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Is there any way to infer the gauge of the wires inside an NM cable from the size and shape of the cable? I reckon a round cable is generally -/3 W/G and a flatter cable -/2 W/G, but I do not know how to distinguish 14/2 W/G from 12/2 W/G without readable markings.

I am mapping house circuits and have run across some NM cable whose markings have faded into nonexistence or were never present (might be 1980s cable, and I am ignorant of 1980s cable manufacture practices). Some text was embossed into the cable sheathing during manufacture, but the visible run is short and I cannot read the desired information off the cable.

I have a bunch of 20A breakers and am concerned the cable running out might be 14/2 W/G rather than 12/2 W/G, which I understand would mean the cable is undersized for the circuit amperage.

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3 Answers

up vote 2 down vote accepted

I don't think you can safely infer it, but what I would recommend is to find an area in your house that has some cable with slack and the wires exposed - maybe the breaker panel. Take note of the diameter and overall appearance of the cable, and then check the wires themselves. As @lqarry said, you can use wire strippers to see what gauge the wire fits. Now compare that to the cable in question.

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I will open up the breaker panel and gauge the wires then. I could use the same trick to gauge the 2-wire no ground fabric-coated stuff, as well, rather than trusting they ran 14/2 W/G to 14/2 no ground. Thanks! –  Jeremy W. Sherman May 16 '12 at 5:01
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Today's manufacturers rules has NM or romex color coded. 10 gauge is orange, 12 gauge is yellow and 14 gauge is white. Inspectors really love this so they don't have the problem you are having.

Older wire, you have to get a good light and a magnifying glass. The only thing I can suggest is de-energizing the circuit and using calipers on the wire to compare differences, or using a set of wire strippers designed for solid wire and see how they measure up.

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Using a magnifying glass is a good idea. I don't have one handy, but that's a cheap fix. I did manage to read the embossing on several more cables by turning out all the lights and then shining a flashlight on angle at the embossing, so shadows filled the letters. Some of it was still too worn away to read. –  Jeremy W. Sherman May 16 '12 at 4:59
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Has it not occurred that you can measure the cross sectional area of the copper itself to give you the cable size. If you have one to hand use a micrometer. kinda looks like a large pair of pliers but can measure incredible small sizes. Failing that pull out the old measuring tape or if times are really hard a ruler perhaps.

AWG 12 (Diameter):

0.0808 (Inches) 2.05 (Millimetres)

AWG 14 (Diameter):

0.0641 (Inches) 1.63 (Millimetres)

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I wanted to work out wire gauge from mid-run cable. It seems that's not really possible, so per @Steven's answer, I'll just use an AWG wire gauge directly on already exposed wire in the breaker box. –  Jeremy W. Sherman May 17 '12 at 4:39
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