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NEC wire size calculator
Pick the smallest conductor that carries your load per NEC Table 310.16. Handles the termination-temperature rule (NEC 110.14(C)) that most online calculators get wrong — terminations over 100 A are usually 75°C-rated, which limits the effective ampacity column even when your insulation is 90°C-rated.
Recommended conductor
1/0 AWG (copper)
Carries 150 A at the 75°C column. Termination cap for 150 A: 75°C (per NEC 110.14(C)). Your THHN insulation is rated 90°C, but terminations limit the effective column to 75°C.
Important caveats
- NEC Table 310.16 ampacities assume no more than three current-carrying conductors in a raceway and a 30°C ambient. Real installs almost always need conductor-bundle and ambient-temperature derating per NEC 310.15(B).
- This tool is a sanity-check for the common “what size wire for X A” field question. It does NOT replace a stamped engineering design. Verify the final selection with your EOR.
- Continuous loads (≥ 3 hours per NEC 215.2(A)(1)) must be sized to 125% of the load — the calculator above does not apply this; sometimes load entered is already the 125% value.
- Voltage-drop check (NEC-recommended 3% branch / 5% total) is separate — long runs need voltage-drop verification even when ampacity passes.
Built by Forward
Want this answer over text? Forward’s built into the field-foreman workflow.
Forward is a phone number your field crew texts. The bot reads from Procore, Autodesk Construction Cloud, and OneDrive, runs the same NEC + IBC + IMC calcs you see above, and replies with cited answers in seconds. Sheet pulls, RFI status, submittal lookups, change orders, code calcs — no app, no login.
Calculator data: NEC 2023 Table 310.16 ampacities for common insulations, ≤2000 V. Termination-temperature rule per NEC 110.14(C). Source code lives in
apps/api/app/llm/code_lookups.py.