Four MC Cables Run in 3" Conduit

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Smart $ said:
[310.15(B)(3)(a)(4)(a)] You would need unjacketed #12 to not have to derate....
[330.10(A)(11)] but you need jacketed in the 16" above the roof to qualify for the wet location usage...
...so you use jacketed and strip off the jacket for all but the 16" above the roof, and
[310.15(A)(2) Exception] use the adjacent rule to not have to derate that 16".
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I think I will use the 10-3 jacketed, strip the last 16" of jacket and see what happens. It just seems stupid to have to use #8 where #12 wire would be large enough. I'm picturing a discussion with the inspector as follows:

Inspector. You used #10 wires and therefore they must be derated. Put in #8.
Me. How about I install #12 instead?
Inspector. OK that will work
 
mkgrady said:
I think I will use the 10-3 jacketed, strip the last 16" of jacket and see what happens. It just seems stupid to have to use #8 where #12 wire would be large enough. I'm picturing a discussion with the inspector as follows:

Inspector. You used #10 wires and therefore they must be derated. Put in #8.
Me. How about I install #12 instead?
Inspector. OK that will work
Click to expand...
Do not strip the portion that goes in the 16" above the roof. The below the roof portion don't need a jacket... it's not in a wet location. Or because you are supposed to derate #10 anyway, just leave the jacket on the whole distance in case you have to pull it out and pull in #8... or #12. :D
 
mkgrady said:
I think I will use the 10-3 jacketed, strip the last 16" of jacket and see what happens. It just seems stupid to have to use #8 where #12 wire would be large enough. I'm picturing a discussion with the inspector as follows:

Inspector. You used #10 wires and therefore they must be derated. Put in #8.
Me. How about I install #12 instead?
Inspector. OK that will work
Click to expand...

Would#12 work? According to table 430.250 a 15 HP motor @ 440-480 volts is 21 amps, 21*125%= 26.25 minimum conductor size.
 
mkgrady said:
If I have to derate I could use 8-3 MC. With derating 50% (of 50 amps) I would still exceed the 21 amp circuit requirement.

I cant use a subpanel. They want the VFD drives on the floor below the roof.

Interdict sounds interesting. Never used it. Not even sure what it is.
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http://www.carlonsales.com/pdfs/HDPE/HDPE_Brochure.pdf

Innerduct is basically conduit within conduit. Instead of having a single 3" conduit to pull all your wire through, you could install several 1/2" to 1" innerduct conduits inside the 3" one. I dont know if this would get you past derating, tho innerduct can be used to run power and communications circuits thru the same pipe as the innderduct is a barrier between the two. You might want to check with Carlon and/or the AHJ to see if this would be acceptable in your install.
 
JFletcher said:
http://www.carlonsales.com/pdfs/HDPE/HDPE_Brochure.pdf

Innerduct is basically conduit within conduit. Instead of having a single 3" conduit to pull all your wire through, you could install several 1/2" to 1" innerduct conduits inside the 3" one. I dont know if this would get you past derating, tho innerduct can be used to run power and communications circuits thru the same pipe as the innderduct is a barrier between the two. You might want to check with Carlon and/or the AHJ to see if this would be acceptable in your install.
Click to expand...

I think the engineers concerns about inner duct is it will not provide a metallic shield between the four different vfd outputs to the motors. Theoretically there old be some issue with that? I think I have read that but I don't understand what problems it could cause
 
mkgrady said:
I think the engineers concerns about inner duct is it will not provide a metallic shield between the four different vfd outputs to the motors. Theoretically there old be some issue with that? I think I have read that but I don't understand what problems it could cause
Click to expand...

I dont know either but you should be able to get 4 MC cables thru 4 3/4" innerduct conduits... assuming the MC jacket is a sufficient shield.
 
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