NMD 3

[Alectric]

Whats the deal with NMD 3 conductor? Is it the #16awg (or so) ground? or the fact it's not rated for 90deg. ? I've heard tales of removing\replacing it before tapping into it or adding to it. Is it only 'safe' for plugs + switches and not lighting? ...Any help?

[neil seidner]

Some of the older NMD did not have a bonding conductor or "ground wire". The newer NMD has a bonding (ground). Doing work on some of the older wiring may require complete removal of the existing wiring, because the code presently requires a bonding conductor (it would depend on the extent and type of work being done). The term "2 conductor" or "3 conductor" refers to the number of current carrying ("circiut") conductors. An older 2 conductor NMD would have 2 circiut conductors and no bonding "ground" conductor. A newer 2 conductor NMD would have 2 circiut conductors and one bonding "ground" conductor. A newer NMD 3 conductor will have 3 circiut conductors and one bonding "ground" conductor. Some of the older NMD may only be rated for 75C (you would have to read the markings on the wire). The newer NMD is marked NMD-90, meaning it is rated for 90C. 75C is not suitable for lighting, because of the heat from the bulb getting into the octagon that the light is mounted on.

[Ellement]

Great reply.
So what to do when changing light fixtures on older buildings that have NMD2, are we to suspect a 75 deg. rating and change the run (very costly for a fixture change)or just replace the existing fixture and walk away? Is it the old saying "Once you touch it's yours." Where does the liability end, and where does it begin?
Any opinion would be welcome.
Thanks

[neil seidner]

Just a clarification here, You said NMD2. The 2 only refers to the number of circuit conductors. Not to the temperature rating, or whether it has a bonding (ground) conductor. The temperature rating is marked on the sheath of the cable (NMD90 means 90C, other markings may require research). It may not be practical to examine the sheath of the cable at the light, but looking at other wiring in the building may be able to tell you what codes the electrician was following at the time, (why would an electrician use 90c wiring on plugs, and use 75c wiring in lights?), there are no absolute guaranties on this, hence the need for liability insurance.

[Ellement]

I understand, I guess my concern was that if you suspect NMD75 (most older wiring is very difficult to read the jacket) are we to insist on changing a line run to accommodate a new light fixture? How about knob and tube wiring, what's the temp. rating on that cable?

We do a lot of renovations on older buildings (between 60 to 120 years old) and come across a lot of different wiring practices that has been done over the decades, which sometimes makes it difficult to establish origin and spec. without opening up the wall or ceiling in question. When did they switch from 75 to 90 degree wire and is there a way to tell other than the possible markings on the jacket?

Thanks again.

[neil seidner]

Can I break the Code? NO!!! However because you are dealing with several inter-related code rules, certain exceptions may apply.
30-306(2),30-308(3),30-310(2)(b),30-310(3),30-400(c),30-404,30-408(1),30-410(1),30-910(1),30-910(2), are the rules that I found. Can I install a wall sconce on an octagon that is wired with NMD75, and the sconce does not have a 90C sticker on it? 30-408 only says ceiling, in the absence of a 90C specification from the manufacturer, 30-306(2) is not broken either. The list of different applications could go on and on and on.

If the fixture has a 90C warning sticker from the manufacturer on it. Follow it to the letter, They would not put a sticker, that makes it difficult or impossible to install there product, for no reason.

[Alectric]

If the '3' in NMD 3 means there's three current carrying conductors. What does the '7' in NMD 7 mean? I have a piece of each in hand and the '2 wire plus bond' [green sheathed] NMD 3 seems to have a #16 bond wire whereas the '2 wire plus bond' [brown sheathed] NMD 7 seems to have a #14 bond wire. I've also been informed that NMD 7 is acceptable for lighting loads?

[electric-ed]

The NMD type designations have changed over the years, and the numbers don't have anything to do with the number of wires in the cable.

NMD-3 meant 60 degree insulation.
NMD-6 meant 75 degree insulation.
NMD-7 meant 90 degree insulation.

What was NMD-7 is now called NMD 90.

Ed

[neil seidner]

Apparently, I was "barking up the wrong tree"