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NEC Art.450
Validated against the standard.
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$2.99 Pro
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC Art.450 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Transformer overcurrent protection has its own table, NEC 450.3(B), and the allowed percentages change depending on whether you protect the primary only or both the primary and the secondary. This app calculates the primary and secondary full-load amps, then sizes the protection and conductors correctly for either approach.
The 450.3(B) rules (1000 V and less)
- Primary only (primary current 9 A or more) — protection not over 125% of primary FLA, with a next-standard-size-up allowance.
- Primary and secondary — primary device up to 250%, secondary device up to 125% of the respective FLA.
- Conductors — sized to the transformer FLA per 240.21(C), typically at 125%.
Worked example — 45 kVA, 480 V to 208Y/120, three-phase
Primary FLA = 45,000 / (480 x 1.732) = 54.1 A. Secondary FLA = 45,000 / (208 x 1.732) = 124.9 A.
Primary-only protection: 125% x 54.1 = 67.7 A → next standard size 70 A.
With secondary protection added, the primary may go up to 250% and the secondary device is sized at 125% of 124.9 A.
Related calculators
Size the feeders with the wire size calculator and the grounding with the grounding size calculator.
Transformer FAQ
Primary-only or primary-and-secondary protection?
NEC 450.3(B) allows either. Primary-only protection is limited to 125% of primary FLA. If you add secondary protection, the primary device may go up to 250% and the secondary device up to 125% of its FLA.
Where do the currents come from?
Full-load amps are calculated from the kVA and voltage: for three-phase, FLA = kVA x 1000 / (voltage x 1.732). The app computes both primary and secondary values for you.
How are the conductors sized?
Transformer conductors are sized to the calculated FLA under 240.21(C), typically at 125%, and then chosen from the ampacity tables.
Does it cover single-phase transformers?
Yes. Choose single- or three-phase and the calculator uses the correct formula and the 450.3(B) percentages for each.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
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