# do you derate?



## ICE (Apr 3, 2013)

I'm pretty sure that the customer didn't envision this when she hired the contractor.












This is the old can that's now a J-box.  This job got a lot of corrections.  Some that I hardy ever write; like all of the LBs with no gasket.  And check it out, some of the LBs can't be accessed unless conduit is removed.






They must think that I don't care.






So back to the question.  Do you require a reduction in ampacity for heat and more than three current carrying conductors in a conduit?


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## gfretwell (Apr 4, 2013)

The practical number is 9 conductors with "small" THHN (using the 90c column and 240.4(D)) but it is clear that this installation has moved into the 45-50% category.

You certainly have other issues there.


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## gfretwell (Apr 4, 2013)

You might even have an ambient temperature thing going on there depending on where that "concrete jungle" really is.


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## Dennis (Apr 4, 2013)

Here is how it works.  If you have more than 3 current carrying conductor's then you must derate however when using 90° C wire you can use #12 at 30 amps.  As stated above, even with 9 current carrying conductor's the factor is 70% .  30 amps * 70% = 21 amps.  Same is true with 14 gauge used at 25 amps.  So unless there are 10 current carrying conductor's then it is not an issue unless ambient temps come into play.

Are those emt connectors wp.  Compression fittings are not wp.  The wp have gaskets inside.


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## steveray (Apr 4, 2013)

You guys are soooo picky...I mean...this has been in a building for like 20 yrs and it hasn't burned down YET! That pesky code is just too dang strict!


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## steveray (Apr 4, 2013)




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## ICE (Apr 4, 2013)

Dennis said:
			
		

> unless ambient temps come into play.


Conduit in air, in direct sunlight = 113º F


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## ICE (Apr 4, 2013)

The cover doesn't fit tight.  The door latch was removed...probably to make room for a conduit entry.


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## BSSTG (Apr 4, 2013)

Greetings,

The pics aren't coming through. I get a big red X. However, attempting to derate conductors if you don't have a proper load analysis is mute. If each #12 only has couple of amps on it you are ok anyway usually. BTW, it's got to look pretty bad for me to question derating on residential. Commercial can be tricky.

 We had 14 motors fed from one conduit where I used to work in a production facility. Quite by accident they were ok by the Code back then. By todays standards they would not be. That said, the conduit stayed really toasty all of the time on the warm side of the wall it penetrated. The cold side of the wall was about -25 F. most of the time. Obviously not a big deal. It stayed like that for the 10 years I worked there and never was an issue. In fact, that warm conduit was a great way to warm your hands if you had been in the freezer.

I was doing a little side job for a retail store that had just been built out years ago. While stringing some speaker wire I brushed up to a conduit that was so hot it blistered my arm. The conduit came off of a mutltigang switch box that fed the whole store lighting. That was in Katy, Tx. I called the elect inspector on that one and it got corrected so I was told. That happened 20 years ago and It made an impression on me.

BSSTG


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## chris kennedy (Apr 4, 2013)

BSSTG said:
			
		

> However, attempting to derate conductors if you don't have a proper load analysis is mute. If each #12 only has couple of amps on it you are ok anyway usually.


Where does anything in 310.15 mention anything about actual load?


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## electriclese (May 30, 2013)

Dennis said:
			
		

> Here is how it works.  If you have more than 3 current carrying conductor's then you must derate however when using 90° C wire you can use #12 at 30 amps.  As stated above, even with 9 current carrying conductor's the factor is 70% .  30 amps * 70% = 21 amps.  Same is true with 14 gauge used at 25 amps.  So unless there are 10 current carrying conductor's then it is not an issue unless ambient temps come into play.Are those emt connectors wp.  Compression fittings are not wp.  The wp have gaskets inside.


OK, correct me if I am wrong but isn't the 90c column only for derating as long as the final amperage does not exceed the 60c amperage for the conductor gauge.  Regardless of conductor insulation temperature rating.  Otherwise we would all just use 12ga NM-B on a 30 amp feed.  I just dont see derating 12 gauge wire to 21 amps as 21 > 20.


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## Dennis (Jun 2, 2013)

electriclese said:
			
		

> OK, correct me if I am wrong but isn't the 90c column only for derating as long as the final amperage does not exceed the 60c amperage for the conductor gauge.  *Regardless of conductor insulation temperature rating.*  Otherwise we would all just use 12ga NM-B on a 30 amp feed.  I just dont see derating 12 gauge wire to 21 amps as 21 > 20.


I agree with the first sentence. Not sure what the bolded section refers to.

All I am saying is that you could have 9 current carrying conductor 's and still not have to protect #12 below 20 amps.  Simply because the calc is 21 amps does not mean you would use that ampacity.  20 amps is the highest.


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## electriclese (Jun 2, 2013)

I see what you are saying.  My point about insulation as you had stated that with 90c insulation that you could use #12 @ 30 amps.  The chart has that number but you can't use #12 @ 30a.  All temperature columns above 60c (70 & 90) are only for use in derating calculation.

It's just a wording thing.  Wouldn't want someone to wire #12 on a 30a OCPD regardless of temp rating.

I could bust out my #12 ZW 150c wire and run into a heater but not exceeding 20a.


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## Dennis (Jun 2, 2013)

electriclese said:
			
		

> I see what you are saying.  My point about insulation as you had stated that with 90c insulation that you could use #12 @ 30 amps.  The chart has that number but you can't use #12 @ 30a.  All temperature columns above 60c (70 & 90) are only for use in derating calculation.It's just a wording thing.  Wouldn't want someone to wire #12 on a 30a OCPD regardless of temp rating.
> 
> I could bust out my #12 ZW 150c wire and run into a heater but not exceeding 20a.


That is correct.  There are situation when the overcurrent protective device can be larger than the ampacity of the conductor.  A/C units have built in overload protection so you could have an overcurrent protective device rated 175% of the fla .  I have has #12 wire on 30 amp overcurrent protective device and it is compliant-- look at 240.4(D) which refers to 240.4(G)


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