PUIL in Indonesia), utility rules, and short-circuit studies. Have a licensed electrician or engineer verify the final selection, overcurrent protection, and grounding before you buy or install anything.