Transformer Sizing Calculator — kVA for 1φ & 3φ

Short answerkVA = (√3 × V × I) ÷ 1000 for three-phase, or (V × I) ÷ 1000 for single-phase. Size to the connected load plus growth, then round up to the next standard rating.

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How it's calculated

A transformer is rated in kVA (apparent power), independent of power factor. Compute the load in kVA from the line-to-line voltage and full-load current, add margin for future load and inrush, and pick the next standard rating. Undersizing causes overheating and voltage sag; oversizing wastes capital and runs at poor efficiency.

3-phase: kVA = (√3 × V_LL × I) ÷ 1000 1-phase: kVA = (V × I) ÷ 1000 Full-load amps (3φ): FLA = (kVA × 1000) ÷ (√3 × V_LL)

Worked example

208 V, 3-phase, 100 A load.
kVA = (1.732 × 208 × 100) ÷ 1000 = 36 kVA.
+25% growth → 45 kVA → next standard = 45 kVA.

Reference tables

Standard dry-type transformer ratings (kVA)

Small	Medium	Large
15	75	300
30	112.5	500
45	150	750
	225	1000

Full-load amps per kVA (three-phase)

Voltage (L-L)	FLA per kVA
208 V	2.78
240 V	2.41
480 V	1.20
600 V	0.96

A number the tables do not give: at 480 V three-phase, a 45 kVA transformer delivers about 54 A of full-load current.

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FAQ

Why kVA and not kW?

Transformers are limited by voltage and current (heat), not by power factor. kVA captures both; kW would understate the current for a poor power factor load.

How much spare capacity should I add?

A common rule is 20–25% for growth, with more if large motors cause inrush. Continuous loading above ~80% of nameplate shortens life.

Single-phase vs three-phase formula?

Drop the √3 for single-phase: kVA = V × I ÷ 1000. Three-phase multiplies the line-to-line voltage and current by √3.

How do I get full-load amps back out?

FLA = kVA × 1000 ÷ (√3 × V) for three-phase. Use the per-kVA table for a quick estimate.

Does this account for impedance and fault current?

No — it sizes for load. Short-circuit, impedance and protection are a separate engineered study; verify against the NEC and manufacturer data.

This page is a reference aid. Always verify against the current edition of the NEC (NFPA 70) and the authority having jurisdiction. Figures shown are for orientation, not a stamped design.

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