TradesHow this works
A transformer is picked by kVA, but the numbers that matter in the field are amps. This tool turns your connected load into the smallest standard kVA size, then works out the full-load amps on both the primary and secondary and the NEC 450.3(B) primary-only overcurrent device. That gives you the size, the currents to run conductors from, and the breaker in one place.
Watch the headroom. Sizing right at the load is cheapest, but a continuous or growing load wants the next size up so it runs near 80 percent and stays cool. The secondary conductors still need their own protection, and a licensed engineer should sign the final design.
Common questions
How is transformer kVA related to amps?
For three-phase, full-load amps equal kVA times 1000 divided by the line voltage times the square root of three. For single-phase, drop the square root of three. So the same kVA draws very different amps on the primary and secondary because the voltages differ.
Should I add spare capacity?
Usually, yes. A continuous load should sit near 80 percent of the transformer rating, and any growth on a right-sized unit means a full replacement. Stepping up one standard size gives headroom and lets the transformer run cooler, which extends its life.
What protects the secondary?
The primary-only device sized here protects the transformer, not the secondary conductors or panel. Those need their own overcurrent protection under 240.21(C). Treat this tool as a planning starting point and have a licensed engineer complete the design.
Sources: kVA / FLA relations (FLA = kVA*1000 / (V*sqrt3) three-phase); standard kVA ladders; NEC 450.3(B) primary-only OCPD (125% + Note 1) and NEC 240.6(A) standard ratings (NEC). Engine version 1.0.0. Risk tier 2.