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NEC Art.240
Validated against the standard.
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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.240 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
An overcurrent device has to protect the conductor and carry the load without nuisance tripping. This app sizes breakers and fuses using the standard ratings in NEC 240.6(A), the continuous-load rule, and the conductor-protection rules in 240.4 — including the small-conductor limits and the next-size-up allowance.
The rules it applies
- Continuous load (210.20(A) / 215.3) — the device must be at least 125% of the continuous load plus 100% of the non-continuous load.
- Standard sizes (240.6(A)) — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A, and so on.
- Conductor protection (240.4) — the device cannot exceed the conductor ampacity, except the next-size-up allowance in 240.4(B) for 800 A and below.
- Small conductors (240.4(D)) — 15 A max on #14, 20 A on #12, 30 A on #10 copper, regardless of ampacity.
Worked example
A 40 A continuous load. Required device = 125% x 40 = 50 A.
The conductor must carry at least 50 A, so #8 copper at 75°C (50 A) works. The 50 A breaker is a standard size in 240.6(A), so no rounding is needed.
The app also handles the next-size-up case: a 38 A non-continuous load on a conductor rated 40 A can use a 40 A device under 240.4(B).
Related calculators
Coordinate with the wire size calculator and start from the demand with the electrical load calculator.
Breaker size FAQ
How do continuous loads change the size?
For a continuous load (three hours or more) the overcurrent device and the conductor must be sized at 125% of that load, plus 100% of any non-continuous load on the same circuit.
What are the standard breaker sizes?
NEC 240.6(A) lists the standard ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and up. The app always lands on a standard size.
What is the next-size-up rule?
Under 240.4(B), when the conductor ampacity does not match a standard device rating, you may use the next standard size up — for devices 800 A and below and where the circuit has no receptacle outlets for cord-and-plug loads.
Why can't a #12 always take more than 20 A?
The small-conductor rule in 240.4(D) caps protection at 15 A for #14, 20 A for #12, and 30 A for #10 copper, regardless of the higher ampacity in the tables.
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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