130' 100amp feeder to retail run on flat roof design

[Ravenvalor]

Hello,

I am considering installing a 3-phase, 208-volt, 100-amp feeder circuit to a retail space on a flat roof in central North Carolina. I rarely see circuits this big and this long on roofs. Can you guys please give me some design tips and do's and dont's when designing and installing this. The roof is only 15' off of the ground and a roof installation will be much simpler than an installation inside the building. I imagine conductor size will be much larger due to the temperature. I can see emt rusting while pvc being expensive due to all of the expansion joints. Are those the reasons why roof top larger circuits are rarely seen in NC?

Thanks for all the help.

 

[George Stolz]

I imagine conductor size will be much larger due to the temperature.

At a quick guess, you'll be looking at #1 CU instead of #3.

#1 CU = 130A @ 75?C

130A * .82 (310.15(B)(3)(c)) = 106A

 

[Ravenvalor]

Conductor Size

Conductor Size

I was considering using a #1 CU for this size anyway given the distance and the fact that other 100amp retail feeders are #1.
I was thinking of putting it in a 2" raceway.

Thanks.

 

[ramsy]

I can see emt rusting while pvc being expensive due to all of the expansion joints. Are those the reasons why roof top larger circuits are rarely seen in NC?

Cal-MCJ4w - No Conduit Needed.
Alcan Stabiloy is Compact aluminum PVC-Coated MC cable with aluminum conductors.
1/0 AL does 100A under 70?c with 3-balanced CCC's.
Large rooftop circuits are ringing dinner bells for copper thieves. Aluminum has much less scrap value.

 

[Ravenvalor]

Cal-MCJ4w - No Conduit Needed.

Cal-MCJ4w - No Conduit Needed.

"Cal-MCJ4w - No Conduit Needed.
Alcan Stabiloy is Compact aluminum PVC-Coated MC cable with aluminum conductors.
1/0 AL does 100A under 70?c with 3-balanced CCC's.
Large rooftop circuits are ringing dinner bells for copper thieves. Aluminum has much less scrap value. "

Hello Ramsey,

I had not even considered installing an MC cable. Would it have to be attached to something rigid like a piece of pipe installed on 4 x 4 blocks every few feet?

For the security concerns you have an excellent point that I had not even considered. If I use aluminum I will have to label the conduit 'aluminum'.
I imagine someone taking it will automatically assume that it would be copper. Evil can be stupid.

Thanks,

 

[ramsy]

Would it have to be attached to something rigid like a piece of pipe installed on 4 x 4 blocks every few feet?

Roger that. Strap it per ? 330.30.
Aluminum is identified at terminations, but a Cable assembly would also be much lighter in weight than copper.

 

[kwired]

Ambient temperature adjustment for raceways on roofs is going to be dependent on geographical location and distance from roof to raceway. See 310.15(B)(3)(c).

I also would not be too concerned about using aluminum over copper because of copper thieves. They will not necessarily know if you used copper or aluminum until damage has already been done.

EMT is just fine if not directly on the rooftop, it should not rust any more than it would anyplace else outdoors and non corrosive.

PVC, you can put all the expansion fittings you want on it, it dust doesn't stand up to the sunlight and heat very well and gets brittle and breaks. Even though PVC is superior for corrosion, it will not take any physical abuse once it has had some heat and sun deterioration.

 

[Ravenvalor]

ambient temp derating

ambient temp derating

It looks like conduit 4" above a roof in the Raleigh/Durham NC region will have to be derated by .67.

1/0 CU = 150A @ 75degreesC

150 * .67 = 100.5A

Thanks for the input.

 

[augie47]

You might consider something like this:

http://www.cooperindustries.com/content/public/en/b-line/brands/Dura-Blok.html

 

[conmgt]

EMT will work until the HVAC guy sits on it to eat lunch. Or space the supports close together.

 

[augie47]

EMT will work until the HVAC guy sits on it to eat lunch. Or space the supports close together.

It is amazing the punishment to which some roof top conduit systems seem to be subjected.

 

[norcal]

It is amazing the punishment to which some roof top conduit systems seem to be subjected.

Who is worse for destruction of electrical? Roofers or Landscapers? In the OP's situation doubt the landscapers will be on the roof, but it seems that rigid or IMC would be the most re$i$tant to damage.

 

[Ravenvalor]

A 4 x 4 block of lumber with a piece of kindorf and strap every 5' should keep it secure.

 

[ptonsparky]

400' of 1AL plus the 2" EMT is worth the trip to a salvage yard. Just takes a few more trips to make groceries.

 

[Ravenvalor]

good point

 

[Ravenvalor]

At a quick guess, you'll be looking at #1 CU instead of #3.

#1 CU = 130A @ 75?C

130A * .82 (310.15(B)(3)(c)) = 106A

Do you reckon that I should derate the circuit to a #1/0 CU in order to account for voltage drop?

Thanks,

 

[Ravenvalor]

You might consider something like this:

http://www.cooperindustries.com/content/public/en/b-line/brands/Dura-Blok.html

Great advice, actually I have tried something like this before. It worked good and looked good also but the cost was high.

Thanks,

 

[jumper]

Do you reckon that I should derate the circuit to a #1/0 CU in order to account for voltage drop?

Thanks,

I do not think it is necessary. You will be under 3%.

 

[kwired]

Do you reckon that I should derate the circuit to a #1/0 CU in order to account for voltage drop?

Thanks,

If you had to derate because of temperature or number of conductors in raceway, you have a new voltage drop value over original size (with no deration). Your run is short enough I would guess without calculation that you are already good on voltage drop - especially if you have already increased from a minimum needed 3 AWG to a 1AWG. But it doesn't hurt to run a VD calc on the new conductor size to verify.