ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

Topic Options
Rate This Topic
#40076 - 07/11/04 09:01 PM derating
ronaldsax Offline
Junior Member
Registered: 06/26/04
Posts: 6
Loc: west palm beach,fl
hi all
I have a question
can anybody explain me please what is derating ? when I have conductors in a raceway?
thank you
Top
Work Gear for Electricians and the Trades
#40077 - 07/11/04 09:05 PM Re: derating
Ryan_J Offline
Moderator
Registered: 08/19/03
Posts: 1374
Loc: West Jordan, Utah, USA
Ampacity adjustment is required when there are more than 3 current carrying conductors in a raceway. If you look at table 310.16, you will notice that that is what the table values are based on. After that, see 310.15(B)(2)(a).
_________________________
Ryan Jackson,
Salt Lake City
Top
#40078 - 07/12/04 06:48 AM Re: derating
sabrown Offline
Member
Registered: 12/12/02
Posts: 302
Loc: Ogden, Utah, USA
To expand on Ryan:

When you have a single conductor carrying current there is energy lost in the form of heat. The more current, the more heat. A bare wire can handle a lot of current without any problems for the bare conductor itself because of the high melting point of the metal conductor.

But when you need to insulate the conductor from other conductors, the insulating material usually has a much lower melting point than the bare conductor itself. The insulations lower melting point becomes the limiting factor.

Taking several insulated conductors and placing them in close proximity effectively places several heat producing sources close together. Less of the heat that is generated in each conductor can disipate into the surrounding enviroment causing each conductor to run at a higher temperature. These higher operating temperature conditions combine to make it so each conductor can not carry as much current without aproaching the safety point before melting the insulation.

Shane
Top
#40079 - 07/12/04 07:54 AM Re: derating
C-H Offline
Member
Registered: 09/17/02
Posts: 1497
Loc: Stockholm, Sweden
Shane's description is very good. I only want to add that it is usually not the melting point of the insulation that is the limit. Rather, it is the temperature which will degrade the material over a very long period of time. The melting point of PVC is at least 150C, but it will become brittle by exposure to only 100C for a long period of time.

[This message has been edited by C-H (edited 07-12-2004).]
Top

Member Spotlight
Member Since: 11/17/00
Posts: 2232
New in the Gallery:
SE cable question
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box


Who's Online
0 registered (), 40 Guests and 8 Spiders online.
Key: Admin, Global Mod, Mod
 
New in the Gallery:
SE cable question
 
Top Posters (30 Days)
Admin 47
HotLine1 43
gfretwell 19
Ruben Rocha 12
Trumpy 9
 
Newest Members
Scotto, Freecrowder, clee512, Jdscott2005, FAIZAN

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals