Derating NM cable
- Thread starter ryan_618
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sandsnow
Senior Member
- Location
- Southern California
Re: Derating NM cable
Rodger
Yep, Lot's of holes in this section.
Thanks for that link. More interesting stuff there. Especially the substantiation. Assuming the physics of it all is true, it seems the text of the code falls short for a remedy of the problem.
What about blown in insulation around NM?
Firestopping through other than wood framing is not an issue?
Rodger
Yep, Lot's of holes in this section.
Thanks for that link. More interesting stuff there. Especially the substantiation. Assuming the physics of it all is true, it seems the text of the code falls short for a remedy of the problem.
What about blown in insulation around NM?
Firestopping through other than wood framing is not an issue?
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Derating NM cable
Thats why I am making a proposal to delete the requirement.
macmikeman
Senior Member
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- Planet macmikeman
Re: Derating NM cable
Good for you Ryan. I feel that when code changes happen that are the result of Inspectors in the field finding problems that need to be addressed they generally are good code changes. When code changes come about as a result of wondering what might happen if... , proposed by the guy's with nice shiny new white hard hats and wearing pen protectors in their shirt pocket and never actually ever wore a tool belt, then we end up with stuff like 334.80 wording.
Good for you Ryan. I feel that when code changes happen that are the result of Inspectors in the field finding problems that need to be addressed they generally are good code changes. When code changes come about as a result of wondering what might happen if... , proposed by the guy's with nice shiny new white hard hats and wearing pen protectors in their shirt pocket and never actually ever wore a tool belt, then we end up with stuff like 334.80 wording.
- Location
- Illinois
- Occupation
- retired electrician
Re: Derating NM cable
I think that the section needs to stay with corrected wording. There have been a number of studies done, but rejected by the CMP that show problems with NM when used in any type of insulting material. There is one that says fully loaded NM in normal wall insulation will exceed the 90 degree C rating of the insulation in may cases. Maybe this is one of the reasons that we "need" AFCIs. The fire stats used to justify the AFCI requirement showed that 40% of the dwelling unit electrical fires originated in the power distribution system (the building wiring).
Don
I think that the section needs to stay with corrected wording. There have been a number of studies done, but rejected by the CMP that show problems with NM when used in any type of insulting material. There is one that says fully loaded NM in normal wall insulation will exceed the 90 degree C rating of the insulation in may cases. Maybe this is one of the reasons that we "need" AFCIs. The fire stats used to justify the AFCI requirement showed that 40% of the dwelling unit electrical fires originated in the power distribution system (the building wiring).
Don
macmikeman
Senior Member
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- Planet macmikeman
Re: Derating NM cable
Don, while I agree with you on the part about nm cable wiring in contact with any type of insulation can exceed the 90 deg rating of the sheathing, I still have a problem with the wording of 334.80 as stated. Specifically there is no mention of the size of the hole in the wording. Since the problem seems to arise from the fire proofing putty and the insulation contact, then perhaps its time for an upgrade to the cable such that it will safely carry its rated current in a higher temperature such as maybe 120 deg. In my world framing carpenters do not allway's leave enough room between stud cavity's to drill multiple holes thru the top plate, and I do not have any control over whether or not insulation will be installed in any given installation. As far as firestopping is concerened, if it is going to be required to install the putty into holes drilled for the purpose of installing the cable , then manufacturer's of the cable should be required to provide cable that can withstand the associated rise in temperature of the enviroment.
Don, while I agree with you on the part about nm cable wiring in contact with any type of insulation can exceed the 90 deg rating of the sheathing, I still have a problem with the wording of 334.80 as stated. Specifically there is no mention of the size of the hole in the wording. Since the problem seems to arise from the fire proofing putty and the insulation contact, then perhaps its time for an upgrade to the cable such that it will safely carry its rated current in a higher temperature such as maybe 120 deg. In my world framing carpenters do not allway's leave enough room between stud cavity's to drill multiple holes thru the top plate, and I do not have any control over whether or not insulation will be installed in any given installation. As far as firestopping is concerened, if it is going to be required to install the putty into holes drilled for the purpose of installing the cable , then manufacturer's of the cable should be required to provide cable that can withstand the associated rise in temperature of the enviroment.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Derating NM cable
I think the fact that NM cable is already limted to it's 60 degree ampacity, and the rules of 240.4(D), it is protected already.
I also think that the rest of the copper conductor of the circuit (besides the three inch length in the fire stop) should have enough heat sinking properties to not make this an issue.
I think the fact that NM cable is already limted to it's 60 degree ampacity, and the rules of 240.4(D), it is protected already.
I also think that the rest of the copper conductor of the circuit (besides the three inch length in the fire stop) should have enough heat sinking properties to not make this an issue.
- Location
- Illinois
- Occupation
- retired electrician
Re: Derating NM cable
Ryan,
The tests results submitted to the CMP showed that when 20 amps was applied to multiple NM cables in a framing member with fire or draft stopping that the conductors exceed their 90 degree C rating at that point. They were correcty applied per the 2002 code, but they were above their maximum rated temperature.
Don
Ryan,
The tests results submitted to the CMP showed that when 20 amps was applied to multiple NM cables in a framing member with fire or draft stopping that the conductors exceed their 90 degree C rating at that point. They were correcty applied per the 2002 code, but they were above their maximum rated temperature.
Don
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Derating NM cable
But doesn't the rest of the conductor act as a heat sink and nullify the problem?
[ June 22, 2005, 05:05 PM: Message edited by: ryan_618 ]
But doesn't the rest of the conductor act as a heat sink and nullify the problem?
[ June 22, 2005, 05:05 PM: Message edited by: ryan_618 ]
macmikeman
Senior Member
- Location
- Planet macmikeman
Re: Derating NM cable
I will be more than willing to change my attitude about this matter if there is any Historical evidence to support the change other than the way it was tested in the lab, at least where dwellings are concerened. I have seen smoked up romex in commercial situations where they kept adding to the load with more fixtures, but I am also guessing they had exceeded the ampacity of the cable at the same time.
I will be more than willing to change my attitude about this matter if there is any Historical evidence to support the change other than the way it was tested in the lab, at least where dwellings are concerened. I have seen smoked up romex in commercial situations where they kept adding to the load with more fixtures, but I am also guessing they had exceeded the ampacity of the cable at the same time.
- Location
- Illinois
- Occupation
- retired electrician
Re: Derating NM cable
Ryan,
Don
Ryan,
The tests had the termocouple embedded with the NM cables in the firestopping and showed that the maximum permittee temperature was exceeded at that point. I don't see any part of the code that permits you to exceed the maximum permitted temperature for any part of the installation.
Don
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