A five-step cable sizing procedure

There are numerous ways to go about doing this. Here’s one that is sure to work. It’s concise: five steps . . . .

What follows is a general cable ampacity derating procedure you can use. It assumes you are specifying a cable rated for 90ºC that will terminate to equipment that’s rated for less than that. This should easily demonstrate your reasoning process in a mod package evaluation while abiding by NEC (2014) Article 310.15.3(B):

  1. State the equipment rating (we’ll assume it’s 75ºC).
  2. Start with the cable’s 90ºC ampacity at either 30ºC or 40ºC ambient.
  3. Apply any applicable derating factors required to adjust its ampacity.
  4. Compare the adjusted (derated) ampacity to the 75ºC ampacity at the cable’s basis ambient temperature.
  5. If the adjusted ampacity is less than the unadjusted ampacity available to a 75ºC cable, then the adjusted value can be used. An ampacity greater than what’s allowed for a cable rated for 75ºC cannot be used, but over-conservative cable sizes don’t have to be selected by starting with the 75ºC ampacity (which assumes there is no difference, and thus no benefit, between a cable rated for 90ºC and one rated for 75ºC).

See Square D, “Wire Temperature Ratings and Terminations,” March 2002. Lexington, KY. Data Bulletin No. 0110DB9901R2/02 for a specific example that illustrates this procedure. The bulletin demonstrates the advantage of using a cable rated for higher temperatures. It shows that a smaller 250 MCM cable can be used if rated 90ºC in a particular application instead of a larger 300 MCM cable that’s rated 75ºC in the same application.

The cable’s insulation characteristics determine its temperature rating. Cables with CSPE jackets are common in Class 1E-rated cables due to the material’s ability to withstand high-temperature environments without degrading. It is also a rugged material so it is less likely to be damaged during installation.

If a cable’s insulation is better able to withstand high temperatures for a longer period of time without degrading, then the conductors that it protects will be able to be heated to higher temperatures without the worry of damaging the insulation. Higher currents produce higher conductor temperatures. To make use of the extra insulating capacity afforded by the premium insulation, ampacities are increased.