Need Help With Conduit Fill Calculations - Electrical - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Need help with conduit fill calculations
 User Name Remember Me? Password
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

 Thread Tools Display Modes
07-28-2009, 04:08 AM   #1
Newbie

Join Date: Oct 2007
Posts: 19
Rewards Points: 10

## 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?

### Advertisement

07-28-2009, 07:33 AM   #2
Member

Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,487
Rewards Points: 2,350

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.

### Advertisement

 07-28-2009, 08:40 AM #3 Member   Join Date: Jan 2009 Location: South of Boston, MA Posts: 17,248 Rewards Points: 2,000 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

07-28-2009, 11:09 AM   #4
Member

Join Date: Jun 2007
Posts: 3,699
Rewards Points: 58

Quote:
 Originally Posted by InPhase277 Multiple runs of conduit should be used, in my opinion.
Thats how it should be done.

 07-28-2009, 11:48 AM #5 Newbie   Join Date: Oct 2007 Posts: 19 Rewards Points: 10 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.
 07-28-2009, 12:27 PM #6 Electrical Contractor     Join Date: Jun 2004 Location: Newnan GA Posts: 6,778 Rewards Points: 150 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. __________________ "The problem isn't that Hillary Clinton lies. We all know she lies. The problem is that her supporters don't seem to care"
07-28-2009, 01:22 PM   #7
Newbie

Join Date: Oct 2007
Posts: 19
Rewards Points: 10

Quote:
 Originally Posted by jbfan 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.

### Advertisement

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post tns1 Electrical 25 08-29-2011 06:56 PM jamiedolan Electrical 73 11-19-2008 12:36 AM diy157 Electrical 6 09-04-2008 11:08 PM capndave Electrical 37 03-26-2008 03:16 PM jambud Electrical 17 03-12-2008 10:31 AM

Top of Page | View New Posts

 Terms of Use

Search Engine Friendly URLs by vBSEO 3.6.1