Derating conductors

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Flex

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poestenkill ny
I would like to put 12 -#8's and a #10 ground in an 1 1/2 emt. Is this legal cause im not sure how to derate. I know code says 22 #8 thhn in a 1 1/2 emt but Im sure thats not gonna fly in my situation.
 
See 310.15(B)(2)(a) and Table 310.15(B)(2)(a) for your answer.

Roger
 
Flex said:
Does that mean 11 would be the max?
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No, but you would have to adjust the conductors per the percentages in the Table.

Roger
 
Flex said:
I would like to put 12 -#8's and a #10 ground in an 1 1/2 emt. Is this legal cause im not sure how to derate. I know code says 22 #8 thhn in a 1 1/2 emt but Im sure thats not gonna fly in my situation.
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I am sure that once you apply the derating rules you will want to change your plan.

Can you tell us about the 12 - 8s, how they are connected and what they are running?
 
Flex said:
Im confused on what you mean. would i have to downsize the wire?
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Downsize the breaker or up size the wire.

Flex said:
They are 4, 480v 3phase ckts feeding roof top units.
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That is 12 current carrying conductors in a single raceway.

According to Table 310.15(B)(2)(a) 12 current carrying conductors get a 50% reduction in amapcity

THHN is 90 C wire, 8 copper is rated 55 amps with a 50% derating that results in the 8 AWG have a 27.5 amp rating.
 
Flex said:
such a huge help man. ive never been taught how to derate, thanks alot
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No problem.

My suggestion is to split it into two raceways, with 6 current carrying conductors in each the derating would only be 80% instead of 50%. Depending in the actual load may get you back in the right.

More important question.

Do you know the name plate data from the units? You should find the MCA (minimum circuit ampacity) and the MOCP (maximum over current protection)

If the MCA is say 27 amps or less and the MOCP is 50 you could use the 8s on the 50 amp breaker.
 
iwire said:
No problem.

My suggestion is to split it into two raceways, with 6 current carrying conductors in each the derating would only be 80% instead of 50%. Depending in the actual load may get you back in the right.

More important question.

Do you know the name plate data from the units? You should find the MCA (minimum circuit ampacity) and the MOCP (maximum over current protection)

If the MCA is say 27 amps or less and the MOCP is 50 you could use the 8s on the 50 amp breaker.
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mca is 29.2 amps
 
Flex said:
such a huge help man. ive never been taught how to derate, thanks alot
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Flex, Sub-panel Man here. :wink: You might also consider simply running a single set of feeders in that 1-1/2" EMT, and placing a 12-space panel at the point where the circuits would otherwise branch out. It can be more economical and efficient.


As for derating, you sort-of have to work backwards. Start with the required circuit ampacities for the loads, and the quantity of required CCC's. Then choose the conductor sizes, and then the conduit size required for those conductors.
 
LarryFine said:
Flex, Sub-panel Man here. :wink: You might also consider simply running a single set of feeders in that 1-1/2" EMT, and placing a 12-space panel at the point where the circuits would otherwise branch out. It can be more economical and efficient.
.
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SP man, in all seriousness that would rarely work out to be true on commercial jobs.

The RTUs are usually too far apart to make that really pan out. If the RTUs are in a tight group, and there is a place to put the panel, and the EE or customer will accept a panel in some odd ball location then I agree the sub panel could work out.

Truth be told we generally run separate cables or raceways to each RTU as per plan.:smile:
 
LarryFine said:
Then what would be the reason for running them all in the same conduit? :-?
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Good point:smile: but where would you locate this panel? On the roof?

I don't really want to keep guessing as we know darn little about the job, if it has an EE as most commercial jobs do you can't really just start changing panel locations, adding panels, etc.
 
iwire said:
Truth be told we generally run separate cables or raceways to each RTU as per plan.:smile:
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Just run one big pipe down the middle of the place, box at each RTU, stuff the pipe full and call it a day. ;)
 
I just did a job where we were feeding 8 rooftop units. We just ran 2 pipes and set junction boxes then we used one of the tap rules and flexed out to each unit. THis would not work obviously if your units are spread out.
 
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