conduit question

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  #1  
Old 10-13-05, 11:47 AM
Tamarin
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conduit question

When is it necessary, not necessary to run conduit?

I know that any exposed wire needs to be protected.

Specifically, I am running approx 30ft of 3/3 copper from one panel to another in my basement. I can run the wire through 2x10 floor joists and in my mind there is no way the wire can be damaged. If I run conduit I'm guessing I'll have to build a raceway rather then setting the wire into the floor. Both solutions are fine and I would like to do it the right way.
 
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Old 10-13-05, 12:00 PM
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You are not allowed to run 3/3 between panels in the same structure unless it has a large enough ground conductor. Plus, #3 NM cable would only have a 60 degree ampacity of 85 amps I think. As I recall from your previous post you are installing a 100 amp feeder. Interior subpanels need a 4 wire feed, so you would need something like 3-3-3-5 SER copper or 2-2-2-4 SER copper cable depending on 60 or 75 degree terminal ratings. The application of the rules vary a little by jurisdiction, so I recommend you either run #3 THHN in conduit or check with your inspector to see what type and size of cable assembly he will allow; I'm pretty certain that 3/3 NM-B will not be allowed however.

To answer your original question, conduit is needed if you are using individual conductors (THHN) instead of cable assemblies (romex, NM, SER); if the wiring is exposed to damage (basement walls, garages, etc); or for unfused feeders such as the service drop from the power company. Conduit is very useful if you need to run several circuits to the same location or if you plan to upgrade, add, or modify circuits in the future where simply pulling new wires would be easier than ripping down drywall. Exposed cable assemblies #8 and larger can be run on the bottom of basement ceiling joists without additional protection.
 

Last edited by ibpooks; 10-13-05 at 12:19 PM.
  #3  
Old 10-13-05, 02:47 PM
Tamarin
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I'm getting a wire called AWG/3 NMD90 (NM-B in US). It is a 4 wire cable. I think it is 3-3-3-6. In addition to this site which has been very helpful I was lucky enough to run into an electrician at Home Depot (there are electricians on staff but very rare to ever see one, this guy was just some random electrician picking up a breaker). He said that the wire was suitable for a 100a subpanel and reminded me to remove the bonding screw.

Next we come to terminals. All of the current stab-lok circuit breakers have a 40 degree celsius (104 fahrenheit) mark on them. Is this what you mean by 75 degree terminals? Are we talking celsius or fahrenheit? It looks like most/all of the wire is rated at 90 (celsius). If all the current breakers are rated at 40 degrees I'm going to go out on a limb and assume that the panel is rated for this type of breaker.
 
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Old 10-13-05, 04:00 PM
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Originally Posted by Tamarin
I'm getting a wire called AWG/3 NMD90 (NM-B in US). It is a 4 wire cable. I think it is 3-3-3-6.
Usually, NM cable is only allowed to be used at its 60 degree C rating, which is 85A for #3, 95A for #2 and 110A for #1. If you are allowed to use NM up to the 75 degree C rating, then #3/3 will be rated at 100A and you will be okay. It would be wise to check with your local building department on this before you fail inspection after purchasing expensive wire. There are a few professional Canadian electricians on this board who may chime in if code differs from the US.

Next we come to terminals. All of the current stab-lok circuit breakers have a 40 degree celsius (104 fahrenheit) mark on them. Is this what you mean by 75 degree terminals? Are we talking celsius or fahrenheit?
In order to use the 75 degree rating of the cable, the termination lugs at both ends of the cable must have at least equal rating of 75 degrees C. All newer breakers and panels will carry a 75 degree rating, but since you are using older equipment it would be good to investigate. Older stuff is sometimes only rated to 60 degrees, so you would have to select the cable based on a 60 degree rating.

I honestly don't know if there are breakers terminals rated at only 40 degrees, but I suspect not. There may be another marking somewhere on the breaker that give the temp ratings of the lug. All temp ratings for wire and terminals are in celcius and usually 60, 75, or 90 degrees.
 
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