Transformer Calculators

Full-load current is the current a transformer delivers when operating at its rated kVA and voltage. It's used to size conductors, select overcurrent protection, and verify that cables and terminations are properly rated.

Start by calculating the required kVA from your load current and voltage, or by totaling the connected load. Then add enough capacity for future expansion, motor starting, and the application's operating conditions. In many cases, the next standard transformer size provides the right amount of headroom, though high harmonic or motor-heavy applications may require additional capacity or a K-rated transformer.

Transformer impedance (%Z) is a measure of how much the transformer resists the flow of current under fault conditions. It directly affects available fault current and voltage regulation. Lower impedance results in higher fault current, while higher impedance reduces fault current but increases voltage drop under load.

These calculators use standard industry formulas and are intended for preliminary sizing, planning, and budgeting. They assume nominal operating conditions and don't account for factors like utility source strength, harmonics, ambient temperature, altitude, or site-specific requirements. Final equipment selection should always be verified by an engineer.

A larger transformer may be appropriate if you expect future load growth, have frequent motor starting, operate in high ambient temperatures, or serve nonlinear or harmonic-producing loads. While oversizing provides additional capacity, it also increases no-load losses, so the goal is to select the right transformer for the application — not simply the largest one.