Can someone please explain the origin of wire sizes. !0 gauge, or 12 gauge I can see. 3/0 or 4/0 is mildly confusing. What is an "O" ? Now I hear there is a 250 size. What the hell is that about? Is there any physical relation of the numbers to the size? Is it in mm, inches, or whatever? Thanks

The smaller wires are sized per American Wire Guage (AWG) standards. As the number gets smaller, the wire cross-sectional area (thickness) gets larger. 10ga is larger than 12ga, 8ga is larger than 10ga etc.

As you go up in size, you get to 1ga, then 0 ga.. To avoid confusion with the letter O, it is referred to as 1/0 (commonly pronounced "one-ought"). Beyond 1/0, you start adding 0's, so 2/0 = 00, 3/0 = 000 and 4/0 = 0000.

After that, the addition of "oughts" would become rediculous and confusing because eventually you would get to 10/0! So instead, the convention switches to using the measured cross sectional area expressed in Circular Mils (CM) representing 1/1000 of a circular inch.. 4/0 is 212kCM, so the next size manufactured beyond that is

250kCM, then 300, then 350, 400, 500 600 etc., and goes up to 2000kCM. For all practical purposes though, 750kCM is the largest used in common installations becuse of the stiffness, so larger ampacities are typically done with paralelled runs of smaller conductors. As mentioned by others, kCM used to be referred to as MCM, but that officially changed in the NEC 2 years ago.

This will be on the Final Exam. Class is dismissed

As I understand it- Originally, gauge was the number of balls of that (wire) diameter needed to make up a particular unit of weight - in mfg the wire, the ball was a handy/usual way to check the wire die diamter

(I think it is the ounce for wire. Shotguns use the pound)

10 balls of ten gauge diamter to make an ounce, 12 balls of twelve gauge diameter to make an ounce, etc.

(it correlates rigorously to cross-sectional area)

The correct nomenclature is either "MCM" (uppercase) or "kcmil" (lower case). They both mean thousands of circular mils, and can be used interchangeably.

The NEC has used "kcmil" since the 1990 edition.

"MCM" is still alive and well in the electric wire industry and among electricians.

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