The National Electric Code (NEC) requires that ampacity corrections be made for cables exposed to ambient temperatures higher than nominal values. (Nominal is 30 deg C; 86deg F) Current passing through a wire generates additional heat, which adds to the temperature from outside the wire, further heating up the wire, insulation and jacketing.
The ampacity is defined as the allowable current-carrying capacity of a conductor measured in amps. Ampacity correction is the application of a factor to adjust for ambient temperatures that are other than 30 degF. The ampacity correction factors are published in NEC tables 310.16 through 310.20. Do not forget to derate the terminals as well.
In the industrial heating industry, most standard wire can withstand continuous temperatures up to 450 deg F. These wires may have a PVC, Teflon or Silicone insulation and may have metalic armor to protect the conductors insulation material. Up to 450 deg F, wires are commonly UL / CSA approved.
High temperature wire can withstand continuous temperatures up to 1200 deg F at maximum 600 Volts. To withstand these temperatures, high temp wires have different coatings than standard wires. Common features of high temperature wire are insulation with MICA tapes and a fiberglass braided jacket which is treated with high temperature saturant. For extra mechanical protection, high temperature wire can be ordered with a stainless steel jacket.
Here are a couple ampacity correction factors based on:
100 degC = .93, 200 degC = .82, 400 degC= .52, 600degC = .26, 800 degC = .22, 1000 degC = .20
If this single conductor is in a conduit, an additional correction factor of .89 must be used.
|
