Planning tool

Transformer Sizing & Overcurrent Protection Calculator

Size a transformer and its protection in one place — full-load current, kVA selection, primary and secondary overcurrent protection per NEC 450.3(B) or CEC Section 26, the available secondary fault current for downstream AIC / SCCR, and a NEMA enclosure suggestion. For the US (NEC) and Canada (CEC).

Size your transformer

Describe the transformer and get sizing, protection & fault current.

Enter a kVA rating or a load to size it. Every result is shown on this page — no sign-up required.

1 · Transformer

Code reference sets the protection table (NEC 450.3(B) for the US, CEC Section 26 for Canada); the percentages apply to systems 1000 V and less. Construction sets the typical impedance used for the fault-current estimate.

2 · Optional

Good to know

Transformer sizing, protection & fault current

Common questions about full-load current, NEC 450.3(B) / CEC Section 26 overcurrent protection, available fault current and enclosure selection — for the US (NEC) and Canada (CEC).

What size transformer do I need for a given load?
Enter the load in kVA, amps or kW (with power factor) and the calculator divides it by a design loading factor (0.80 by default) and rounds up to the next standard kVA. As a rough guide, a 220 kVA load points to a 300 kVA transformer; a 90 kVA load points to a 112.5 kVA transformer. It also returns the primary and secondary full-load current so you can size the upstream and downstream gear. Confirm the final rating, taps and temperature rise for your application.
How do you calculate transformer full-load current (kVA to amps)?
For a three-phase transformer, full-load current I = kVA × 1000 ÷ (V × √3); for single-phase, I = kVA × 1000 ÷ V. For example, a 300 kVA three-phase transformer with a 208 V secondary draws about 833 A on the secondary, and about 289 A on a 600 V primary. The calculator returns both winding currents from the kVA and the two voltages.
How is transformer overcurrent protection sized under NEC 450.3(B)?
For transformers 1000 V and less, NEC Table 450.3(B) sets the maximum overcurrent device as a percentage of the winding’s rated current. With primary protection only: 125% where the primary current is 9 A or more (the next standard size up is permitted by Note 1), 167% for 2–9 A, and 300% below 2 A. With both primary and secondary protection: up to 250% on the primary and 125% on the secondary (167% below 9 A). The calculator applies the table to the full-load current and selects the standard device from NEC 240.6(A).
What fuse or breaker size do I need for a 300 kVA transformer?
Take the winding full-load current and apply the NEC 450.3(B) percentage. A 300 kVA, 600 V three-phase primary draws about 289 A; with primary-only protection at 125% that is about 361 A, so the next standard device is 400 A (Note 1). The exact device depends on inrush, coordination and whether the secondary is separately protected — the calculator shows the maximum and the recommended standard size for both windings.
How do I find the available fault current at a transformer secondary?
The infinite-source estimate is the secondary full-load current divided by the per-unit impedance: I_sc = secondary FLA ÷ (%Z ÷ 100). For a 300 kVA, 208 V secondary (≈833 A) at 5% impedance, that is roughly 16.7 kA. Enter the nameplate %impedance and the calculator returns the available fault current and the next standard interrupting rating, so you can confirm the AIC of downstream breakers and the SCCR of panelboards and switchgear. A finite utility source lowers it and motor contribution raises it — confirm with a full short-circuit study.
Does the calculator work for Canada (CEC Section 26)?
Yes. Switch the code reference to CEC 2024 and the protection percentages follow CSA C22.1 Section 26 instead of the NEC. For dry-type transformers 750 V and under, Rule 26-254(1) caps the primary device at 125% of rated primary current; Rule 26-254(2) allows the primary feeder to go up to 300% when a secondary device is set at no more than 125% — versus the NEC ceiling of 250% on the primary. Liquid-filled and over-750 V units fall under 26-250 / 26-252, so confirm those against CSA C22.1 and your AHJ / ESA. This makes it one of the few transformer sizing tools that covers both NEC and CEC.
Is this a code-compliant calculation I can submit for permit?
No — it is a free estimate for early planning, not a permit or construction document. Overcurrent percentages, conductor ampacity, secondary-conductor protection (NEC 240.21(C)), grounding and a full short-circuit study are separate design steps. Use it to scope and sanity-check, then confirm the numbers with a registered engineer (PE / P.Eng) and your AHJ / ESA.

The method behind the numbers

These guides explain what the calculator computes and why — useful for a submittal, a spec or learning the rules.