Can't run Romex in conduit?

In Home Depot yesterday noticed large signs warning that Romex must not be run in conduit, according to NEC.
I wasn't planning to do so, but I'm wondering why not.
-=- Alan
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Article 334.15 (B) NM - Exposed work (B) Protection from physical damage The cable shall be protected from physical damage where necessary by conduit, EMT, schedule 80 PVC NMC, pipe, guard strips, listed surface metal or nonmetallic raceway, or other means.
Looks like it is fine for exposed work and I can not find anything that explicitly denies it. However, if you run it in conduit you must use the width rather than the thickness for the conduit fill calculations.
Nate
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when I lived in Bay Area CA. but never saw it in writing. They will still let you put romex in short runs of conduit in exposed areas below 7'. An electrician did this in my unfinished garage bofore I moved in. He used just enough conduit to get up to 7' above floor where it does not have to be protected. He used conduit cable clamps on the ends and all passed inspection. I later rewired the whole garage using EMT and THHN wire. Kevin
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This is Turtle.
Yes it is because of it adds to the heat build up inside the wire as the power is flowing through it. In conduit the thinner coating will let heat out of wire and in conduit housing and then transmitt to the outside of conduit. The extra coating on the wire slows the heat down and wire will run hotter than normal if it just had the thinner coating on it. Now it does not seem to be a big deal but you got to follow the NEC because the next fellow coming behind you may change something else and add more trouble to the problem that you left behind of Romex in the conduit. Then the next fellow adds more till , Then Bam Bam you have a fire. Follow the NEC rules and just stay out of trouble all together.
Some of the stuff may seem stupid but everyone has a reason for it.
TURTLE
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As a practicable matter... Try pulling some romex through conduit - especially through a bend in conduit. Not easy to say the least.
Maybe because it is so hard to pull, someone could pull too hard and the wire would get damaged/break. Or in a bend one side would have too much tension on it since the jacket tends to hold the whole works together.
Loose individual wires will "slide against each other" at a bend in conduit.
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That has been my experience too. Ive torn the outer jacket trying to pull it around a bend.

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Turtle is right. I recently ran electrical to a detached garden shed, and questioned the city inspector about this subject during his inspection of my work. He said the heat build up is the concern.
He also said he will allow a small run of it from a interior wall into a J-Box. The conduit run should be less than a foot. This was my case. I had a exterior mounted j-box where my run to the shed began and I tapped off an existing interior wall box. So I ran romex out from the existing interior wall outlet to a hole in the exterior wall, where a short piece of conduit (2-3 inches) protruded into the wall space leading to my exterior j-box. Then in the exterior j-box wired the romex to standard THHN wire, which made the run through the conduit under the ground to the shed.
He said in the case I just outlined this is the prefered method. The romex is used in the interior cavity of the wall, but comes into a short run of conduit that seals out the elements as it exits the exterior wall into the j-box. He said in this case the conduit is more of an extension of the j-box since it's only a couple of inches long. Mark
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