Cable Sizing Derating
A cable’s headline ampacity rating is a best-case number. Here is what actually reduces it in a real installation, and why both corrections matter together.
Every cable ampacity table has a headline number for each conductor size — but that number is a best-case figure measured under specific, standardized reference conditions. Real installations almost never match that reference exactly, which is why derating factors exist and why skipping them is such a common source of undersized cable.
Cable ampacity tables assume a specific reference ambient temperature, commonly 30°C. As actual installation temperature rises above that reference, the cable's real capacity to shed heat drops — and since current-carrying capacity is fundamentally about how much heat the conductor can safely dissipate, capacity has to drop with it.
| Where this shows up in practice |
|---|
| Rooftop conduit runs, especially under solar panels |
| Hot ceiling or attic spaces |
| Tropical outdoor installations without shading |
| Enclosed switchboard or panel spaces with poor ventilation |
A cable's rated ampacity assumes it's dissipating heat on its own, typically in free air or a lightly loaded raceway. Bundle several current-carrying conductors together in the same conduit or tray, and they heat each other — the group's combined heat has to escape through a shared, more limited surface area, so each individual conductor's safe current-carrying capacity drops as more conductors join it.
| Where this shows up in practice |
|---|
| Multi-circuit conduit runs from a panel to several loads |
| Multiple PV strings routed through one combiner conduit |
| Cable tray installations with many circuits sharing the same tray |
Temperature and grouping are independent physical effects, and their correction factors get multiplied together, not added or applied selectively:
Derated ampacity = Base ampacity × Temperature factor × Grouping factor
A hot, grouped installation — a rooftop conduit carrying several PV strings together, for instance — can see its effective capacity drop substantially below the table's headline number once both corrections are properly stacked. Applying only one correction and assuming "close enough" is exactly how an undersized cable ends up passed as compliant.
OhmWorks applies both corrections automatically for every cable sizing result — see them broken out explicitly in the Cable Sizing calculators.