ToolnozaOverview
Transformer nameplates are commonly expressed in kilovolt-amperes (kVA), which is apparent power. For a current and voltage calculation, power factor is not needed to find kVA; power factor becomes relevant when converting apparent power to real power in kW.
Formula and calculation
Single-phase kVA = volts × amps ÷ 1,000. Balanced three-phase kVA = √3 × line-to-line volts × line amps ÷ 1,000. To solve for current, rearrange the same formula: single-phase amps = kVA × 1,000 ÷ volts; three-phase amps = kVA × 1,000 ÷ (√3 × volts).
Worked example
For a 480 V balanced three-phase load drawing 30 A: kVA = 1.732 × 480 × 30 ÷ 1,000 = about 24.94 kVA. This is the load calculation, not an instruction to select the smallest possible transformer rating.
How to use the calculator
Choose single-phase or balanced three-phase. Enter line voltage and current to calculate kVA, or enter kVA and voltage to estimate current. For three-phase, use line-to-line voltage and line current unless your engineering method explicitly says otherwise.
Common mistakes and practical limits
A kVA result does not check inrush, harmonics, continuous loading, ambient temperature, enclosure, insulation class, impedance, fault current or local electrical code. Motors and nonlinear loads can require extra design margin. Always select equipment using manufacturer and code requirements.
Frequently asked questions
Do I multiply kVA by power factor?
Not to calculate kVA from volts and amps. Power factor is used when converting kVA to kW: kW ≈ kVA × power factor.
Why is √3 used for three-phase?
In a balanced three-phase system, total apparent power from line quantities is √3 × line voltage × line current.
Technical reference
Schneider Electric — Transformer selection formulas
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