Cable Derating — Bounded US NEC Example
US NEC-related table arithmetic, not BS 7671. Ambient excerpt edition is unverified: this is not a normative ampacity or wire-size recommendation. Supported only: copper 14/12/10 AWG, 1–3 current-carrying conductors in raceway, ambient 30–55°C.
US NEC-related table arithmetic, not BS 7671. Ambient excerpt edition is unverified: this is not a normative ampacity or wire-size recommendation. Supported only: copper 14/12/10 AWG, 1–3 current-carrying conductors in raceway, ambient 30–55°C.
No current calculation
Enter and review the parameters, then calculate. The fixed example below is not your current result.
Method and boundaries
Uses the selected 60/75/90°C copper column and the upper-bound ambient band (31–35, 36–40, 41–45, 46–50, 51–55°C); never nearest-temperature rounding. The estimate is the minimum of corrected table ampacity, selected terminal-column ampacity and the small-conductor limit (14/12/10 AWG: 15/20/30 A), divided by 1.25 only for a continuous-load planning example. Grounding, fault current, voltage drop, installation exceptions and local adoption are not assessed. Verify all applicable code and manufacturer requirements with a qualified electrician.
Fixed worked example — not a live result
Fixed example: 12 AWG copper, 75°C column 25 A, ambient 40°C factor 0.88 => 22 A. A 60°C terminal-column cap and 20 A small-conductor limit give 20 A; continuous-load planning gives 16 A. A 100°C ambient case is unsupported and returns no estimate.
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Worksheet version 2026-10-15. Verify source applicability and all omitted requirements before any real-world decision.