Can I run one or more cables through conduit?

Question

I'm running outlets in my workshop. The wall has modular insulation blocks, which have a wiring channels already designed into them, so running cable through the wall is not difficult; I've already done so on the wall opposite the (surface mounted) subpanel and installed boxes for flush outlets.

But for the near wall, I am now thinking it would be easier, and more flexible, to run surface mounted conduit for the MWBC to supply 120V and/or 240V to the larger power tools.

However, I have already purchased NM-B (not NMC, as originally written) 12/3 cable for the circuits. Wasting that investment would not be a disaster, but copper isn't cheap these days.

Question:

1. Can I use that cable inside metal conduit, and if so could I have more than one in a single section of conduit?

2. If the answer to either is no, would it be completely insane to strip the outer cable sheathing and use its component individually insulated wires within the metal conduit? Again, could I run multiple MWBC's thru the same conduit?

3. Or do I need to/should I purchase single wires with a different kind of insulation (or at least certification) for this application?

Answer 1

1. Can I use that NMC inside metal conduit, and if so could I have more than one in a single section of conduit?

Yes. As far as more than one, you do have to be concerned about the number of current carrying conductors, much the same as if you used individual wires. Off the top of my head, I would say two cables would be fine, more than that and you should run the calculations to see whether it is OK or not. And definitely make sure you are OK on conduit fill, as that is often a problem with cables.

2. If the answer to either is no, would it be completely insane to strip the outer NMC sheathing and use its component individually insulated wires within the metal conduit? Could I run several thru the same MC?

3. Or do I need to purchase single wires with a different kind of insulation for this application?

NM cable normally does not have individual wires appropriately rated and marked for use without the outer sheath.

If you are using conduit, individual wires (THHN, THWN, etc.) generally works better. Conduit fill is less of a problem. Physically running the wires through is easier. Depending on usage, you may be able to use smaller wires to get the same job done because they are rated at 75 C where NM cables are typically rated at 60 C. For example, the Southwire Ampacity Chart shows that for 20A circuits you can use 14 AWG wire for 20A instead of 12 AWG cable, though it is all dependent on all components meeting the higher temperature specification.

Answer 2

When you start talking about pulling NM-B cables in to conduit you are knocking the door of ratings and calculations hell.

1. You can run NM-B in conduit, but NEC Chapter 9 Table 8 Note 9 says for multi-conductor cable the maximum fill calculation shall be based on using the major diameter of the ellipse as a single conductor. So based on Southwire's published major dimension of 12/3 of 0.524 one cable (53% max allowed) requires 3/4" conduit, two 12/3 cables (31% max fill) requires 1.25" conduit, and 3 cables (40%) would require 1.5" conduit. (Conduit size based on EMT. Inner dimension of SCH 80 PVC is smaller requiring 1", 1.5", 1.5"). Amperage Derating not a concern until you exceed 4 12/2 or 12/3 cables.

2. The individual conductors insulation is not marked for insulation, temperature, or UL/CSA/ETL rating.

3. Purchasing THWN-2 individual conductors would allow much more flexibility allowing 9 conductors in 1/2" EMT or 6 conductors in 1/2" SCH 80 without additional derating and would definitely be easier to install.

EDIT: I forgot to mention, there is an odd NEC section that requires securing most cables that enter cabinets through conduit be secured to the cabinet. I've never seen that section enforced and never seen a fitting that serves this purpose.