Transformer Sizing Calculator (kVA ⇄ Amps)
Convert between transformer kVA rating and full-load amperage, either direction, single-phase or three-phase.
How this is calculated
A transformer's kVA rating, voltage, and full-load amperage are related by simple power-law math — this is standard electrical calculation, not a code table lookup:
Single-phase: kVA = (V × I) ÷ 1000
Three-phase: kVA = (V × I × 1.732) ÷ 1000
Where V is line voltage, I is full-load current in amps, and 1.732 (√3) accounts for the phase relationship in a balanced three-phase system. This calculator rearranges the formula to solve for whichever value you don't have.
Standard transformer sizes (kVA)
3 · 5 · 7.5 · 10 · 15 · 25 · 37.5 · 45 · 50 · 75 · 100 · 112.5 · 150 · 167 · 225 · 300 · 500 · 750 · 1,000
When sizing for a real installation, it's standard practice to round up to the next standard kVA size above your calculated requirement — plus headroom for future load growth, typically 20-25%, which this calculator does not add automatically.
How this calculator works
kVA = V × I ÷ 1000 (single-phase), × √3 for three-phase, rounded up to standard distribution transformer ratings: US sizes, or in the European mode the ratings of EU Regulation 548/2014, Annex I, Table I.1 (25–3 150 kVA); kW are turned into kVA with kVA = kW ÷ cos φ.
Worked example
A 75 kVA three-phase transformer with a 208 V secondary.
Full-load current = 75,000 ÷ (208 × 1.732) = 208.2 A per phase.
Working the other way: a 150 A three-phase load at 208 V needs 208 × 150 × 1.732 ÷ 1,000 = 54.0 kVA, so the next standard size up is 75 kVA.
Frequently asked questions
How do I convert kVA to amps?
Single-phase: amps = kVA × 1,000 ÷ volts. Three-phase: amps = kVA × 1,000 ÷ (volts × 1.732). For example, a 25 kVA single-phase transformer at 240 V delivers 104.2 A.
Why is kVA used instead of kW?
Transformers are rated in kVA because their heating depends on current and voltage, regardless of the load's power factor. kW is only equal to kVA when the power factor is 1.
How much spare capacity should I leave?
Common practice is to round up to the next standard size and allow about 20–25% for future load growth. This calculator does not add that margin automatically.
Do I use the primary or secondary voltage?
Use the voltage of the side you want the current for. To find the current your load draws, use the secondary voltage; to size the feeder to the transformer, use the primary.
Common mistakes
- Using √3 on a single-phase transformer. Single-phase: amps = kVA × 1,000 ÷ V. Only three-phase divides by 1.732 × V.
- Treating kW as kVA. kVA = kW ÷ power factor. A 40 kW load at 0.8 power factor needs 50 kVA.
- Using the wrong side's voltage. The current on each side depends on that side's voltage. Use the secondary voltage for the load current and the primary for the feeder to the transformer.
- Sizing to the exact load. The calculator rounds up to the next standard rating but adds no growth margin. Allow for future load separately.
What it does not cover
Load diversity, motor inrush, harmonics, ambient derating and NEC Article 450 protection.
Sources
- NFPA 70, National Electrical Code (free read-only access)
- Commission Regulation (EU) No 548/2014, ecodesign requirements for transformers (EUR-Lex)
Sources, checks and limits for this calculator →
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