Resistance is Futile

Dear Swami,

How does one find the cross sectional area of a cable? In the book, "Electricity for Entertainment Electricians & Technicians," there is a formula in Appendix 4 to calculate voltage drop, and it uses the cross sectional area of the cable. Can you point me in the right direction?

Signed,
Looking for Direction

Dear Looking,

The universe is a bountiful source of information, but the internet puts it at your fingertips. For American Wire Gauges (AWG), you can find the cross sectional area of solid wire in Wikipedia on this page and for international wire sizes, they are spelled out in IEC 60228.

Two things to remember: 1. The cross-sectional area of stranded wire is calculated by adding all of the cross-sectional areas of the individual strands of solid wire that make up the stranded wire, and 2. The resistance of copper wire varies according to its temperature. The wire gauge charts typically use 30 degrees C (about 86 degrees F) as the ambient air temperature. When it's hotter, the resistance increases.

Also, the values listed in these charts also assume DC or low frequency (50 or 60 Hz), but there is something called the "skin effect" that causes the resistance to increase with higher frequencies. The skin effect causes the current of higher frequency to flow more along the outside of a conductor than through the middle. The higher the frequency, the greater the skin effect and the greater the resistance. That's important when you have non-linear loads that generate harmonic currents, which are currents of higher frequencies than the 50 or 60 Hz fundamental frequency.

Signed,
Swami Candela of the Third Millennium

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