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Old 08-17-2006, 05:00 PM   #1
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conduit size and NEC

Hi, is there a chart or a rule that the NEC mandates for conduit size relative to the number and size of wires your running thru it?

I have to pull (4) 6 ga THWN wires thru PVC Sch # 40 conduit and I wasnt sure what size conduit I should use?

Also, I know it can be very tough sometimes to get the fish thru the conduit, and even harder to pull the wires back even using lube.
Ive heard of people glueing the conduit sections together as thru run the wires thru at each section. The only down side I see here is that the pvc glue will get on the wires and either eat or weaken the covering, or the wires will glue themselves to the pvc cement and if u ever had to pull them out, your pretty much SOL.

Any Suggestions?


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Old 08-18-2006, 06:49 PM   #2
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I saw your question a couple of days ago and decided not to answer due to the fact that I am in Canada and don't know the NEC.

So my answers come from the Canadian Electrical Code, but i beleive wire size and conduit fill are very similar.

First of the code does tell you how many wires you can put into a pipe. They allow you to fill it to 40%.

According to the Canadian code, you can fit 4 #6 THWN into a 3/4 pipe. If you are worried about haveing trouble pulling those in you could always jump up to 1" pipe and have more play.

Also you may have to derate those cables because you have 4. Here in Canada with 4 cables in a raceway you can only load the wire to 80% of the capacity. Up here #6 TW(with a 60 degree C rating) is rated for 55A. In your pipe you could only put 44A on those wire.(55x.8)

So I hope this helps. Hopefully someone who knows more about the NEC will chime in and they can tell you for sure if I our codes are similar on this.


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Old 08-18-2006, 09:43 PM   #3
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Four #6's??? Is this two hots, a neutral and a ground? If so, the ground wire size need only be a #10 and you can save a little cash.
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Old 08-19-2006, 02:13 PM   #4
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sorry to answer so late. I have been having puter problems. Even as I type I am having to open actual books to look up the proper references to be sure that I am giving you the right answer.. Yes actual bound paper things.

I think it is a bad idea to sleve the conduit over the wires cause the glue could eat into the insulation on the wires.

3/4 pvc can handle 4 number 6 and in this particular case you gain little by downsizing the ground, it however may be allowed and save you money on wire.

I reccomend that you go up one size to 1 inch your labor will offset the material cost.

De-rating, in the US is for 4 or more current carrying conductors. Since you only have 4 to start, it will not be an issue. I am still curious what the voltage phase requirements are for what you are doing.
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Old 08-20-2006, 09:44 AM   #5
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Thanks for all your help. I will go with the 1" conduit to make it easier to pull the wires thru.
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