Size transformers by calculating the required kVA from load current, voltage, and phase configuration for single-phase and three-phase installations.
Transformer kVA is apparent power: the product of voltage and current, scaled to thousands. For a single-phase transformer, kVA equals voltage times current divided by 1000. For three-phase, you multiply by the square root of three as well.
Sizing a transformer means matching its kVA rating to the connected load with headroom, then selecting primary and secondary overcurrent protection. The calculator lets you set primary and secondary voltages independently and recommends an overcurrent device.
A three-phase load drawing 40 A at 208 V needs about 40 times 208 times 1.732, divided by 1000, which is roughly 14.4 kVA. You would select the next standard size up, a 15 kVA transformer, giving a small margin for the load.
Transformer overcurrent protection is set by NEC Article 450 and CEC Section 26, which give the maximum primary and secondary protection percentages based on transformer size and impedance. Conductor sizing to and from the transformer follows the ampacity rules in NEC Article 310 and CEC Section 4. These are pointers; verify against the current edition.
For single-phase, multiply voltage by current and divide by 1000. For three-phase, also multiply by the square root of three (about 1.732). The result is the apparent power in kVA.
Calculate the load kVA, then select the next standard transformer size above it to leave headroom. A load of 14.4 kVA would use a 15 kVA transformer.
Primary and secondary protection percentages come from NEC Article 450 or CEC Section 26 and depend on the transformer size and impedance. The calculator recommends a device based on these rules.
Yes. The calculator lets you set primary and secondary voltages and conductor materials independently, since the two sides of a transformer are separate circuits.