NM cable (romex) in EMT (conduit) allowed?


Old 01-20-09, 12:01 PM
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NM cable (romex) in EMT (conduit) allowed?

Would like information on potentially adding receptacles to an unfinished basement. 1/2" EMT condiut would be run from existing lighting boxes to the new metal receptacle boxes. May NM cable (14-2 romex) be run thru this EMT, or must separate wires (THHN?) be pulled? Any explanation accompanying the answer would be appreciated.

Thanks and best regards
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Old 01-20-09, 12:22 PM
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A single piece of NM-B cable may be pulled through conduit, but it is not considered to be good practice. Typically you would use THHN conductors.

The reason is because of conduit fill and flexibility. The cross-sectional area of three THHN conductors is must less than that of a romex cable with the added bulk of the outer jacket and inner paper wrappers. Code limits the conduit fill to 40% which is roughly either 10 individual conductors or one romex cable.

Furthermore, the individual conductors pull more easily through the conduit requiring less force to pull and therefore less chance of kinking or skinning the conductors.
Old 01-20-09, 12:30 PM
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You are allowed to pull NM cable through EMT indoors as long as you don't exceed the allowable fill. There has been some confusion on this issue for historical reasons, but it is definitely allowed as of the 2005 edition of the NEC. For elliptical Romex cable, the fill is calculated by considering the Romex to be a single conductor whose diameter is the major diameter of the ellipse.

Individual wires will be easier to pull, however. I don't see any reason to use Romex unless you happen to have a bunch of it left over from another job.
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