02:39PM | 11/17/04
Member Since: 11/16/04
1 lifetime posts
I am moving and upgrading an overhead service panel attached to the exterior of a masonry home. Due to structural issues, I cannot easily bring the house wires through the wall and into the back of the new panel location. Instead I intend to run exterior conduit out the side of the panel up through the roof blocking to a junction box in the attic. Most of the house wires are NM 12/3.

My questions:

1) What type (rigid, IMF, other?) and size (diameter) of conduit should I use?

2) How many wires of what size and type can I run through that conduit?

3) Are there other issues to consider? Could it be better to deal with the structural issues in order to get the house wires through the back of the box?


04:24PM | 12/04/04
Member Since: 12/03/04
2 lifetime posts
I would use IMC. If you know how to look it up in the NEC code book you can get the exact size conduit,but the quick and dirty wayis. Estimate the area of a circle that the wires tightly bundled together occupy. The area that the wires occupy for the new conduit should be only 40% of the total area of the new conduit.

EX: You guess the bundle of wires is 2" in Diameter. You need a larger conduit. 40%(Area of new conduit)=(Diameter of bundled wires) so 2"/.4= 5" you need a 5" conduit

doug seibert

03:47AM | 12/07/04
Member Since: 08/10/02
842 lifetime posts
......You need to calculate the AREA......not the diameter


10:32AM | 12/07/04
Member Since: 09/17/02
524 lifetime posts
And if you run more than 9 current carrying conductors in one conduit you need to start derating. There are 2 wires on a circuit so after 4 circuits you should run a new conduit. Ground wires don't count. They don't carry current.

Derating means that a #14 wire rated for 15 amps is no longer big enough. You need to switch to #12. That would be for all the conductors not just the ones after 9.

You also can't run NM cable in conduit.


08:11PM | 12/19/04
Member Since: 12/19/04
62 lifetime posts

You can use PVC or Emt but do not install the cables in the conduit. Run the conduit to the attic and terminate it in a "pull box". Do not install the pull box in an area that will be covered by insulation. It must be accessible. Calculate the amount of circuits you will be needing to run to the attic and using table 4 & 5 from the code book you can calculate the size conduit you will need. You can then break the number of circuits needed into three wire circuits (two hots with one neutral) bring them to the pull box using thhn type wire and then go from the pull box with nm-b cable. Check with your local inspection dept as to whether or not you have to derate the circuits. In wisconsin we do not have to derate wiring for residential circuits. Whatever size conduit you calculate it to need, go one size larger and you'll never regret it. Good luck.

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