ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
Ground fault remover :-)
by dsk - 11/30/20 03:51 AM
From an outsider- How does tipping work?
by Trumpy - 11/30/20 03:50 AM
Happy Thanksgiving
by Trumpy - 11/30/20 03:32 AM
Where is Everyone?
by Trumpy - 11/30/20 03:11 AM
What does your work place look like?
by Trumpy - 11/30/20 02:45 AM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
0 registered members (), 18 guests, and 15 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 3 1 2 3
Why is NM restricted to 60 deg ampacity? #101178 02/15/07 03:09 AM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline OP
Member
Does anyone know why NM cable is singled out to be restricted to 60 degree ampacity? AC and MC can use the 75 degree ampacity; what is "special" about NM?

2017 / 2014 NEC & Related Books and Study Guides
Re: Why is NM restricted to 60 deg ampacity? #101179 02/15/07 12:12 PM
Joined: Jun 2005
Posts: 821
S
ShockMe77 Offline
Member
Besides NEC article 334.80 telling us we have to... I would guess because of the non-metallic sheathing compared to the metallic jacket of an AC or MC cable.

[This message has been edited by ShockMe77 (edited 02-15-2007).]

Re: Why is NM restricted to 60 deg ampacity? #101180 02/15/07 01:34 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline OP
Member
To clarify my question, I understand that 334.80 is where it says this in the NEC. What I am wondering is if anyone here can explain to us what the thought process was that led those who write the Code to decide that this restriction was necessary for NM, but not for other types of cable. For example TC, which is rather similar to NM, has no such restriction.

[This message has been edited by SolarPowered (edited 02-15-2007).]

Re: Why is NM restricted to 60 deg ampacity? #101181 02/15/07 03:12 PM
Joined: Nov 2005
Posts: 1
L
little dog Offline
Junior Member
If you look at article 110.14 C . It tells you that most wiring terminals and lugs for smaller wiring are rated only at 60 degree c. Even though there might be other reasons this is reason enough.

Re: Why is NM restricted to 60 deg ampacity? #101182 02/15/07 04:42 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline OP
Member
I don't think that's it, because #10 and smaller is already restricted to the 60 degree ampacity or less by the limits to 30, 20 and 15 amps. Thus the 60 degree limitation only has relevance for #8 and larger, where you certainly do find lots of 75 degree terminations.


[This message has been edited by SolarPowered (edited 02-15-2007).]

Re: Why is NM restricted to 60 deg ampacity? #101183 02/15/07 05:04 PM
Joined: Dec 2003
Posts: 751
E
earlydean Offline
Member
I think it is because the NM outer sheathing has been rated for 60 degrees; similiar to the requirement for the plastic bushing to have the same or greater temperature rating than the conductors passing through.


Earl
Re: Why is NM restricted to 60 deg ampacity? #101184 02/15/07 05:38 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline OP
Member
That also doesn't appear to be the reason, as you are permitted to use the 90 degree column for derating. That will permit the cable to get hotter than allowing the 75 degree column for fusing would allow.

Re: Why is NM restricted to 60 deg ampacity? #101185 02/15/07 05:41 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Quote
That will permit the cable to get hotter than allowing the 75 degree column for fusing would allow.

No, I do not believe so as even though the derating can start at 90 C the final derated ampacity can still not exceed the 60 C rating.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Why is NM restricted to 60 deg ampacity? #101186 02/15/07 06:28 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline OP
Member
Let me rephrase that. Derating using the 90 degree ampacity is designed to keep the temperature of the wire below the temperature limit of the 90 degree insulation. If you have a bundle or a raceway with, say, 20 CCCs, you could get up to the 90 degree limit, even with ampacity that's less than the non-derated 60 ampacity.

Whereas, if you were to use the 75 degree column for ampacity, in a context that doesn't require derating, the wire will never heat to more than the 75 degree limit.

Thus I contend that it doesn't appear that protecting the jacket from overheating is the reason for the restriction to 60 degree ampacity. And again, why the limit for NM, and not for TC cable? What is the difference between these two cable types that requires the difference in treatment?


[This message has been edited by SolarPowered (edited 02-15-2007).]

Re: Why is NM restricted to 60 deg ampacity? #101187 02/15/07 06:46 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline OP
Member
To expand on an earlier post of mine, this rule is irrelevant for #10 and smaller wires, because those are already limited to 30, 20, and 15 amps, which are at or less than the 60 degree ampacity. So in almost all cases where this rule applies, we are talking about either the feed for a stove or cooktop, or to a subpanel.

So, does anybody have any recollection about what you might have read or heard about the CMPs' deliberations on this subject?

[This message has been edited by SolarPowered (edited 02-15-2007).]

Page 1 of 3 1 2 3

Featured:

2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
TOOL_5150
TOOL_5150
Bay Area
Posts: 61
Joined: August 2007
Show All Member Profiles 
Top Posters(30 Days)
Trumpy 17
Popular Topics(Views)
272,108 Are you busy
206,006 Re: Forum
194,036 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3