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Posted by on August 27, 2007, 12:16 am
On Sun, 26 Aug 2007 18:40:06 -0400, "Paul"
>I'm confused by a couple of online calculators I've found.
>
>I'm trying to determine the proper gauge wire to carry 20A, 120VAC over a
>distance of 300 ft. with an approx allowable 2% (per NEC) voltage drop.
>This UF cable will be buried for most of it's length.
>
>This calculator:
>http://www.stealth316.com/2-wire-resistance.htm
>advises a 6 ga. wire would be sufficient (voltage drop 2.414V)
>
>However, this calculator seems much more conservative:
>http://www.vihon.com/Vihon_Calculators/Vihon_Wire_Gage_Calculator/body_vihon_wire_gage_calculator.html
>It says a 0 (zero) gauge wire is required !
>
>Also would like some real world advice on the voltage drop recommendations
>of the NEC. At what point does one see excessive/dangerous wire heating?
>2% ? 5% ? 10% ? 20% ?
>
>-- Paul
>
A lot really depends on the expected load. If you are really just
running a few lights out there you don't really care that much about
voltage drop. If it is voltage sensitive things like motors you need
the real FLA before you do the calc. The NEC suggestion (it is not a
rule) is 5% total drop from service to load.
#8 with a 16a load (reasonable max on a 20a circuit) at 300' comes
out to a tad bit over 6%. That is still overkill if the load doesn't
care about a little voltage drop. #6 should bring you in no matter
what.
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