Forums | Home Repair | Home Improvement | Painting | Interior Decorating | Remodeling | Landscaping


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 01-26-2008, 08:34 PM   #16
Member
 
Join Date: Jan 2008
Posts: 309
Share |
Default

PVC conduit ,how do I end it?


Thank you Chris. I didn't realize that temperature effected amperage.

In a typical household installation, wouldn't you have this problem where the numerous cables are bunched together leaving the Main Panel? Or is it only over a distance that this becomes a problem.

FOUND ON WEB:

NEC 310.15.B.2.A table says that if you install 3 current carrying conductors or less the ampacity of a conductor in a conduit can be rated at 100 %. That same table says 4 to 6 current carrying conductors in a conduit must be derated down by 20 %. That same table says 7 to 9 current carrying conductors in a conduit must be derated down in ampacity by 30%. That same table says 10 to 20 current carrying conductors in a conduit must be derated down in ampacity by 50 %. It gets worse from there.

Using that table if you install that 4th 12 awg copper current carrying conductor in that conduit that 20 amp rated wire now must be protected with a maximum of a 15 amp breaker. If you install that 7 th 12 awg copper conductor in a conduit you now are not allowed to install that 12 awg conductor anywhere in the home with voltage rated over 50 volts at all because it is not even 15 amp rated which is the smallest ampacity rating allowed in a dwellng for 120 volts or more.

The reason for the derating of ampacity is because these current carrying conductors are heating each other and unable to cool due to the close proximity of other current carrying conductors in that conduit.


Last edited by Randell Tarin; 01-26-2008 at 08:40 PM.
Randell Tarin is offline   Reply With Quote
Old 01-26-2008, 08:49 PM   #17
Member
 
Join Date: Mar 2006
Posts: 682
Default

PVC conduit ,how do I end it?


close randell.

The starting point for calculations is the 90 deg C columb.

So start your derating of number 12 wire at 30 amps.

You still may only use 12 wire at the 60 deg columb, but you can do your calculations starting at the 90 deg columb.
__________________
Master Electrician
jwhite is offline   Reply With Quote
Old 01-26-2008, 09:23 PM   #18
Power Gen/RS Engineer
 
Join Date: Jun 2006
Location: Oak Park, Illinois
Posts: 712
Default

PVC conduit ,how do I end it?


Quote:
Originally Posted by jwhite View Post
The starting point for calculations is the 90 deg C columb. So start your derating of number 12 wire at 30 amps. You still may only use 12 wire at the 60 deg columb, but you can do your calculations starting at the 90 deg columb.
Jwhite points out an often-misunderstood part of the code, one of which I didn't understand correctly when I first read it. The de-rating table that you quoted is used in conjunction with the ampacity tables presented in article 310. Notice how different the listed ampacities are between 310.16 and 310.17 for the same size/insulation type wire? It makes sense when you think about a wire trying to dissipate i^2R heat in a conduit versus one that is run outside of a conduit, i.e. in free air (this is one of the reasons that the POCO's service drop conductors are typically much smaller than those downstream from the service point. Another is that they're cheap bastards while yet another is the fact that most residences rarely pull anything near their rated capacity.).

And yes, the hotter the wire or ambient temperature surrounding it, the higher resistance. The greater the resistance, the higher the heat dissipation. More heat dissipation, more...you get the idea!

Also, if you're spending your retired life reading the NEC (in which case I'll have to proclaim you a "poor bastard!" ), you'll want to check article 240.

At the end of the day, the NEC is trying to prevent everything from melting. Maybe the Wicked Witch should have given it a read??

Take care,
Jimmy
__________________
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves,"You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." - George Carlin

Last edited by BigJimmy; 01-26-2008 at 09:25 PM.
BigJimmy is offline   Reply With Quote
Old 01-26-2008, 09:40 PM   #19
Member
 
Join Date: Jan 2008
Posts: 309
Default

PVC conduit ,how do I end it?


Nah, I'm not a poor bastard. I think a fork is a voltage tester.

Thanks for sharing you knowledge.
Randell Tarin is offline   Reply With Quote
Old 01-26-2008, 09:44 PM   #20
Power Gen/RS Engineer
 
Join Date: Jun 2006
Location: Oak Park, Illinois
Posts: 712
Default

PVC conduit ,how do I end it?


Quote:
Originally Posted by Randell Tarin View Post
Nah, I'm not a poor bastard. I think a fork is a voltage tester.

Wait-It isn't??

__________________
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves,"You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." - George Carlin
BigJimmy is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pvc Conduit bobo60 Electrical 1 11-14-2007 09:52 PM
Conduit issues - help needed nymackem Electrical 19 11-05-2007 06:45 AM
Conduit through basement wall. bdalekid Electrical 13 01-31-2007 08:35 PM
pulling wire through old flexible conduit cbray Electrical 1 05-20-2005 03:55 PM
Running Conduit in new basement framed walls tben Electrical 3 12-10-2004 11:14 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.