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NM Cable Limitations

Posted By: George Little

NM Cable Limitations - 06/30/05 09:49 PM

Why does the '05 NEC limit the use of NM cable to Residential and buildings of Types III, IV and V construction?
Posted By: renosteinke

Re: NM Cable Limitations - 06/30/05 11:56 PM

The code greatly expands the allowed uses of NM, and you're complaining?

One of the objections to the acceptance of NM has been its' contribution to the "fire load." So limiting the use to small or fire resistant buildings seems consistant.

I have always seen NM associated in use with wood balloon-frams (stud type) buildings. Oddly enough, the prior limitation on the use of NM was three stories....the highest you can go with that sort of framing. It seems everyone just understood NM wasn't for use with block, steel frame, masonry, tilt-up concrete, or whatever other methods were out there.
In like manner, balloon framing was associated with residential construction- not commercial. Of course, today we see that line crossed- a lot.

Personally, I think NM is limited more by the fact that there are limited wires in the stuff. This in turn limits your flexibility- anything more elaborate than 20 amp light/receptacle circuits are best run using other methods (the odd stove/ water heater circuit notwithstanding).
Posted By: Ryan_J

Re: NM Cable Limitations - 07/01/05 12:48 AM

Why? Because that is where the cable Gods and the raceway Gods met in the middle.

Take a look at the ROP/ROC for the 2002 if you think I am joking. It was just a tradeoff for the 3 stories rule. [Linked Image]
Posted By: resqcapt19

Re: NM Cable Limitations - 07/01/05 02:13 AM

You can't stop with just looking at the ROP/ROC for this one. The 3 story rule was upheld in the ROP, the ROC, the floor of the May meeting, but eliminated by the Standards Council.

[This message has been edited by resqcapt19 (edited 06-30-2005).]
Posted By: George Little

Re: NM Cable Limitations - 07/01/05 11:59 AM

I suspect Ryan has it correct about the metal versus non-metallic battle. However they allow smurf tubing in Types I, II and III construction, given the consitions of 15 minute finish rating etc.

The State of Michigan has for at least 20 years amended the NEC and allowed NM cable above 3 floors in all types of construction. So the limitations of the '02 and especially the '05 are a step backwards as far as Michigan is concerned. Michigan's record is spotless and the use of NM cable does not cause fires when installed properly. I will concede the fire load issue but if we are in a Type I, II or II construction the chances of fire are lessened. One could argue that NM cable would be less of a fire load risk in I, II and II construction.
Posted By: cpal

Re: NM Cable Limitations - 07/02/05 11:41 AM

Does anyone recall if NM was used in the MGM fire in Vegas??

[This message has been edited by cpal (edited 07-02-2005).]
Posted By: Tiger

Re: NM Cable Limitations - 07/02/05 02:11 PM

The use of NM wasn't mentioned in the NFPA report of the fire. A waitress reported blue sparks coming from a keno board.

Posted By: resqcapt19

Re: NM Cable Limitations - 07/02/05 02:45 PM

The MGM fire originated at a FMC connection to a display cooler in the Deli area.
From the The MGM Grand Hotel Fire Investigation Report
The metal raceway (tubing) acted as the “grounding” conductor in the side stand. Through examination of the raceway in and over the side stand, it was determined that a ground fault condition did exist prior to the fire. The O/Rs observed several loose connections between the junction box connectors and the flexible aluminum raceway.
Portions of the raceway were unwound and unconnected. Arcing was observed where the raceway and E.M.T. chase came in contact with each other and where the raceway came in contact with each other and where the raceway came in contact with the metal studs. ... Improper installation and exposure to the warm atmosphere surrounding the raceway caused the insulation surrounding the copper conductors to loosen and deteriorate, exposing the bare copper conductors. Short circuiting occurred in the exposed copper conductors because only two conductors existed (no ground). Installation of the raceway to the junction boxes was faulty, and the ground could not function the way it was intended. The delayed action caused the electrical current to attempt to seek a ground; when it could not find a ground, it caused the copper conductor to become warm and eventually hot. This in turn caused the raceway to overheat to the point of glowing metal. This caused additional heat build up in the area approximately 45 in. from floor level in the soffit area. The lack of proper ground equipment for the conductors to provide a low impedance rate to carry the fault current to trip the breaker allowed a longer period of time for arcing to occur. This arcing produced that amount of heat needed for ignition temperature. Electrical fires can occur even when properly operating circuit protection is installed. As an example, loose connections at termination points can cause this effect.

(For the purposes of this report, the term O/R or O/Rs designates the officer or officers reporting, terminology which is in fact applied to Clark County Fire Department Fire Investigators in all written communications.)

[This message has been edited by resqcapt19 (edited 07-02-2005).]
Posted By: Ryan_J

Re: NM Cable Limitations - 07/02/05 05:09 PM

Don thanks for that link.
Posted By: richard

Re: NM Cable Limitations - 07/02/05 10:05 PM

so if that compressor had been fed with rx, chances are there would not have been a fire there?
Posted By: resqcapt19

Re: NM Cable Limitations - 07/03/05 12:29 AM

If the installation had been code compliant there would not have been a fire. The wiring method used does not make any difference. There are any number of noncompliant ways of installing NM that could have had the same result.
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