Cable Sizing Derating
A deeper, standalone explanation of the grouping derating concept, with the thermal reasoning spelled out.
Grouping derating gets mentioned throughout this series as one of the two universal correction factors — here's the underlying thermal reasoning spelled out on its own.
A current-carrying conductor generates heat proportional to its resistance and the square of its current (I²R losses). A single cable in free air sheds that heat efficiently into the surrounding environment. Bundle several current-carrying conductors together in the same conduit or tray, and each one is now partly surrounded by other heat-generating conductors instead of open air — reducing each individual conductor's ability to shed its own heat.
More conductors sharing a confined space means proportionally less "clear" surface area per conductor for heat to escape through — which is why grouping correction factors get progressively smaller (more restrictive) as the number of current-carrying conductors in the group increases.
A cable rated at, say, 40A in free air might have a meaningfully lower real capacity once installed as one of six current-carrying conductors sharing a single conduit — the table's headline figure was never describing that grouped condition.
Every OhmWorks cable sizing calculator applies this correction automatically based on your entered conductor count.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks