Sheathed Cable in Conduit

[snydl0ga]

I don't want to start a firestorm but I am having trouble understanding the NEC's position on using jacketed NM cables in conduit. I can understand it being allowed for short distances for protection but what is a simple rule of thumb on this?

[Jim Baird]

Lance,

I can't(won't) quote you chapter and verse but I think it has to do with limits on filling conduit volume and with the conduit maybe interfering with heat dissipation.

If you're an electric engineer, seems like your grasp would exceed that of most of us.

[hausdok]

Hi,

If you're serious about being a true professional in this kraft, you really need to get Doug Hansen's book, Electrical Inspection of Existing Dwellings. It's written specifically with the questions of home inspectors in mind and has the answers to virtually every question that will ever come up.

Here's what Douglas says about this:

Page 5.12

NM cable is not designed for installation inside of conduit as a wiring system, but conduit may be used to protect cable where necessary. Metal or PVC conduit can be used to protect cable on a garage or basement wall [336-6b]. If the conduit goes all the way to a panel enclosure, the cable sheathing should not be stripped off inside the conduit. Instead, the cable sheath must extend at least 1/4 inch into the panel, and the cable must be secured with a clamp at the other end of the conduit [373-5cX].

Conduit should not enclose NM cable as a complete wiring method from one pull point to another; it is only allowed as a means of protecting the NM cable system in localized areas where it is exposed to damage. If several cables are installed in a conduit, the conduit could entrap heat given off by the cables, which could damage their insulation. An example of an area where cable might need protective conduit is a garage wall surface.

When NM cable makes a transition to another wiring method, a box is needed. The conductors inside NM cable may not be stripped out for use as individual conductors inside conduits. Individual conductors in a conduit system require markings, such as temperature ratings, on the individual conductors. In figure (Sorry) the sheathing was tripped off the cables and the conductors were pulled through a conduit. The proper way to do this would be to install a box, terminate the Romex in the box, and install properly rated conductors inside the conduit.

I think that answers your question, no?

ONE TEAM - ONE FIGHT!!!

Mike

[snydl0ga]

I purchased his check list publication and loved it. Will try this as well....thanks!

[Jim Katen]

Originally posted by snydl0ga

I don't want to start a firestorm but I am having trouble understanding the NEC's position on using jacketed NM cables in conduit. I can understand it being allowed for short distances for protection but what is a simple rule of thumb on this?

Try pulling it sometime and you'll know right away why it's a bad idea. The conductors in Romex are solid. If you try to pull them through any significant distance or around bends, something's going to get damaged.

Other than that, everything that Douglas says in the quotation that Mike posted.

- Jim Katen, Oregon