PVC Conduit ,how Do I End It? - Electrical - Page 2 - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum PVC conduit ,how do I end it?
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

01-26-2008, 08:34 PM   #16
Member

Join Date: Jan 2008
Posts: 309
Rewards Points: 250

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.

 01-26-2008, 08:49 PM #17 Member   Join Date: Mar 2006 Posts: 682 Rewards Points: 500 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
01-26-2008, 09:23 PM   #18
Power Gen/RS Engineer

Join Date: Jun 2006
Location: Oak Park, Illinois
Posts: 785
Rewards Points: 566

Quote:
 Originally Posted by jwhite 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
__________________
Well, now, there's what's right and what's right and never the twain shall meet.

Last edited by BigJimmy; 01-26-2008 at 09:25 PM.

 01-26-2008, 09:40 PM #19 Member   Join Date: Jan 2008 Posts: 309 Rewards Points: 250 Nah, I'm not a poor bastard. I think a fork is a voltage tester. Thanks for sharing you knowledge.
01-26-2008, 09:44 PM   #20
Power Gen/RS Engineer

Join Date: Jun 2006
Location: Oak Park, Illinois
Posts: 785
Rewards Points: 566

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

Wait-It isn't??

__________________
Well, now, there's what's right and what's right and never the twain shall meet.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post bobo60 Electrical 1 11-14-2007 09:52 PM nymackem Electrical 19 11-05-2007 06:45 AM bdalekid Electrical 13 01-31-2007 08:35 PM cbray Electrical 1 05-20-2005 03:55 PM tben Electrical 3 12-10-2004 11:14 PM

Top of Page | View New Posts