# NM Resi. vs Comm.



## jumper (Mar 22, 2010)

What is the reasoning of why NM is allowed in drop ceilings in residential, but not commercial buildings? I am talking about commercial buildings where is it okay to use NM in walls because of the correct type of construction.


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## raider1 (Mar 22, 2010)

Re: NM Resi. vs Comm.

My understanding is that the big problem is with the toxic smoke that the PVC jacket of NM cable can produce.

The amount of NM cable in drop ceilings of homes would be a lot less that then there would be in a commercial dropped ceiling.

Chris


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## Paul Sweet (Mar 22, 2010)

Re: NM Resi. vs Comm.

The NEC used to allow NM above dropped ceilings.  A few years ago they eliminated the 3-story  height restriction for using NM, and this was the trade-off.


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## jumper (Mar 23, 2010)

Re: NM Resi. vs Comm.



			
				raider1 said:
			
		

> My understanding is that the big problem is with the toxic smoke that the PVC jacket of NM cable can produce.The amount of NM cable in drop ceilings of homes would be a lot less that then there would be in a commercial dropped ceiling.
> 
> Chris


Makes sense. I thought it would be something like that, but I never really knew..


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## jumper (Mar 23, 2010)

Re: NM Resi. vs Comm.



			
				Paul Sweet said:
			
		

> The NEC used to allow NM above dropped ceilings.


Did know that. I admit that I was never taught the correct rules for where you can use NM in non residential apps. Basically, we used MC in anything that was not residential. I am trying to learn the construction tables.



> A few years ago they eliminated the 3-story  height restriction for using NM, and this was the trade-off.


Don't we have a four story limit in VA. I added my location to my profile, forgot to do it when I joined.


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## McClary's Electrical (Mar 23, 2010)

Re: NM Resi. vs Comm.

I personally believe it is because in a commercial environment, it is subject to physical damage compared to a residential environment. The NEC states in other than dwelling units, NM must be concealed. There's no way around that, and a dropped ceiling is no exception.


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## Paul Sweet (Mar 24, 2010)

Re: NM Resi. vs Comm.

Jumper, Virginia's amendment appears to limit NM to 4 stories above grade, unless the buildings are allowed to be types III, IV & V construction, which probably aren't allowed to go much above 4 stories anyway.

It looks like the 2002 NEC first prohibited NM above dropped ceilings in other than 1&2 family dwellings and dropped the limit on number of stories.  I always preferred EMT or MC in commercial buildings of noncombustible construction, but NM was allowable.


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## Pcinspector1 (Mar 25, 2010)

Re: NM Resi. vs Comm.

McClary's Electric,



> I personally believe it is because in a commercial environment, it is subject to physical damage compared to a residential environment. *The NEC states in other than dwelling units*, NM must be concealed. There's no way around that, and a dropped ceiling is no exception.


Basement ceiling only not required to be concealed?

Walls in a residential basement would need to be concealed or protected with conduit, true?


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## raider1 (Mar 25, 2010)

Re: NM Resi. vs Comm.



			
				Pcinspector1 said:
			
		

> McClary's Electric,
> 
> 
> 
> ...


Basement ceiling only not required to be concealed?

Walls in a residential basement would need to be concealed or protected with conduit, true?

Here is what 334.15© says about NM in unfinished basements.



> © In Unfinished Basements and Crawl Spaces. Where cable is run at angles with joists in unfinished basements and crawl spaces, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables shall be run either through bored holes in joists or on running boards.* NM cable installed on the wall of an unfinished basement shall be permitted to be installed in a listed conduit or tubing or shall be protected in accordance with 300.4. Conduit or tubing shall be provided with a suitable insulating bushing or adapter at the point the cable enters the raceway. The NM cable sheath shall extend through the conduit or tubing and into the outlet or device box not less than 6 mm (¼ in.). The cable shall be secured within 300 mm (12 in.) of the point where the cable enters the conduit or tubing. Metal conduit, tubing, and metal outlet boxes shall be connected to an equipment grounding conductor.*


Chris


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## McClary's Electrical (Mar 25, 2010)

Re: NM Resi. vs Comm.



			
				Pcinspector1 said:
			
		

> McClary's Electric,
> 
> 
> 
> ...


Basement ceiling only not required to be concealed?

Walls in a residential basement would need to be concealed or protected with conduit, true?

Again,,,my response is for

"other than dwelling unit."

I was not speaking of an unfinished basement


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## Pcinspector1 (Mar 25, 2010)

Re: NM Resi. vs Comm.

Just needed reassurance, thanks raider & McCleary Elect.


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## peach (Mar 25, 2010)

Re: NM Resi. vs Comm.

The OP asked about drop ceilings in residential, not commercial.

How oftten do we see grid ceilings in residential?  (Like never).

Romex is ok in some commercial installations.

Remember that all electrical code (including the electrical provisions in IRC) are governed by NFPA and the IAEI.. or eieio).. not the ICC.


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