TL;DR
Ampacity derating is the mandatory reduction of a conductor's allowable current when real-world conditions trap heat: more than three current-carrying wires bundled in one conduit, ambient temperatures above 30 C, or cables run within insulation. NEC 310.15 supplies the correction factors; for example, seven to nine conductors in a raceway drop each to 70 percent of its table value.
What it means
Ampacity derating is the mandatory reduction of a conductor's allowable current when real-world conditions trap heat: more than three current-carrying wires bundled in one conduit, ambient temperatures above 30 C, or cables run within insulation. NEC 310.15 supplies the correction factors; for example, seven to nine conductors in a raceway drop each to 70 percent of its table value. The practical effect is upsizing wire, common in attic runs, solar circuits crossing hot roofs, and EV charger feeders sharing conduit. Skipped corrections are a frequent rough-in inspection failure.
Where it sits in the glossary
Ampacity derating is part of the Trade jargon group inside the ProFix Directory glossary. Browse every term in this category from the glossary index.
Why Ohio homeowners should know it
This is a term Ohio homeowners encounter when reading contractor quotes, hiring paperwork, or inspection reports. Understanding it well enough to ask one good follow-up question is usually all the protection a homeowner needs.
ProFix Directory keeps definitions short on the index page and saves the longer context — Ohio-specific rules, where the term comes from, and which ProFix tools touch it — for these per-term pages so the term is easy to cite and easy to share.
ProFix tools that touch this term
See also
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