# derating wire or conduit?



## 480sparky (Sep 20, 2007)

In order to derate conduit, it must have an ampacity.

So, what's the ampacity of conduit?

Suppose it's PVC conduit?


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## sparks134 (Jan 30, 2009)

so you can only fill a conduit with as many wires as you can, but not exceding the fill and also using the derating chart in art. 310 if neccesarry.


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## 480sparky (Sep 20, 2007)

Raceway fill is simply a mathematical formula: How many little circles (the conductors) can you fit into one big circle (the raceway)?

Derating simply means you limit the ampacity of a conductor to limit the amount of heat it produces. 

If you want to maintain the same ampacity for the circuit, you use a larger conductor. Using larger conductors _may_ require a larger raceway.


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## sparks134 (Jan 30, 2009)

I guess thats not an issue for us ein chicago,because we are only aloud to put 9 CCC in one conduit. So we never have to derate the conductors.


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## 480sparky (Sep 20, 2007)

sparks134 said:


> I guess thats not an issue for us ein chicago,because we are only aloud to put 9 CCC in one conduit. So we never have to derate the conductors.


Chicago lives in (or at least, _wires_) it's own little universe. However, I feel sorry for you if you need to travel outside the city limits, where the long arm of the AHJ cannot prevent you from coming in contact with the NEC and the 'outside world', evil as we may be.


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## sparks134 (Jan 30, 2009)

Im learning the NEC now, there are not that many differences though.


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## wareagle (Jan 10, 2009)

If you were under the NEC, the 9 conductors would have to be derated to 70% of its ampacity. Conduit size has nothing to do with the reduction of ampacity. I knew Chicago had some different rules but I thought the rules were more stringent than the NEC.


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## goose134 (Nov 12, 2007)

Bear in mind that 134's work jurisdiction extends outside of the city of Chicago to places that use the NEC. In addition, Federal buildings inside the city have their own inspectors that use the NEC.


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## Wireless (Jan 22, 2007)

sparks134 said:


> I guess thats not an issue for us ein chicago,because we are only aloud to put 9 CCC in one conduit. So we never have to derate the conductors.


Ever? or say 9 12AWG THHN's on 20 amp circuits? out of Chicago that is the max also if you want to put more you have to lower the amperage or use a larger size conductor.


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## sparks134 (Jan 30, 2009)

goose134 said:


> Bear in mind that 134's work jurisdiction extends outside of the city of Chicago to places that use the NEC. In addition, Federal buildings inside the city have their own inspectors that use the NEC.


I cant remember ever derating the conductors, do you?


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## 480sparky (Sep 20, 2007)

sparks134 said:


> I cant remember ever derating the conductors, do you?


 
So, you're just starting in the trade then?


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## goose134 (Nov 12, 2007)

> I cant remember ever derating the conductors, do you?


I have for ambient temperature, and have upsized for voltage drop. But it rarely comes up.


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## goose134 (Nov 12, 2007)

It should be noted that the fill max (except for control wiring) is nine wires. The fill tables for Chicago stop there. Two three phase networks and a ground is all we can put in.


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## NolaTigaBait (Oct 19, 2008)

I have haven't really had to it either. For the most part in residential(), the wire is already derated.


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## wareagle (Jan 10, 2009)

NolaTigaBait said:


> I have haven't really had to it either. For the most part in residential(), the wire is already derated.


What do you mean by "the wire is already derated"?


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## NolaTigaBait (Oct 19, 2008)

wareagle said:


> What do you mean by "the wire is already derated"?


Romex. Example: for branch circuits you are limited to a 20 amp ocp for #12. But, you can derate from 30 amps. You have no choice, thus it is already derated.
Oh, and I'm sorry to see you are an Auburn fan:bluh......


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## Bob Badger (Apr 19, 2009)

wareagle said:


> What do you mean by "the wire is already derated"?


Due to 240.4(D) 14, 12 and 10 AWG are in essence already derated from their T310.16 rating.


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## NolaTigaBait (Oct 19, 2008)

Bob Badger said:


> Due to 240.4(D) 14, 12 and 10 AWG are in essence already derated from their T310.16 rating.


Thanks Badger:thumbsup:.


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## Bob Badger (Apr 19, 2009)

No problem Bait.


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## wareagle (Jan 10, 2009)

NolaTigaBait said:


> Romex. Example: for branch circuits you are limited to a 20 amp ocp for #12. But, you can derate from 30 amps. You have no choice, thus it is already derated.
> Oh, and I'm sorry to see you are an Auburn fan:bluh......


OK. I am with you. I was not sure what you were inferring.


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## sparks134 (Jan 30, 2009)

goose134 said:


> It should be noted that the fill max (except for control wiring) is nine wires. The fill tables for Chicago stop there. Two three phase networks and a ground is all we can put in.


OK,thats what I thought, tHANK YOU!


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## wareagle (Jan 10, 2009)

Bob Badger said:


> Due to 240.4(D) 14, 12 and 10 AWG are in essence already derated from their T310.16 rating.


I have tried to find the answer to why the NEC reduces the rating of these 3 conductor sizes but no luck. Anybody have and answer?


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## 480sparky (Sep 20, 2007)

wareagle said:


> I have tried to find the answer to why the NEC reduces the rating of these 3 conductor sizes but no luck. Anybody have and answer?


The so-called 'small conductor rule' was inroduced in the 1956 NEC concurrent with the introduction of type RHH insulation.

In 1956, the following was added to the bottom of Table 1, Chapter 10 (what our current 310-16 was back then):
"The current-carrying capacities for Type RHH conductors for sizes 14, 12 and 10 shall be the same as designated for Type RH conductors in this Table." This put 14, 12 and 10 at 15, 25 and 40 amps respectively.

In '59 the Table was relocated and became Table 310-12. The ampacites were also changed for Type RH conductors, so this put 14, 12 and 10 at the ratings of 15, 20 and 30 amps we know today.


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## sparks134 (Jan 30, 2009)

Do we need to derate 14, 12, 10 awg?


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## 480sparky (Sep 20, 2007)

sparks134 said:


> Do we need to derate 14, 12, 10 awg?


For more than 4 CCCs, ambient temp, and rooftops, yes.


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## william1978 (Sep 21, 2008)

sparks134 said:


> Do we need to derate 14, 12, 10 awg?


 Yes.


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## wareagle (Jan 10, 2009)

sparks134 said:


> Do we need to derate 14, 12, 10 awg?


Yes. You can use the 90C rating to derate the conductor but you may not exceed the 75C rating after the derating.
The rule for #14, 11 and 10 does not apply to all installations.


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## manchestersparky (Mar 25, 2007)

Derate from the 90 degree column
do not exceed the ampacity in the 75 degree column
UNLESS 
it's NM Cable then do not exceed the ampacity in the 60 degree column
as article 334.80 dictates this to us

Confused ? Welcome to the NEC


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## wareagle (Jan 10, 2009)

When installing a recessed fixture, the instructions say to use 90C conductors in the junction that is attached to the fixture. If you remove 12 inches of the covering on romex, can you consider that 12 inches as 90C?


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## 480sparky (Sep 20, 2007)

wareagle said:


> When installing a recessed fixture, the instructions say to use 90C conductors in the junction that is attached to the fixture. If you remove 12 inches of the covering on romex, can you consider that 12 inches as 90C?


 
Only if the conductors are marked as such.

Good luck with that one. :whistling2:


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## Saturday Cowboy (Nov 26, 2009)

Most romex today(NM-B) has 90 deg wire inside.


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## william1978 (Sep 21, 2008)

Saturday Cowboy said:


> Most romex today(NM-B) has 90 deg wire inside.


 It is not labeled 90 deg.. Open up a piece of romex and take a look.


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## 480sparky (Sep 20, 2007)

Saturday Cowboy said:


> Most romex today(NM-B) has 90 deg wire inside.


 
Per the NEC it's required to be. But take the sheath off the cable, and you end up with nothing more than copper with an unknown cover.


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## Saturday Cowboy (Nov 26, 2009)

Since romex is a cable assembly, an't it required only to be "marked/ labeled" on the cable jacket?

And so it goes, R


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## Magnettica (Jan 23, 2007)

One thing I have not seen mentioned on this thread that should be mentioned is the reason for derating "more than three current-carrying-conductors" and that reason is heat dissipation. High current, high heat build up within the conductor. Add a few of these high current conductors in the same conduit and then run through a warm room (like a boiler room, or attic), and now you have an issue. 

think heat :thumbsup:


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## 480sparky (Sep 20, 2007)

Magnettica said:


> One thing I have not seen mentioned on this thread that should be mentioned is the reason for derating "more than three current-carrying-conductors" and that reason is heat dissipation. High current, high heat build up within the conductor. Add a few of these high current conductors in the same conduit and then run through a warm room (like a boiler room, or attic), and now you have an issue.
> 
> think heat :thumbsup:


 
Mebbe post #4?


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## joebell (Sep 1, 2007)

manchestersparky said:


> Derate from the 90 degree column
> do not exceed the ampacity in the 75 degree column
> UNLESS
> it's NM Cable then do not exceed the ampacity in the 60 degree column
> ...


 
I thought 110.14(C)(1) requires it not to exceed the 60 degree column for 100 amps or less or size 14 thru #1, and 75 degree for over 100 amps, unless of course you are using romex and then you are limited to the 60 degree column. 

I guess unless the equipment is dual rated 60/75 degree C.............NEVERMIND:icon_cheesygrin:


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## wareagle (Jan 10, 2009)

joebell said:


> I thought 110.14(C)(1) requires it not to exceed the 60 degree column for 100 amps or less or size 14 thru #1, and 75 degree for over 100 amps, unless of course you are using romex and then you are limited to the 60 degree column.
> 
> I guess unless the equipment is dual rated 60/75 degree C.............NEVERMIND:icon_cheesygrin:


110.14(C)(1) say *"unless the equipment is listed and marked otherwise"*. All new equipment is rated 75C so you can use 90C when derating and 75C for conductor rating except romex.


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## nolabama (Oct 3, 2007)

Derating of wire as taught to me in New Orleans. I asked a contractor "Do you derate?" contractor answers "Depends on the inspector."


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## sokoservices (Feb 23, 2010)

Let me get this right.... I have a panel, I am going to stub up 5' to get above the drop ceiling with two pieces of 2" emt to a 8x8x24 trough. The wire coming to the trough is 12/2, 12/3, 12/4 MC. If I calculate it right I would have to use #8 from the breaker up to the trough 5' to maintain the 20 amps????

What is the standard table annex C in the back for that says I can put 110 #12 THHN's in the pipe if they are all the same conductor, but hold on thats not taking in consideration the De-rating. So there for if I have 25 conductors to keep the 20amps I would need to multiply the #10 at 40amps X adjustment factor of .45= 18 amps, So thats not even enough, I would need #8 to stub up 5'?????

Thanks
Soko.


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## BBQ (Nov 16, 2010)

Run more pipes.

Derating is a killer.

Of course you may be able to apply this exception


2008 NEC


> 310.15(A)(2)
> 
> Exception: Where two different ampacities apply to adjacent
> portions of a circuit, the higher ampacity shall be
> ...


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## sokoservices (Feb 23, 2010)

BBQ said:


> Run more pipes.
> 
> Derating is a killer.
> 
> ...


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## Salvatoreg02 (Feb 26, 2011)

I believe everyones conFUSED. all of today's equipment is rated for 75' celisius or above specifically rec & sw panelboards are rated 90 cel also most lighting eq. Is 90'


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## BBQ (Nov 16, 2010)

Salvatoreg02 said:


> I believe everyones conFUSED. all of today's equipment is rated for 75' celisius or above



No not all equipment is rated 75 C.




> specifically rec & sw panelboards are rated 90 cel also most lighting eq. Is 90'


I am going to ask you to show me a any of those items rated at 90 C.


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## Salvatoreg02 (Feb 26, 2011)

Lighting fixtures 90'
Sw & Rec 75
panelboards 75 & 90'


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## desertbucks (Feb 19, 2011)

I had more than 3 #12 CCC in a homerun conduit on 20 amp breakers and the inspector wanted these circuits on a 15 amp breaker or he would not green tag the job.


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