EMT for new circuits

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  #1  
Old 07-16-07, 10:19 AM
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EMT for new circuits

I need to add two new circuits (20A). The easiest routing is through an overceiling crawlspace which will get me to an outside wall. I plan to run it through the wall to an exterior junction box and from there along the exterior of the house (stucco) using EMT with compression fittings to the service panel.

I'm using NMB 12/2 Romex inside to the box. From there, I assume I can use the individual insulated 12 gauge white-black-green wires through the EMT to the service panel, one set for each circuit. Can I use just one green (ground) through the EMT, or do I need separate green wires for each circuit? Also it would be convenient to put in a couple of extra lines for future use. How many wires can I fit in 1/2" or 3/4" EMT?
 
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  #2  
Old 07-16-07, 11:11 AM
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The relevant Code Article on the question of a single Equiptment Grounding Conductor (EGC) is 250.122 (C), Multiple Circuits.-------

"When a single EGC is run with multiple circuits in the same raceway ( EMT ), it shall be sized for the largest (rating / amps) of the (circuit-breaker) that protects conductors in the raceway"

If you had 3 Un-grounded conductors in the same run of EMT, one 15 amps, one 20 amps, and one 30 amps, you would need a single #10 Green copper conductor for the EGC. -- Table 250.122
 
  #3  
Old 07-16-07, 11:41 AM
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> Can I use just one green (ground) through the EMT

Yes, the ground size should match the largest hot size (#12).

> How many wires can I fit in 1/2" or 3/4" EMT?

In either case, 4 circuits (hot-neutral pairs). This is based on the heat rating, not the conduit fill so the limit is the same for either 1/2" or 3/4" conduit.
 
  #4  
Old 07-20-07, 08:43 PM
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Thank you.

re: no. of wires in EMT, 4 20-Amp circuits means nine #12 wires. That's a pretty tight fit in 1/2" EMT, definitely no fun to pull! Stanley's publication "Advanced Wiring, Pro Tips and Simple Steps", copyright 2002, says "in general, 1/2 inch conduit is large enough for five or fewer wires; 3/4 inch conduit is used for more than five wires." Have you actually pulled 4 circuits in 1/2" EMT?!?
 
  #5  
Old 07-20-07, 09:11 PM
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Good case for a multiwire circuit with the neutral shared for each pair of circuits. Just need to know the cautions on multiwire circuits to make sure they stay on opposite busses in the panel.

That would only need 7 wires for 4 circuits.
 
  #6  
Old 07-20-07, 10:26 PM
ddr
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I would definitely use 3/4" EMT for 9 wires. Even though code allows 9 #12 wires in 1/2" EMT, it is the maximum allowed and, as you said, a tight fit. Using the 3/4" will give you some breathing room.

Also, since you are running the conduit outside, I believe you must use wire rated for wet locations (THWN). You will find that most THWN wire comes dual rated as THHN/THWN; this is fine to use.
 
  #7  
Old 07-25-07, 10:06 AM
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OK. I took all my 1/2" stuff back to Home Depot and got 3/4". The junction box 'm using is a "2-gang aluminum outlet box" similar to the one shown at this site:

http://electrical.hardwarestore.com/14-50-weatherproof-boxes/aluminum-outlet-box-gray-605355.aspx

My plan is to fit the 4 NM (yellow) cables coming from inside the home out through a 1" hole in the stucco into the hole in the back of the box. The box will be attached with its back flush to the outside stucco wall. How can I anchor the cables to the box and still have it flush with the wall? Can you suggest the appropriate hardware (maybe a picture from the same web site above would help). Or would an LB box be better? I can't anchor it on the interior side of the wall without removing a big chunk of drywall. Any suggestions?

Thanks again!
 
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Old 07-25-07, 10:48 PM
ddr
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walks,

The pros will have to confirm anything and everything I say below, but you have a couple of issues to contend with:

First, having 4 12/2 cables coming in and being spliced to outgoing individual wires would exceed your allowed box fill. You need 2.25 cubic inches (ci) per #12 wire entering the box (all grounds count as one), so in your case you have 8 hots (4 in, 4 out), 8 neutrals (4 in, 4 out), plus 1 for the grounds which is 17 x 2.25 or 38.25 ci needed. A standard 4-11/16" x 4-11/16" x 2-1/8" box is 42 ci and listed for 18 #12 wires. The box you indicated is not as big, so you can't put all those circuits in the way you plan.

Second, even if the box was big enough, I don't believe NM cable is allowed outdoors AT ALL, not even entering an exterior junction box to splice to THWN so, if I'm right about this, you would need to make your splices inside anyway. If this is the case, you could use a short piece of threaded 3/4" rigid conduit (enough to get you inside the house to another box), thread it to the back of the exterior box, drill a hole just large enough to accommodate the conduit, slide the conduit in until the back of the box is flush to the outside, and secure it. (You could use an elbow on the exterior as well). I believe that rigid conduit requires bushings on the threads to avoid damaging the wires.

As I said, I could be wrong about this, so don't do anything until the pros confirm what I said. Hopefully they'll give you a better way to run the wiring.
 
  #9  
Old 07-26-07, 10:03 AM
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The box I'm using is 4-1/2" by 4-1/2" by 2" which is 40.5 cu. in. which looks big enough according to ddr's calculations (although the thickness of the metal where the holes are located may reduce that a little). The electrical 'specialist' at Home Depot said the box would be big enough for this application. I'd like to confirm this, though, before I install it. Any comments from the pros?

When I explained my plan in the 1st post of this thread, none of the respondents suggested it's against code to take the NM wires directly to the exterior junction box, so I'd like to confirm that as well. The main problem with this wiring project is that the interior wall near the junction box is inaccessible without some serious drywall demo & repair, which I'm trying to avoid.

If there's no legal way to do what I proposed, I suppose I could run rigid conduit (or EMT? or other type of conduit?) from the back of the exterior box, inside through the crawl space to a more accessible location where I could put a junction box, probably about 20' away.

Thanks again for all comments!
 
  #10  
Old 07-26-07, 10:08 AM
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An exterior box mounted on the wall must be just that, exterior and waterproof. It must also be a surface mount box. You can then have NM cable enter the box from the inside of the house/wall.
 
  #11  
Old 07-26-07, 12:47 PM
ddr
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walks,

Sorry about giving you the wrong info about NM cable feeding to an exterior j-box. That's why I tell people to wait for the pros to check in unless I'm absolutely sure about something.

Good luck with your project.
 
  #12  
Old 07-26-07, 01:30 PM
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I don't know of any box clamp that is rated for 4 NM cables, so your box will need at least two entrances in the back for the NM. It'll be a tight fit, but do-able.

What is the intended purpose of these circuits? As MAC suggested multi-wire circuits may be a better option. You could reduce from four 12/2 cables down to two 12/3 cables, and down from 9 wires in the conduit to 7.
 
  #13  
Old 07-26-07, 05:02 PM
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Thumbs up

Three of the 4 circuits will be used for a kitchen remodel (1 for dedicated refrigerator, 1 for dedicated microwave and 1 for lighting), and 1 for future expansion (probably garage receptacles).

Would you happen to have a link to a web site that shows a picture of the clamps you're talking about? (or see the link I cited previously)... The more I think of it, I'm nearly resigned to opening up the drywall and try to staple the NM wires to the wall framing at the back side of the box. It'll add some extra time, but I'd like to make sure it's done correctly.

Other than the clamping, I think I've got a good handle on this project now, thanks to all the advice. This is by far the best web site I've ever found for getting advice and others' opinions in trying to solve everyday problems. Thanks everyone!
 
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