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John Livingston's

The Nuclear Electrical Engineer

An educational resource for electrical engineers in the nuclear power industry

Here you can find design tips on technical skills like cable and breaker sizing, short-circuit evaluations and more, PLUS deep-dive analysis of nuclear-specific topics like design basis, Tech Specs, UFSARs, and safety-related SSCs — built from primary source documents.

Sizing cable · Feature

Cable temperatures and physics

John Livingston
December 11, 2014 · 2 min read

It’s helpful to be reminded of the basic physics underlying the basis for the temperature rating of conductors . . . .

A conductor sitting at room temperature, with no current running through it, will be the same temperature as the room. But when the cable is loaded down with a current, that current will begin to generate heat internally within the conductor. The heat generated inside the conductor will be transferred into the surrounding insulation, jacket, and environment.

The heat-transfer relationship between those four physical elements is complex.

Consequently, the overall cable temperature will begin to rise above the ambient temperature. It’s this temperature rise above ambient that impacts a cable’s ampacity: how many degrees above ambient temperature the cable can operate at before degrading (see Mukund R. Patel, Shipboard Electrical Power Systems, p.70).

A cable rated at 90ºC is, roughly speaking, able to generate an extra 50 degrees of internal heat before damaging the insulation if the ambient temperature is 40ºC. A cable rated at 75ºC is able to generate an extra 35 degrees of internal heat if the ambient temperature is 40ºC.

To look at it another way, a cable rated for 60ºC that is installed in an environment whose ambient temperature is 60ºC has zero ampacity.

This paper prepared by the CDC presents a detailed technical analysis of cable heat transfer. It develops differential equations and compares the calculated results to experimental results.