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NEC Art.310
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.310 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
Picking a conductor is more than reading one number off a chart. You start with the ampacity in NEC Table 310.16, then respect the temperature rating of the terminals (110.14(C)), then derate for high ambient temperature and for bundling more than three current-carrying conductors. This app does all of that and gives you the smallest legal wire for the job.
The three checks
- Ampacity (310.16) — the base current a conductor can carry at a given insulation temperature (60, 75, or 90°C column).
- Termination rating (110.14(C)) — most equipment is listed for 60°C up to 100 A or 75°C, so you cannot use the 90°C column for the final size even if the insulation allows it.
- Derating (310.15) — multiply by the ambient-temperature factor and the conductor-count factor; the 90°C column is used as the starting point for derating only.
Worked example
A 40 A load, THHN copper, 75°C terminals, six current-carrying conductors in the raceway.
#8 THHN has a 90°C ampacity of 55 A. Derate for 4–6 conductors (80%): 55 x 0.80 = 44 A — still above the 40 A load.
Check the terminals: #8 at 75°C = 50 A, also above 40 A. #8 copper passes on both the derated ampacity and the termination limit.
The app covers copper and aluminum, THHN/THWN-2 and other insulations, ambient temperatures, and conductor counts, and flags when the termination temperature — not the insulation — is the limiting factor.
Related calculators
Confirm the run length with the voltage drop calculator and check the raceway with the conduit fill calculator.
Wire size FAQ
Which temperature column should I use?
Size the final conductor from the column that matches your terminal rating — usually 60°C for breakers and lugs up to 100 A, or 75°C. The 90°C column is only the starting point for applying derating factors.
When do I have to derate?
Derate for ambient temperatures other than 30°C using Table 310.15(B)(1), and for more than three current-carrying conductors in a raceway using Table 310.15(C)(1). The app applies both automatically.
Does it handle aluminum?
Yes. Choose copper or aluminum and the calculator uses the correct ampacity values from Table 310.16, including the smaller selections aluminum requires.
Does it account for continuous loads?
Yes. You can mark a load as continuous and the app applies the 125% factor so the conductor and the overcurrent device are coordinated.
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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