Transformer Sizing Calculator

Enter the load as kVA, as kW with a power factor, or as a secondary current, add the two voltages and how much spare capacity you want, and this returns the smallest standard kVA rating that covers it along with the primary and secondary full-load currents. A second mode takes the nameplate impedance and returns the bolted fault current at the secondary terminals, either assuming an infinite source or combining with the fault current available at the primary. A third applies NEC Table 450.3(B) to give the largest overcurrent device permitted on each side, with the rule and the arithmetic shown. Everything here is transformer protection at 1000 volts or less; conductor sizing and the secondary tap rules are separate work.