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Nonmetallic Sheathed Cable

Throughout the vast majority of the U.S.A., nonmetallic sheathed cable (NM cable) has been the most common type of residential wiring system since about the early 1950’s.  And it’s had a pretty good track record of success.  It consists or two or more insulated wires along with a bare ground wire, wrapped in paper and then contained inside a thermoplastic outer sheathing.  It’s often called Romex®, but that’s a brand name and not all NM cable is Romex®.  It comes in large spools of different wire sizes, it’s easy to install, it’s easy to cut to length, and it goes up pretty quickly.

Despite its long history of successful usage throughout most of the country, in much of the Chicagoland area this type of wiring isn’t allowed.  Chicago and many of the surrounding suburbs require that wires be pulled through some type of raceway, which usually is electrical metallic tubing (EMT) and normally called “conduit”.

So when you see NM cable in a part of Chicagoland where it’s not allowed, what should you think?  And what should you do?

The first thing to note is that if this issue is coming up when you’re buying a house then it’s likely to come up again when you sell.  So keep that in mind.

The biggest problem with NM cable in Chicagoland is that it’s often installed not by a good qualified electrician but by a handyman or homeowner.  In this case it’s not the material that’s in question but the installation methods.  When it’s exposed, NM cable needs to be run closely along the surfaces of the building finish to provide support and protection.  It needs to be supported at least every 4.5 feet and secured within 12 inches of its ends.  Where the cable enters any type of panel enclosure or junction box it needs to be clamped to the box.  These are very common defects when NM cable is installed by an amateur.

This house is in Chicago so there shouldn’t be any NM cable. Still, this is run closely along the building surfaces, it’s supported properly, and it’s clamped into this junction box for the light. It’s installed well.

This cable isn’t clamped to the box. Not done by a licensed electrician.

NM cable can only be used inside.  It can’t be used outside or exposed to sunlight.  It can’t be buried underground (there’s a special kind of cable for that) or encased in concrete or plaster.

NM cable shouldn’t be used outside. No self-respecting electrician did this.

NM can’t be used with a plug, making it into an extension cord.  It isn’t designed to be bent back and forth many times like an extension cord is.

A double whammy — NM cable run through a concrete wall and used with a plug as a glorified extension cord. Not done by a good electrician.

In an attic NM cable needs to be protected from physical damage.  Often times the cable is just run across the tops of the framing members, right where you want to step.  NM cable needs to be protected within six feet of any attic access opening, and if the attic has a permanent ladder or stairs then it needs protection throughout the attic up to a height of seven feet.

When NM cable is run in an unfinished basement along the bottoms of floor joists it needs to be attached to a running board (usually a 1×4 board fastened to the bottoms of the joists).  Or it can be installed through holes bored in the middle of each floor joist.

Cable that’s run along the bottoms of floor joists is susceptible to physical damage. First a 1×4 running board should be installed, and then the cable is attached to that.

NM cable is much more susceptible to damage from nails than is conduit.  So when NM cable is run through a wall stud hole that’s less than 1-1/4 inch from the front edge it needs to be protected with a steel strike plate to stop an errant nail from piercing the wire.

And dealing with the equipment grounding conductor (the ground wire) is quite a bit different with NM cable than with conduit, so that has to be taken into account.

So if the NM cable in your house is installed well then it’s likely to not pose a hazard, even though it wasn’t installed under a building permit like it should have been.  But if it has any of these defects described above then the risk of problems rises greatly.  And that can lead to shock, electrocution, fire, or other problems.  Electrical wiring is not the place you want to see amateur workmanship in your