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Old 07-28-2009, 04:08 AM   #1
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Need help with conduit fill calculations


I need to bring a bunch of NM-B cables into a junction box and transition all of them to THHN inside EMT. I have a few questions as I've never done anything on this scale before.

Here's what I have:

2 6/2 AL
1 10/3 CU
1 10/2 CU
1 12/3 CU
2 12/2 CU
5 14/2 CU

I'll convert the 6/2 aluminum to copper in the junction box, so those become #8 with #10 ground.

What I came up with is:

4 #8
9 #10
10 #12
15 #14

Based upon entering those numbers into a conduit fill excel spreadsheet I downloaded, it says I need 1-1/2" EMT. Is there more to this than I have presented? The calculator did a 40% fill. What about derating?

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Old 07-28-2009, 07:33 AM   #2
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Need help with conduit fill calculations


I think you should seriously redesign the whole plan. First, you only need one ground, sized to the largest circuit. So that leaves you with:

4- #8
6-#10
7-#12
10- #14

A total of 28 wires. Now, that is a helluva pull, just in purely mechanical terms, but derating kills the whole thing. Your conductors derate to 45%. Multiple runs of conduit should be used, in my opinion.

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Old 07-28-2009, 08:40 AM   #3
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Need help with conduit fill calculations


Conduit fill calc I use:
http://www.cerrowire.com/default.aspx?id=49

Interesting - just noticed it doesn't have #14 listed


How far are you pulling the wires?
How many bends in the conduit?
As said that's a lot of wires

I usually upsize one conduit size depending upon # of wires, distance & bends

As said derating with that many wires will kill you
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Old 07-28-2009, 11:09 AM   #4
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Need help with conduit fill calculations


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Originally Posted by InPhase277 View Post
Multiple runs of conduit should be used, in my opinion.
Thats how it should be done.
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Old 07-28-2009, 11:48 AM   #5
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Need help with conduit fill calculations


The conduit will only be about 25ft long with two 90s and an LB. I was thinking about upsizing to 2" since the destination already has a 2" concentric knockout. I figured it would make things easier and allow for future pulls if needed.

Let me give you some background on what I am proposing here...

The main panel in my house is located on the southeast corner of the garage. All of the NM cables go from the panel into a wooden chase that runs the length of the garage so that they can enter the house at the northeast corner of the garage and branch out from there into the house. The face of the chase was covered in drywall. One day I removed some of the drywall so I could inspect the wiring. I was curious as to how it was done. What I discovered is that rats had run up and down the chase and chewed on pretty much all of the NM cables, every few feet. Some of the cables were down to the bare copper.

As a temporary fix, I wrapped all of the exposed portions in electrical tape. I don't believe this is a permanent solution and I want to correct it. I realize that replacing all of the wiring inside conduit stills leaves all of the NM inside the attic susceptible to rats, but I've been in the attic and as far as I can tell none of it has been messed with.

Multiple runs of conduit may be an option. How many would I need and of what size to avoid derating?

Can someone explain derating?

I also don't understand why I would only need a single ground wire.
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Old 07-28-2009, 12:27 PM   #6
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Need help with conduit fill calculations


A big issue I see is the AL to Copper connection.
I would try to run multiple conduits, and have multiple jboxs.
With the conduit runs, you can use thhn so you can use smaller conduit.
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Old 07-28-2009, 01:22 PM   #7
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Need help with conduit fill calculations


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Originally Posted by jbfan View Post
A big issue I see is the AL to Copper connection.
I would try to run multiple conduits, and have multiple jboxs.
With the conduit runs, you can use thhn so you can use smaller conduit.
For the AL to CU I am planning on using Polaris connectors. I never planned to run NM inside conduit.

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