# Voltage drop

• posted on July 10, 2007, 10:29 pm
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.
http://www.csgnetwork.com/voltagedropcalc.html
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.

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• posted on July 11, 2007, 12:25 am

The maximum load on a 20A circuit is... any guesses? ...wait for it... 20A.
16A is the maximum *continuous* load on a 20A circuit.
--
Regards,
Doug Miller (alphageek at milmac dot com)

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• posted on July 11, 2007, 7:35 pm
On Jul 10, 8:25 pm, snipped-for-privacy@milmac.com (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 before tripping.

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• posted on July 11, 2007, 8:44 pm
nosmo snipped-for-privacy@yahoo.com wrote:

Hm. Now you've got me curious; have to go to Federal Pioneer and get the Stab-Lok series data sheets. Wonder what the instantaneous trip rating is?
Yours aye, W. Underhill
--
"Take sides! Always take sides! You may sometimes be wrong - but the man
who refuses to take sides must *always* be wrong! Heaven save us from

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• posted on July 11, 2007, 6:51 pm
Terry wrote:

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)