The history of wire gauges goes back into British industrial history -- and can get involved.
The original wire standard was exclusively for steel. It was devised for it. Copper wire -- for electric use came generations later.
While not exact, #12 in steel is pretty close to #12 in copper, and so forth. #10 is bigger in cross section for both, etc.
( Over a century ago, they actually were the same sizes. Then better steels caused them to re-standardize the steel gauge somewhat. Pure copper stayed copper.)
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The numbering system got started by way of the technique:
A #1 (steel) wire had been drawn through the first (largest) die.
It was then annealed.
Next it was drawn through the #2 die, the second die, and it grew longer and thinner.
It was then annealed.
And so forth.
The 'oughts' ( 2/0 ) were the first attempts at back sizing upwards.
4/0 was as big as they needed, so it stopped there.
Last edited by Tesla; 03/29/14 08:11 PM.