#12 wire is not rated to carry 40A. #10 is not even rated to 40A. What are we up to - #8? Man - that'd be one hefty cord. "Sure hon, you can help - just coil up the power cord for the table saw while I sweep up, will ya?"...
You have to be reading something wrong in the code book. If this was true, there is not a house in America that is wired to code. Think of how many of your circuits contain more than 35' of #12 wire. I'd look at it myself but I don't have my code book (also an older edition) available right now. Watch those current ratings.
Again, my previous post was based on information from the 1978 Code. Doubt the laws of physics have changed all that much but...
Here's relatively current (no pun intended) information on ampacity from the NEC Code Table 310.17. It appears that #12, depending on type of wiring IS rated to up to 40 amps and #10 as high as 55 amps depending on specific factors.
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Table 310.16, again dealing with ampacity, allows up to 30 amp and
40 amp for #12 and #10, respectively.
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>Remember too that Tim was, I believe, referring to a 240 volt circuit.
Remember we're talking a 20 amp LOAD on a 120 v circuit and discussing voltage drop. Again, it may not be a 3% drop as someone else mentioned when they posted the link to the calculator, but neither the properties of copper nor the laws of physics have changed since 1978 AFAIK.
Amen. In a shop setting particularly, bigger is better. You'll never have a problem running most power tools on dedicated, 240v circuit fed with #12 or #10 but you may well have a problem with the #12 at 120 v and that table saw you lust after.
1999 National Electrical Code. The numbers are for 2% or less drop at the rated load. We seldom actually load circuits anywhere near their rated load, but it doesn't hurt. According to their ratings most houses are under-wired because of the length of the wire runs.
"* Small Condutors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for
12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied."
-- Regards, Doug Miller (alphageek at milmac dot com)
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