NEC · 2020 National Electrical Code
§310.15 · Ampacity
Effective in NEC 2020 · NEC 2023 NW Indiana.
Plain-English explainer
310.15(B) sets conductor ampacity: apply the temperature-correction and ampacity-adjustment factors of 310.15(B)(2)(a) and 310.15(B)(3)(a). More than three current-carrying conductors in a raceway or cable triggers an adjustment factor on the table value.
Why inspectors flag this
Adjusting for >3 CCCs in a conduit or for ambient temperature is the most common ampacity mistake; the drafter lifts the rule so the conductor schedule carries the corrected values.
As quoted in the code: Conductor ampacity is determined from Table 310.15(B) with the temperature-correction and ampacity-adjustment factors of 310.15(B)(2)(a) and 310.15(B)(3)(a) applied; more than three current-carrying conductors in a raceway or cable triggers an adjustment factor on the table value.
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