How this calculator works
Transformer thermal loading is described by apparent power in volt-amperes, rather than real watts alone. Single-phase apparent power is voltage times current, while a balanced three-phase load uses the square root of three multiplier with line quantities. The spare percentage is added to present load, so 25% spare means 1.25 times current VA, not 75% utilisation. Example catalogue sizes are illustrative; starting currents, harmonics, cooling and manufacturer derating need separate assessment.
Formula
Ssingle = VI; Sthree = sqrt(3) Vline Iline; Srequired = Sload(1+allowance); kVA = VA/1000.
Worked example
A single-phase 120 V, 100 A load is 12000 VA or 12 kVA. Adding 25% load allowance gives 15000 VA or 15 kVA.
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Frequently asked questions
Is the spare percentage a utilisation limit?
No. It adds a fraction of the present load. A 25% addition corresponds to 80% use of the resulting capacity.
Can I size a motor supply from running current alone?
Starting current and allowable voltage dip can require a larger transformer; check the manufacturer's application data.
Is the suggested size a complete equipment recommendation?
No. It is the next example catalogue rating and does not check voltage ratio, impedance, protection or installation conditions.