Wire Size for 20-amp, 70-foot Run

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Old 04-06-09, 12:39 PM
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Wire Size for 20-amp, 70-foot Run

I'm planning to install a 20-amp, 120-volt circuit out to a shed that I'm building. It will require a total run of about 70 feet in PVC conduit. The guy at the permit counter said that #12 copper wire would be fine, but I've found other references that suggest I should use #10 due to the fairly long run in conduit. What size would you recommend?
 
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Old 04-06-09, 12:50 PM
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The #12 is adequate for a 70' run; you can jump up to #10 if you want but I don't think it's necessary until about 100'.
 
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Old 04-06-09, 05:23 PM
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This is true even for wire inside of conduit for the whole run?
 
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Old 04-06-09, 06:49 PM
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Yep that will cover that issue and if you are under 20 amp worth of load and less than 100' [ 30.4 metre ] you are fine with #12 AWG size

Merci,Marc
 
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Old 04-06-09, 07:14 PM
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The concern here is voltage drop. But there are two complicating factors in the analysis.

(1) Voltage drop does not depend on the theoretical maximum current for the circuit, but rather on the actual current. So if you know what will be running in the shed, you can compute based on that. If you want to plan for anything, then you can use the full 20 amps as a conservative number. But that may be overly conservative if you never approach the full 20 amps.

(2) How much voltage drop you can tolerate depends on what kind of loads you will have. Incandescent lighting and resistance heating can tolerate a lot of voltage drop. All that happens is that lights gets a bit dimmer and heaters produce a bit less heat. Motors can tolerate less voltage drop, and they may overheat or refuse to start is you have too much drop. So consider the types of loads in your analysis.
 

Last edited by John Nelson; 04-06-09 at 07:44 PM.
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Old 04-06-09, 07:19 PM
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Originally Posted by John Nelson View Post
(2) How much voltage drop you can tolerate depends on what kind of loads you will have. Incandescent lighting and resistance heating can tolerate a lot of voltage drop. All that happens is that lights gets a bit dimmer and heaters produce a bit less heat. Motors can tolerate less voltage drop, and they may overheat or refuse to start is you have too much drop. So consider the types of loads in your analysis.

John Nelson just hit the nail on the spot the biggest curpit useally are Motor related issue especally with compressor or large motor like table saw or powerfull skillsaw.

Those load it will make the lights flicker a bit during start up.

If you have alot of pretty powerfull tool maybe you can run in MWBC format so have one circuit for tools on 120 v 20A circuit and second circuit for lights and that will really reduce the flickering or dimming the lights a bit.

Merci,Marc
 
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Old 04-06-09, 07:34 PM
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If you are concerned about a voltage drop and you might be between a 15 to 20 amp load most of the time you might want to just run #10 wire. Your voltage down would never go below 3% and you wouldn't have to worry about anything. But to be honest, if it's just a general circuit I have never had any problems with something like this using #12 wire. Hope I didn't confuse you.

Jim
 
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