A quick question about conduit...

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  #1  
Old 11-20-04, 04:34 PM
SkyKing
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A quick question about conduit...

I have tried to look for tables for appropriate conduit size but cannot find any for nm-b.

I want to run 4: 12-2 NM-B cirucits, 3: #4 THHN wires, and 1: #6 THHN wire.

The wire will be ran though a small portion of attached garage, into the ceiling of my unfinished basement, and a short LB to the disconnect. The 12-2 ciruits will branch off at various locations though a swept union. There will be three 90 bends. (with swept radius' of course)

Can I run all of this in 2 1/2" Schedule 80 Conduit? If not, is there a size (within reason) that allows me to do this?

Thanks!
 
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  #2  
Old 11-20-04, 05:48 PM
SkyKing
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Wow...

I didn't intend to start a riot on this question... so please don't... I didn't realize running NM-B in a closed system was such a big deal...

If anyone wants the argument for either side of the situation, read the this post:

http://www.handymanwire.com/ubbthrea...b=5&o=&fpart=1

I found this information to be extremely useful.. BTW, am I allowed to post a link to another forum's website? If I am violating the rules let me know...

But this article is very enlightening.

John, any thoughts?
 
  #3  
Old 11-20-04, 08:41 PM
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Whether or not it is legal is a moot point. It's a terrible idea. The biggest issue is ampacity derating. You'll have to derate those wires so much that they will no longer be able to serve their intended purpose. Why would you want to use conduit anyway?
 
  #4  
Old 11-21-04, 08:37 AM
SkyKing
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Well, the 12-2 nmb lines are for the basement and have to run exposed into through the garage for about 8 ft and into the unfinished basement ceiling. I was under the impression that you are required to have any lines in an unfinished ceiling in conduit to prevent any physical damage. Only 18" of such can be exposed?

I know the thhn wire I'm running has to be in the raceway and I thought it would be easy enough to run the 12-2 in as well.
 
  #5  
Old 11-21-04, 09:26 AM
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So you are only running this for eight feet? Then go ahead and use the conduit, but use two smaller ones rather than one large one to avoid ampacity derating issues. Of course, the requirement for conduit at all depends on how exposed "exposed" is.
 
  #6  
Old 11-21-04, 06:06 PM
SkyKing
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Well, it's just open floor joist ceiling. I think I have found a solution. I will use the 2" conduit for the short garage run (where the wires would be subject to damage) and then when I get inside the basement I'll use a T 90 deg. LB style box to send the nm-b out of the conduit and the thhn lines continue through sched 80 1 1/2" conduit to the spa disconnect. I'll drill and run the nm-b through holes in the joists in the basement to get to the required spots.

If you have any more suggestions I'd love to hear them.

Thanks,

Sky King
 
  #7  
Old 11-22-04, 04:18 AM
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As John said, it depends upon how 'exposed' thing are and what requirements your local inspector places on mechanical protection.

I would suggest using a 'running board' for the NM. This is just a strip of 1x wood laying flat on the bottom of the joists, with the NM stapled to the bottom of this. Put the loose THHN into a smaller conduit of its own.

Don't put all of these cables into a single conduit for more than 24 inches. When multiple circuits are in a single conduit, you need to derate the circuit because of mutual heating. You are permitted to have multiple circuits in a single short conduit for doing things like going through walls and floors, thus the 24" comment.

-Jon
 
  #8  
Old 11-22-04, 08:52 AM
SkyKing
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Let me clarify.

I HAVE to put sched 80 in the garage because it is/will be subject to physical damage. No if ands or buts.

The basement is my real question. It is a standard 8 1/2' basement with open floor joists that are 12" deep. It's just open. Do I need to put NM-B in conduit?
 
  #9  
Old 11-22-04, 09:05 AM
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All we are saying is to use two smaller conduits rather than one large conduit.

In the basement, if you drill the joists, you don't need conduit for the NM-B. This would be the preferred approach. Drill the holes spaced out at least 3 inches apart, dead center (top to bottom) of the joist, and no larger than necessary to hold a couple of cables. Preferrably neither too close to the supported ends nor too close to the center of the span.

If you prefer to skip the holes and mount the cables to the bottom of the joists, you may use conduit if you want, but running boards are easier. Just run some one-by boards across the bottom of the joists and staple the cable to the one-by.
 
  #10  
Old 11-22-04, 09:31 AM
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In particular, run the #4s in their own conduit, and run the 12/2s in a separate conduit.

Wgoodrich wrote:
NEC 310.15.B.2.A table says that if you install 3 current carrying conductors or less the ampacity of a conductor in a conduit can be rated at 100 %. That same table says 4 to 6 current carrying conductors in a conduit must be derated down by 20 %. That same table says 7 to 9 current carrying conductors in a conduit must be derated down in ampacity by 30%. That same table says 10 to 20 current carrying conductors in a conduit must be derated down in ampacity by 50 %.
Basically, if you put everything into one conduit, then you could only use 15A breakers on your 12/2s, and your #4s could only be protected at 50A or lower. The #4s need to be in conduit the whole way. The 12/2s can remain in conduit (you already noted the discussion about this, and I won't rehash; I'm entirely comfortable with NM cable in conduit as long as you watch for conduit fill and derating), or you could transition out of the conduit using a junction box and suitable clamps.

-Jon
 
  #11  
Old 11-22-04, 09:43 AM
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SkyKing,

Just a minor clarification. With all the talk of derating, if you are using NM-B, ("Romex"), the overall jacket inhibits heat dissipation. Indivudual conductors in conduit dissipate heat much better. The overall jacket on NM-B traps it around the conductors within it. Hence, you are restricted from using the normal full capacity of that #12/20 amp wire. Reduce the amperage and you reduce the heating. Hence the term "derate".

Juice
 
  #12  
Old 11-22-04, 01:55 PM
SkyKing
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I understand what you are saying. Thanks for the info!

I thought about using thhn for all the circuits but didn't want to because of convienience.

I'm actually running the 12-2 lines for 15 amp circuits. In case I ever want to redo the system that is already in place and make it 20 amp.

I'd also rather run all the lines in the same conduit in the garage but I think I'll just remove the drywall and run it inside the wall.

I appreciate your repsonses!
 
  #13  
Old 11-22-04, 05:10 PM
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I say "why wait?" Put in the 20-amp breakers to start with.
 
  #14  
Old 11-22-04, 05:17 PM
SkyKing
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I'm actually balancing a load problem that the previous owner created. So there is exsisting wiring that is 14-2 and 14-3. All the outlets are rated at 15 as well. I really don't feel like upgrading all of the wiring at this time.
 
  #15  
Old 11-22-04, 05:37 PM
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Existing wiring and existing breakers are not important. If you are adding new circuits and new breakers they can be 20 amp circuits, without regard to any existing circuits..
 
  #16  
Old 11-22-04, 05:43 PM
SkyKing
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I don't think I'm explaining this clearly. Probably because this isn't the question. The lines are 14-2 and the outlets are rated for 15 amps. NOT 20 amps. I cannot put 20 amp breakers on these lines because they are not rated for 20 amps.

I'm balancing them because the previous owner overloaded two of the circuits with potential draws (or outlets).

So yes, the wiring is important. All I'm doing is running a new feed to the exsisting cirucits. The only thing that is changing is the feed.
 
  #17  
Old 11-23-04, 05:55 AM
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SkyKing,

Unless you have a dedicated receptacle on a 20 amp circuit, you are permitted to have 15 amp rated receptacles on a 20 amp circuit. For instance, if you have a window air conditioner that requires a 20 amp circuit, and only a single receptacle near the unit that is installed specifically to serve that unit, that receptacle must be rated 20 amps because it was intended to draw 20 amps based on the A/C nameplate.

But if you have a string of general purpose receptacles, it is unlikely that you will pull 20 amps through any one of them, so the NEC permits the 15 amp rated receptacles.

However, you are correct that because the circuit in question is wired with #14, you can't have a breaker controlling that circuit that is greater than 15 amps.

Of course, you can certainly install #12 wire on a 15 amp circuit. But you seem perfectly aware that you can't have #14 on a 20 amp circuit.

Juice
 
  #18  
Old 11-23-04, 08:52 AM
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Because you will have 12-gauge wiring at the panel, you may confuse some future homeowner (or yourself when you get old) into thinking that all the wiring on the circuit is 12 gauge. So please put a note in the panel explaining that there is 14-gauge wire on this circuit to prevent someone from changing the breaker without changing the rest of the wire.
 
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