May 2009 Electrical Apparatus

May 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus May 2009 featured technical article,  by Richard L. Nailen, P.E.     

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.

To order a back issue with the full article, “Thermal rating of circuit conductors” call 312-321-9440 or visit our online webstore.

Share Button