NEC - National Electrical Code charts for just this thing. I only have
an old one but that's what it says for a 20amp, 240 volt single phase
FWIW, checking the voltage without a load in place won't tell you squat.
It has to be under load. As an example that same #12 wire is good for
280' with a load of <5 amps, and a qualified (depending on type of
insulation, conduit or cable, etc) 36' run at 40 amps.
Remember too that Tim was, I believe, referring to a 240 volt circuit.
With a 120 volt circuit #12 is good only for a run of < 35'.
Bob - Not an electrician but I do a much better imitation of one than
#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
NEC Table 310.16, again dealing with ampacity, allows up to 30 amp and
40 amp for #12 and #10, respectively.
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.
You apparently missed the footnote at the bottom of each of those tables:
"* 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."
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
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.
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