Conduit fill calculation

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Old 01-05-06, 02:25 PM
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Conduit fill calculation

Thanks to John Nelson and others in this forum, I am deciding to ignore what my electrical contractor said and did with putting too much wire in an EMT conduit. As long as I am putting in extra conduit, is there a way to figure out how much Romex 12/2 with ground (Essex brand) can fit into conduit?

So far all I have seen is calculators and tables that account for a single conductor, not the usual residential Romex style cable with 2 insulated conductors, a bare ground. For example, I will have 3 #2 AWG copper conductors and a #8 insulated ground wire in 1.25" EMT. How much 12/2 Romex can I safely include in the EMT with all those other wires?
 
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Old 01-05-06, 02:31 PM
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In general. except for very short runs, you are not allowed to put cable ( Romex) inside conduit.
 
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Old 01-05-06, 02:34 PM
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NEC reference?

Is there a NEC requirement about this? I would think it is safer to have Romex in conduit?
 
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Old 01-05-06, 03:09 PM
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The legality of running Romex (NM-B) cable inside conduit has been hotly debated for years. In earlier codes (1999, 2002), the wording was ambiguous and led to all the debate. The wording has been cleared up in the 2005 code, and it is clear (to me at least) that NM-B inside conduit is perfectly fine. That said, it's usually not a great idea.

Putting NM-B in conduit would require much, much larger conduit than the same number of individual wires. And it's harder to pull. So it's almost always better to run individual wires in conduit instead of NM-B.

12/2 NM-B must be treated for conduit fill as if it was a 1/2" diameter round wire. This is covered in the notes in Chapter 9. There are no tables as there are for individual conductors in Appendix C, so you have to do the math yourself. The answer of how many 12/2 NM-B can fit in any given conduit size will come out suprisingly few.
 
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Old 01-05-06, 03:16 PM
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It is not safer at all. In the trade this is considered poor workmanship. For complete conduit runs we use THHN conductors box to box. Having extra sheathing inside the conduit only serves to fill it up much quicker and waste precious conduit fill.

You will not find "typical" NM cable listed in conductor fill tables since this is not expressly allowed not expressly forbidden, and is not typically installed in conduit. I will not get into that now. Let's save that debate for later.

You will have to find the exact volume allowance for the cable you are using and figure the fill that way.
 
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Old 01-05-06, 03:46 PM
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For example, I will have 3 #2 AWG copper conductors and a #8 insulated ground wire in 1.25" EMT. How much 12/2 Romex can I safely include in the EMT with all those other wires
You might not be allowed to put any cable or individual wires with those conductors. It depends on what those three #2 awg and #8 awg serve. Is your example a feeder for a sub-panel?
 
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Old 01-05-06, 04:09 PM
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Sub panel feeders

The three #2 and one ground is indeed for my subpanel. The contractor who did the pull is state licensed and we did check some book which was chained to the wall at Home Depot when we bought the wire and conduit. We were both unsure at the time we were buying the wire if 1" EMT was big enough, but the book we looked at told us it was ok. I don't remember the title of the book, in our hurry I trusted him.

So now I am planning on installing 1.25" EMT from the main to my J-box. Then removing all four conductors from the 1" EMT and pulling it through the new 1.25" EMT. I guess I will leave the 1" EMT for wiring other parts of the house. What a pain! The J-box, by the way, I had to put in to house the splices since we estimated (again, the trust thing) the wire too short. Arrgggh!
 
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Old 01-05-06, 04:30 PM
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Knowing now to avoid Romex in conduit ...

Let's say that I run #12 AWG insulated wire from my panel into EMT conduit which is in my cathedral ceiling. The EMT ends in a large crawl space (I have to stoop and crawl to get to where the EMT ends but then I can stand up) and from there I am planning to connect it to a J-box where I splice it to Romex. The Romex will run for a very short distance in the crawl space and then down a hole in the framing header into the bathroom wall below. The Romex would end in a receptacle box. Sound good?
 
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