Transformer Sizing & Full-Load Current Calculator

A transformer's full-load current on either side follows directly from its kVA rating and that side's voltage: I_FL = S / (√3 · V) for three-phase, or S / V for single-phase. This calculator computes both primary and secondary full-load current from a single kVA rating, and — since transformers are manufactured in a limited set of standard sizes — checks your entered rating against the standard three-phase or single-phase distribution transformer size list and suggests the next size at or above it.

Use it to size overcurrent protection and conductor ampacity on both sides of a new transformer, to check a nameplate FLA figure, or to see how much headroom a standard size gives you over a calculated load. Full-load current is also the base value most transformer protection settings (like 125–250% of FLA for primary fuses) are built from.

This calculator computes rated full-load current only — it doesn't account for inrush current (which can reach 8–12× FLA for a fraction of a second at energization and must be considered when selecting fuse or breaker time-current characteristics), or for any derating from ambient temperature, altitude, or non-standard duty cycle. Confirm the final kVA selection against the actual connected load with diversity and future growth factored in, not just the calculated full-load current alone.