Help with cable size.

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tjacobs

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Moorestown, NJ
I have a customer who is installing a rooftop air conditioning unit. Because of the corrosive environment of the plant and plant specs the conduit needs to be schedule 80 PVC. The AC unit comes with a 400 amp disconnect switch with 350 amp fuses. AC unit requires 480 volt 3 phase. The conduit run will be on the roof on 5" high support blocks. The run is 470' long. I found with NEC 310.15 B 3 there is an exception that says I can use XHHW-2 insulation and I do not need to apply the adjustment Factor for being on a rooftop. I am confused on weather I need to factor in ambient temperature and how to apply this adjustment if needed. Looking for some guidance. Any help would be appreciated.

Thanks.
 
Barbqranch said:
Just curious, how many expansion joints will that require?
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I looked at table 352.44 and determined with a 50 degree F change there should be 2.03 in of change for every 100'. I plan on putting one expansion fitting for every 100'. This should more then cover it. I think I am on the right track with this one.
 
tjacobs said:
I looked at table 352.44 and determined with a 50 degree F change there should be 2.03 in of change for every 100'. I plan on putting one expansion fitting for every 100'. This should more then cover it. I think I am on the right track with this one.
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I never been to NJ but from what I am aware of outdoor temps likely change more then 50, probably more like at least 120 - 130 degree change a typical annual high to low temperature. Maybe it don't get quite as cold as I think it might but I'm still sure it swings more then 50 degrees.

Is it really that corrosive on the roof? Seems like RMC or IMC would be a much better choice on exterior, your corrosion is likely concentrated indoors. PVC won't last all that long on the roof even if you put an expansion fitting every 10 feet.
 
jumper said:
Yes, since you are pulling XHHW-2 you can ignore the rooftop adder.

If the website below is fairly accurate, you can probably ignore any ambient adjustment factor also.

http://www.usclimatedata.com/climate/moorestown/new-jersey/united-states/usnj0328

Ask your AHJ.
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That chart show avg low of 23* and avg high of 88*.


kwired said:
I never been to NJ but from what I am aware of outdoor temps likely change more then 50, probably more like at least 120 - 130 degree change a typical annual high to low temperature. Maybe it don't get quite as cold as I think it might but I'm still sure it swings more then 50 degrees.

Is it really that corrosive on the roof? Seems like RMC or IMC would be a much better choice on exterior, your corrosion is likely concentrated indoors. PVC won't last all that long on the roof even if you put an expansion fitting every 10 feet.
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Agreed; I'd figure a low of 0* and high of 120* if it's on a blacktopped flat roof in direct sunlight.
 
JFletcher said:
That chart show avg low of 23* and avg high of 88*.




Agreed; I'd figure a low of 0* and high of 120* if it's on a blacktopped flat roof in direct sunlight.
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I thought it may get colder then zero in NJ, but maybe being on the coast makes some difference. I don't think you have to go much further inland and it can get much colder then zero. Average low/high don't mean anything IMO but 10 year min/min max is probably pretty good design points.
 
tjacobs said:
I looked at table 352.44 and determined with a 50 degree F change there should be 2.03 in of change for every 100'. I plan on putting one expansion fitting for every 100'. This should more then cover it. I think I am on the right track with this one.
Click to expand...

I would figure at least 100 degree F change.

I would put the expansion joints closer than 100' more like 40'.

Are you aware that the PVC support requirements are closer than EMT or RMC?

Also the supports must allow the PVC to slide.
 
Make sure that your AHJ is okay with you using xhhw-2 in lieu of derating if your current code cycle is not 2014. The 2014 code was when this exception was introduced and personally, I have had some push back in jurisdictions that are not using 2014 yet on this issue (strangely enough).

- Drew
 
Fitzdrew516 said:
Make sure that your AHJ is okay with you using xhhw-2 in lieu of derating if your current code cycle is not 2014. The 2014 code was when this exception was introduced and personally, I have had some push back in jurisdictions that are not using 2014 yet on this issue (strangely enough).

- Drew
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NJ has statewide adoption and is on the 2014 NEC.
 
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