Calculate required transformer kVA, select the next standard size, and determine NEC 450.3 overcurrent protection ratings for primary and secondary.
Transformer sizing begins with the load kVA. The required kVA equals the load apparent power โ calculated from amps times volts divided by 1,000 for single-phase, or amps times volts times โ3 divided by 1,000 for three-phase. Always select the next standard kVA size above the calculated requirement.
Standard dry-type transformer kVA sizes per NEMA: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 45, 50, 75, 100, 112.5, 150, 167, 225, 300, 333, 500, 750, 1000 kVA.
Per NEC Table 450.3(B), the maximum primary overcurrent protection for a transformer rated over 9 amps is 125% of primary FLA when secondary protection is provided, or 250% when primary protection is the only OCP. Always select the next standard fuse or breaker size up.
Secondary OCP is required when the secondary conductors are longer than 10 feet (NEC 240.21(C)(1)), when there is no supervised industrial installation exception, or when the primary OCP cannot protect the secondary conductors due to the turns ratio. Secondary OCP must not exceed 125% of the secondary FLA.
Available fault current at the secondary is approximately kVA รท (โ3 ร V_sec ร Z%). Lower impedance means higher available fault current, requiring higher-rated overcurrent devices and switchgear. Typical dry-type transformer impedance is 2โ6%, while liquid-filled units are often 5.75%.
Only if the turns ratio is 2:1 or less and the primary OCP does not exceed 125% of the primary FLA (NEC 450.3(B) footnote). Otherwise separate primary and secondary overcurrent devices are required.