Cable Ampacity After Derating
Cable Ampacity After Derating
A cable’s nameplate ampacity is rated under specific reference conditions — typically a defined ambient temperature and a single cable in free air. Installed conditions are rarely that ideal, so codes require the rated ampacity to be derated with correction factors for higher ambient temperature and for bundling multiple current-carrying conductors together.Skipping these derating factors is a common and dangerous mistake: a cable that is fine on its own can overheat and degrade its insulation when run in a hot equipment room bundled with several other loaded conductors, even though its nameplate rating looks adequate.
The derated ampacity is I_derated = I_rated·C_t·C_g, the rated ampacity times the temperature and grouping correction factors. where I_rated is the cable’s nameplate ampacity, C_t is the ambient temperature correction factor, and C_g is the grouping (bundling) correction factor.
Inputs
I_rated := 100 A=100 A
C_t := 0.87=0.87
C_g := 0.8=0.8
Applying both correction factors together scales the ideal nameplate rating down to the ampacity the cable can actually carry safely in its real installed environment.
I_derate := I_rated·C_t·C_g=69.6 A
Results
A derated ampacity of about 70 A from a 100 A nameplate rating shows how quickly combined corrections erode the usable current capacity — nearly a 30% reduction here. Whatever load current this cable will actually carry must stay below the derated value, not the nameplate value, or the conductor will run hotter than its insulation is rated for.
Assumptions & limitations
- Correction factors taken directly from code tables for the specific ambient temperature and group size.
- Continuous duty; short-term overload capacity not considered.
- Only temperature and grouping corrections applied — conduit fill and other factors may also apply.