NM Cable in Metal Conduit for protection

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  #1  
Old 07-03-09, 09:19 AM
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NM Cable in Metal Conduit for protection

I know that the topic of Romex in conduit for protection has come up before but I'm seeing conflicting answers so I'm a little confused. Please advise if the following is acceptable practice:

I'm building a new addition and I need to extend 3 12/20 lines from the panel in my garage to a location in the basement. The cables will need to enter the basement at the end of an ajacent wall so the total length of the run inside the garage will be about 40 ft.

The garage walls are finished with drywall so if I wanted to protect the wires by running them through the studs I would have to tear out a lot of drywall to get to them. Instead of doing that, I'd like to exit the top of the panel and go into 1" metal conduit that runs across the length of both walls (at 6' above the floor) and then down to the point where the lines need to enter the basement through a hole in floor joists.

First of all, is it legal to run NM through ungrounded metal conduit for a distance this long? I'm not opposed to using PVC but I was thinking that metal would provide more protection in a garage.

Also, can I go into the wall at the floor with the conduit or do I need to have a section of exposed wire before entering the floor joist? Same question on the other end at the box... is there anything special about the way that the three lines enter the conduit or the panel?

Is 1" metal conduit wide enough for 3 runs of 12/20?

Sorry if this has been asked before but I'm seeing everything from "don't run NM cable in conduit" to "NM in conduit is fine as long as it's only for protection".

My county uses the 2008 NEC without modification.

Thanks!
 
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  #2  
Old 07-03-09, 11:57 AM
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You CAN use conduit, either EMT or PVC for protection of type NM cable but you will have a hard time pulling the NM through the conduit. You will find it next to impossible to pull it through any bends, regardless of how large a conduit you use. You do need to protect the cable from the sharp edge of the end of the conduit and that usually means connectors and bushings although Arlington Industries does make a plastic slip-on bushing for EMT.

Another option would be to build a wooden wireway around the cables to protect them. Myself, I would use the conduit with junction boxes at each end to transition from the NM cable to individual type THHN conductors through the conduit and then back to the cable at the other end of the conduit run.

When going through the wall into the basement you are likely penetrating a code required fire barrier so be sure to use a code approved method for this penetration.
 
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Old 07-03-09, 07:51 PM
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Thanks for the feedback. I've decided to take your advice and run NM into J boxes on both ends but use individual wires inside the metal conduit.

Only rather than running 9 individual #12 wires I think I'm going to place a subpanel in the basement and use it to power the 3 circuits that I mentioned plus 3 others from an old re-model job that are, IMO, a bit messy. This will let me have a single run into the basement from the box in the garage for everything in the basement plus the new addition.

So now I just need to figure out what size subpanel and conductors to buy for 6 15 amp circuits. If I'm reading the ampacity table correctly, I guess this means I'll need to use #4 wire with a 100 amp breaker at the main panel and a 125 amp sub panel (haven't seen any 100 amp panels so I guess they don't make those??).

Does that sound about right if I go this route?

Alternatively, I could go with only 5 circuits (since one was just to have future capacity) and use #6 and a 60 amp breaker at the main box.

Any feedback on this would be a big help. Thanks again!
 
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Old 07-03-09, 08:25 PM
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A 60A feeder will be more than adequate for the six circuits you listed (the sum of the breaker handles is irrelevant). Use 3/4" or larger conduit*. Pull three #6 copper THHN conductors in black, red and white, and a #10 green. Use a 60A double-pole breaker in the main panel. The rating of the actual panelboard can be anything 60A or greater -- a 100A or 125A panel will probably have the number of slots you need. Buy a ground bar kit for the subpanel you select and remove the bonding screw/strap. A main breaker is not required in the subpanel -- main lugs only is okay. Follow furd's advice regarding the firewall penetration.

* Use 1" conduit or larger if you want the ability to expand to 100A in the future.
 
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Old 07-03-09, 09:38 PM
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I completely agree with ibpooks. You could actually have six 20 ampere circuits with the #6 feeders and 60 ampere sub-main circuit breaker without overloading the sub panel. Remember that the total number of branch circuit breakers will be divided between both "hot" wires of the 240 volt circuit. Six 15 ampere circuit breakers would be a maximum of 45 amperes on each of the two "hot" wires. Furthermore, you will likely never approach the maximum of 15 amperes on any one of the branch circuits.
 
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Old 07-03-09, 11:28 PM
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Thanks for the help fellas. This information will be a huge help to me.

After 2 days of flipping through the NEC and trying to think through this, I'm really starting to appreciate how much knowledge a pro really has.

I'll stick to the plan and add 6 15 amp circuits to the sub panel with a 60 amp feeder. It's good to know I have plenty of extra capacity so I may even run an additional dedicated circuit for a treadmill that tripped the breaker down there once before.
 
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Old 08-08-09, 10:10 AM
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alright, I finally got the building permit and next week I meet with the electrical inspector to go over the plan.

I still have a couple questions though so I'm going to just keep it all in this thread.

First, when I transition from conduit to NM in the garage, where the line enters the basement, what's the best way to do that?

Obviously, I'll need a J Box to convert from individual conductors to NM. Is it okay to just surface mount the box in the garage and then have a short section of NM cable (maybe about 6 inches) that enters a hole in the floor joist and then into the basement? If that's okay, what's the best material to use to seal around the cable for insulation / fire resistance? I won't be entering any stud cavities so I don't think that's an issue. ...just need something to seal around the cable. btw, I can tap into the floor joist from the garage because the floor of the garage is about a foot below the floor level of the adjacent room.

If it's not legal to have any length of unprotected surface mounted NM in a garage (can't find this in the NEC), is it okay to go through the joist with the conduit and then mount the J Box inside the basement instead? Alternatively, I could mount the box in the garage but go directly out the back of it, through one of the knock outs, and then into the hole in the joist. That way no wire would be exposed at all. I haven't seen any examples like that though so I'm not sure if it's considered bad practice.

My other question is with regard to the way the wires are brought into the new sub panel in the basement.

Considering that the sub panel will be inside a furnace room that is almost never occupied, is it okay to go over top of the exposed studs and joists before entering the panel or do I still need to drill holes to pull the wires through the wood? If I go over top, each cable would be unprotected for about 2-3 feet before entering the subpanel.

Thanks again for the help. If all goes well, I'll be kicking this off next week. Should be a lot of fun.
 
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Old 08-09-09, 06:19 AM
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If the NM cable is big enough (8-3, 6-3, etc), you don't need to drill holes through the joists. However, at only 2-3 feet, why not run conduit the full length from main to sub? Then you can pull wires the full length and not have to deal with any splices. Less splices means less chance of a splice failing later on.
 
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