Using 2/3 NM-B to connect 100 A subpanel

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Old 03-26-12, 10:36 AM
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Using 2/3 NM-B to connect 100 A subpanel

I plan on running 2/3 NM-B from main to subpanel. The issues I have are at both ends. This cable will exit the panel side and penetrate a sheet rock wall. I want to run it on the outside of the sheetrock and need to protect it.

What size PVC conduit can I run it in? (Is 1 1/4" PVC okay?)

Can I strip off the casing and just run this section (total of ~15-20') through 1 1/4" PVC schedule 80?

The cable will then penetrate another sheet rock wall and go through the joist.I plan to drill the joist and put another 1 1/4" PVC stub through it.

Under the house I will staple the NM-B bundle to the joists.

When it comes back up it is a straight shot through subflooring. I have a 4' section of 1 1/4" PVC Pipe here.

Does the cable need any clamping at either end as it passes through the two boxes?

Thanks in advance.

K
 
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Old 03-26-12, 12:08 PM
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What size PVC conduit can I run it in? (Is 1 1/4" PVC okay?) Best practice if continuous conduit is to use individual conductors such as THHN/THWN. Conduit can be used to sleeve short sections of cable if the cable is protected by other means for the rest of the run. 1" conduit would be adequate.

Can I strip off the casing and just run this section (total of ~15-20') through 1 1/4" PVC schedule 80?
No. That is not permissible because the wires within the cable are not certified for that use. You would need to run the unstripped cable or transition to approved conductors such as THHN/THWN.

Under the house I will staple the NM-B bundle to the joists.
It would be better to use conduit to prevent gnawing by animals.

Does the cable need any clamping at either end as it passes through the two boxes
No, but using conduit the whole run with individual wires would be a better choice in my opinion.

What size breaker are you planing to use at the main? If it is 100 amps or less you can use #8 for the ground (60a=#10 for ground)

Correction: I was thinking conduit when I wrote no clamps. As Zorfdt pointed out boxes would require clamps.
 

Last edited by ray2047; 03-27-12 at 11:38 AM.
  #3  
Old 03-27-12, 10:45 AM
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I agree with Ray that your best bet may be conduit the whole stretch.

But if you do stick with NM-B, it still needs to be clamped when it enters each box. You can use PVC conduit as a sleeve where it may be subject to damage, but there would be no clamp on either side of the PVC, only where it enters the panels.

Also, remember your bend radius on cable that large is pretty big. If you use any LB fittings, make sure the cable can make the bend.
 
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