# Ampacity Derating



## switchleg (Sep 22, 2008)

What is the right way to go about this situation?

Main panel is 20' away from a j-box that has circuits feeding new addition. Circuits are piped over in 2" Sch 40 pvc. There are 8 120v/20amp circuits and 3 240v/30amp circuits in this conduit. How do I go about derating the wire?


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## McClary’s Electrical (Feb 21, 2009)

switchleg said:


> What is the right way to go about this situation?
> 
> Main panel is 20' away from a j-box that has circuits feeding new addition. Circuits are piped over in 2" Sch 40 pvc. There are 8 120v/20amp circuits and 3 240v/30amp circuits in this conduit. How do I go about derating the wire?


 
You have 22 ccc's. That's .45 derating


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## 480sparky (Sep 20, 2007)

mcclary's electrical said:


> You have 22 ccc's. That's .45 derating


.....without using MWBCs.

18 if you do.


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## McClary’s Electrical (Feb 21, 2009)

480sparky said:


> .....without using MWBCs.
> 
> 18 if you do.


 
Good point. But anybody that would wire an addition in this fashion, knows nothing of a MWBC:whistling2:


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## switchleg (Sep 22, 2008)

Where is the .45 reference?


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## 480sparky (Sep 20, 2007)

switchleg said:


> Where is the .45 reference?


He may not know of the '%' over the 5.......Table 310.15(B)(2)(a).


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## switchleg (Sep 22, 2008)

How do I go about sizing my conductors feeding the j box? Table 310.16? Which column do I use?


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## McClary’s Electrical (Feb 21, 2009)

hire a professional


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## switchleg (Sep 22, 2008)

Come on. Help me out. I'm not a diy


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## 480sparky (Sep 20, 2007)

switchleg said:


> How do I go about sizing my conductors feeding the j box? Table 310.16? Which column do I use?


Uh, you size the box to the conductors, not the other way around.

Try 314.16 instead of 310.16


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## switchleg (Sep 22, 2008)

Maybe we are misunderstanding each other. The work is already done. My boss says to use #10's to feed the 20amp circuits and #8 to feed the 30amp circuits. I'm just trying to understand why and how he came up with this?


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## 480sparky (Sep 20, 2007)

switchleg said:


> Maybe we are misunderstanding each other. The work is already done. My boss says to use #10's to feed the 20amp circuits and #8 to feed the 30amp circuits. I'm just trying to understand why and how he came up with this?


 
Derating per 310.15(B)(2)(a).


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## sparks134 (Jan 30, 2009)

why would you even try to put that many wires in a pipe, even though its ok? i just dont understand peoples thinking sometimes.


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## sparks134 (Jan 30, 2009)

switchleg said:


> Maybe we are misunderstanding each other. The work is already done. My boss says to use #10's to feed the 20amp circuits and #8 to feed the 30amp circuits. I'm just trying to understand why and how he came up with this?


 maybe you have voltage drop???


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## switchleg (Sep 22, 2008)

It's only 20' away from the panel. Voltage drop that close? I was thinking conduit fill? Heat/resistance could make the current carrying capacity of the conductors less.


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## 480sparky (Sep 20, 2007)

switchleg said:


> Ok. still clueless. Thanks for the references. I'll just ask him to clear it up for me. Sure that will go over well.


If you used MWBCs, you have 18 CCCs. That puts you in the 50% column of T310.15(B)(2)(a). 

#10 THHN is good for 40 amps per T310.16. 40 X 50% = 20 amps.

*8 THHN is good for 55 amps. 55 X 50% = 27.5 amps, too small for a 30a OCD.


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## rdr (Oct 25, 2009)

switchleg said:


> It's only 20' away from the panel. Voltage drop that close? I was thinking conduit fill? Heat/resistance could make the current carrying capacity of the conductors less.


Actually if he's worth a grain of salt it should go over fine. How do you think all of us learn what we know? If you don't ask he doesn't know you don't know. Once upon a time all of us knew nothing about this trade. I ask questions every day....call me crazy.


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## switchleg (Sep 22, 2008)

480-
I follow that. But why do it in the first place? Is it because of too many conductors in a raceway? Heat/resistance.


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## 480sparky (Sep 20, 2007)

switchleg said:


> 480-
> I follow that. But why do it in the first place? Is it because of too many conductors in a raceway? Heat/resistance.


No conductor is a perfect one. They all have a certain amount of resistance. And that resistance, no matter how small, creates heat. So, all conductors produce heat. 

Heat is the enemy of insulation. Too much heat destroys insulation. Therefore, if you have over a certain number of conductors stuffed into a pipe, you must reduce their heat output by limiting how much amps are allowed in each conductor.


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## rdr (Oct 25, 2009)

switchleg said:


> 480-
> I follow that. But why do it in the first place? Is it because of too many conductors in a raceway? Heat/resistance.


BINGO! :thumbsup:


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## sparks134 (Jan 30, 2009)

480sparky said:


> If you used MWBCs, you have 18 CCCs. That puts you in the 50% column of T310.15(B)(2)(a).
> 
> #10 THHN is good for 40 amps per T310.16. 40 X 50% = 20 amps.
> 
> *8 THHN is good for 55 amps. 55 X 50% = 27.5 amps, too small for a 30a OCD.


 Is that really how its done?


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## 480sparky (Sep 20, 2007)

sparks134 said:


> Is that really how its done?


If you have another way, I'm all ears.















OK, not all ears, but I'm all eyes.


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## sparks134 (Jan 30, 2009)

i was under the impession you were suppose to use a larger conduit, not up size the wire, maybe im wrong though.


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## switchleg (Sep 22, 2008)

sparks134 said:


> i was under the impession you were suppose to use a larger conduit, not up size the wire, maybe im wrong though.


That's what I thought too until he decided to do it like this. One other thing, I thought MWBC's were use less with the 08' code?


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## rdr (Oct 25, 2009)

switchleg said:


> That's what I thought too until he decided to do it like this. One other thing, I thought MWBC's were use less with the 08' code?


Wow maybe I misunderstood.....I thought you were asking why derate wires? Am I the only one under that impression?


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## sparks134 (Jan 30, 2009)

switchleg said:


> That's what I thought too until he decided to do it like this. One other thing, I thought MWBC's were use less with the 08' code?


 what do u mean by useless?


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## 480sparky (Sep 20, 2007)

sparks134 said:


> i was under the impession you were suppose to use a larger conduit, not up size the wire, maybe im wrong though.


You could put four #14s in a 60" raceway, and you still must derate them to 80%.


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## sparks134 (Jan 30, 2009)

480sparky said:


> You could put four #14s in a 60" raceway, and you still must derate them to 80%.


 we always increase the conduit size, not upsize wire!


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## sparks134 (Jan 30, 2009)

480sparky said:


> You could put four #14s in a 60" raceway, and you still must derate them to 80%.


 yes ,therfore you increase conduit size.


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## switchleg (Sep 22, 2008)

sparks134 said:


> what do u mean by useless?


Because of the arc fault cb


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## JayH (Nov 13, 2009)

480sparky said:


> You could put four #14s in a 60" raceway, and you still must derate them to 80%.


Absolutely correct.

Sounds like your owner has planned his job well by installing the 2" C to accomodate the necessary derating.


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## 480sparky (Sep 20, 2007)

switchleg said:


> That's what I thought too until he decided to do it like this. One other thing, I thought MWBC's were use less with the 08' code?


 
MWBCs are not useless, they are just harder to work with. First, you must use 2-pole breakers or handle ties [210.4(B)] and, in dwellings, most must be AFCI protected [210.12]. AFCI protection is hard to do with MWBCs because 2-pole AFCI breakers aren't currently made by most manufacturers. Yet.


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## 480sparky (Sep 20, 2007)

sparks134 said:


> yes ,therfore you increase conduit size.


I can still fit four #14s in a 1/2" raceway.



sparks134 said:


> we always increase the conduit size, not upsize wire!


You got a Code reference there, Skippy?


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## sparks134 (Jan 30, 2009)

switchleg said:


> Because of the arc fault cb


 i dontknow?


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## JayH (Nov 13, 2009)

_My gas tank can go from zero to $50 in fifteen seconds._


That's friggin funny.


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## switchleg (Sep 22, 2008)

I shall be more careful with my wording next time.


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