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Old 08-18-2008, 03:31 PM   #1
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help a complete newbie with conduit

Hi all, first post here, and limited knowledge on what I'm doing. Here is what I am attempting to do:

My main panel is on the outside of my garage, and currently all wiring for the house goes through the garage (up the studs and along a joist) to the house. All electrical runs through the crawlspace. The garage is not sheetrocked and all wiring is exposed. I'm looking to sheetrock the garage now, but I'm also planning on rewiring the entire house one day (the elctrical is pretty dated). So, I'm thinking of putting in conduit for the future wiring pulls, but don't know what is allowable and how much conduit I need. Any ideas or suggestions is greatly appreciated.


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Old 08-18-2008, 03:59 PM   #2
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Welcome to the forum.

The problem with installing a single conduit for the house circuits is the probable need to de-rate the current carrying capacity of the wires due to having more than 3 in the conduit.

A real life example would be installing 10 circuits in that conduit to the house. That is 20 current carrying conductors in the conduit. These wires then must be de-rated by 50%. A #12 THHN wire would thus be limited to 15 amps. More wires would mean even further reductions. Add the circuits up and count the wires you would need. Each circuit normally has two wires.

If you put 40 current carrying wires in the conduit you would be forced to use #10 wire for a 15 amp circuit. And #8 wire for the 20 amp circuits.

The reductions are:
3 or less = 100 of ampacity
4 to 6 = 80% of ampacity
7 to 9 = 70%
10 to 20 = 50%
21 to 30 = 45%
31 to 40 = 40%
41 or more = 35%



Last edited by jrclen; 08-18-2008 at 04:07 PM. Reason: add more info
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Old 08-18-2008, 05:36 PM   #3
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pocketfulladoubles, jrclen speaks wisely. You could cover the "bay" (space between the two joists where the wires run) with a length of plywood (it only needs to be 16" wide) screwed in place and painted the same color as the sheetrock. This way when you need to run new wires, unscrew the plywood, put your new wiring in place, screw the plywood back and you're done. It might be a wise idea to move some of the existing wires further away from the face of the joists to lessen the chances of an errant screw piercing a wire/s.
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Old 08-18-2008, 06:10 PM   #4
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Now THAT is a good idea!
Thank you for saving me a lot of time and money...
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