Derating cable ampacity – count the ground or neutral?

Here are three common scenarios that you need to keep in mind when sizing a new cable. Its ampacity will have to be derated depending on how these three factors apply to you . . . .

A cable’s ampacity will be derated by additional factors depending on its installation configuration and environmental conditions. For example, tables of cable ampacities are often given using some base ambient temperature. If this is 30ºC (86ºF), but the cable will be installed in an environment hotter than that, its ampacity has to be multiplied by some correction factor (given by the code or standard you’re using) to compensate for the difference in base conditions.

Three common cable ampacity derating factors encountered (and the NEC 2014 sections that address them) are:

  • An ambient temperature exceeding that used for the ampacity base value [NEC Article 310.15(B)(2) in conjunction with Tables 310.15(B)(2)(a) and 310.15(B)(2)(b)]
  • More than three current-carrying conductors in a raceway [Article 310.15(B)(3) with Table 310.15(B)(3)(a)]

Ground wires (“grounding conductors”) are not counted as current-carrying conductors. [NEC Article 310.15(B)(6)]

Neutral wires (“neutral conductors”) may or may not be counted as current-carrying conductors, depending on the situation. Circuits that supply nonlinear loads (such as certain kinds of lighting, like fluorescent) must count the neutral as a current-carrying conductor because there are usually substantial currents present on it. It does not usually have to be counted in a wye-connected three-phase system because the loads are (assumed to be) balanced. [Article 310.15(B)(5)]

  • Cable inside a raceway exposed to direct sunlight, such as when installed on a rooftop. [Article 310.15(B)(3)(c) in conjunction with Table 310.15(B)(3)(c)]