I have heard in the trade as a general rule if you are using #12 wire
for a 20A circuit you should go to #10 if the run is over 100 ft.
The fan #14 or #12 thread prompted me to just do a quick check at one
of the voltage drop web pages.
If you use this web page and try using 16A which would be the max on a
20A circuit you get 5% voltage drop in just the branch circuit.
On Jul 10, 8:25 pm, firstname.lastname@example.org (Doug Miller) wrote:
Just to be anal, the maximum load on a 20A circuit can be far greater
than 20 amps, at least for a short period of time. According to the
Trip Curves on the Square D web site, their QO series of breakers can
handle a 2x load (or in this case, 40 amps) for just under 10 seconds
My copy of NEC (1990 -- somewhat obsolete) only mentions voltage drop
in FPN's (non-manditory notes). It recomends a max of 3% on branch
circuits "at the farthest outlet of power, heating, or lighting loads",
and a max of 5% on both feeder and branch circuit.
10ga copper is about 1.25 ohms / 1000', and
12ga copper is about 2 ohms / 1000', and
14ga copper is about 3.1 ohms / 1000'
3% of 120v is 3.6v.
100' of 12ga is actually 200' (hot & neutral) is .4 ohms
so 3.6/.4 = 9 amps for 3% voltage drop at 100'.
10 ga. will give you 14.4 amps at 100' and
14 ga. will give you 5.8 amps at 100'.
Of course you may be willing to withstand greater voltage drops.
(and yes, you get 5.33% voltage drop at 16A on 100' of #12: .4*16/120)
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