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Thermal rating of circuit conductors

The integrity of any electrical circuit depends upon how much current flow is allowable in various conductor sizes. One limitation is the amount of heat, generated by current flow, that will damage conductor insulation. Heat flowing from a conductor through terminations can also cause damage to connected apparatus, and vice versa.

Heat dissipation from conductor to the surrounding air depends on conductor surface area. For round wires, that area is directly proportional to diameter. Heat generation, however, is proportional to cross-sectional area and therefore to the diameter squared. Hence, the smaller the conductor, the greater the allowable current density (ratio of amperes to area).

Cooling also depends upon the surroundings and the ambient temperature. Isolated in unobstructed air, a conductor can carry more current than when bundled with other conductors or confined within a raceway.

Designers often work with the ratio of current-to-area--the current density--which is conveniently expressed in amperes per square inch or per square millimeter. Workers in U.S. electrical trades are likely to use the ratio of area-to-current, with area expressed in circular mils (one circular mil is the area of a circle 0.001 inch in diameter).

Again in the U.S., standard round wires are sized by American Wire Gauge (AWG) numbers such as No. 14. The higher the number, the smaller the wire. Metric wire sizes are designated by their area in square mm, such as 35 or 50. None exactly matches an AWG size, although many are within five percent.

Stranded wire is widely used above about No. 10 metric. Its greater flexibility is helpful in routing wires through conduit, and in circuits subject to vibration. Because of the space lost in bundling multiple strands, any stranded conductor will have an overall diameter exceeding that of a solid wire of the same area. But there will be no difference in current-carrying capability.

Please note correction to this article, printed in our June issue in response to the letter that follows:
No requirement that conductors be solid
Editor: Richard Nailen’s articles are always lucid and well-written. In the article “Thermal rating of conductors” in the May 2009 issue is stated, “For raceway installation, the NEC requires conductors up through AWG No. 6 to be solid.” Article 310.3, Stranded Con­ductors, in the 2008 National Electrical Code actually states, “Where installed in raceways, conductors of size 8 AWG and larger shall be stranded.” That is not to say that smaller conductors must be solid. It is that larger, solid conductors prove difficult to pull through conduit or other raceways.
David Bredhold, MS, Application Engineer, Eaton/Cutler-Hammer, Louisville, Ky.
Editor's response: Thanks for catching this. Actually, our statement was incorrect on two counts. “Up through AWG No. 6” was meant to read “up through AWG No. 10,” which would at least have been consistent. In any event, we agree that conductors smaller than No. 8 aren’t required to be solid, although they usually are.—Editor

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