Conduit fill

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arnettda

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I am running a 250-250-3/0 USE cable with a RHW-2 ratting plus ground in a pipe roughly 60ft under a house slab and comming up into the service panel. I am trying to size the PVC pie for it. I used chapter 9 tables 4 and 5 to determine the size. 355.22 tells me to do this. Is there anywhere else in the code that talks about conduit fill and cables? Or just the respective pipe code articles?
 
I think you have a problem. First off if conductors in parallel are installed then you must count each conductor in the pair as a ccc. 310.15(B)(3)(a) last sentence first paragraph.

Now assuming the neutral is not a CCC you have 6 conductors and must derate 80%. 250 MCM RHW-2 is rated 90C both wet and dry so the ampacity is rated 230. 230*3= 690 * .8= 552 so you can go to 600 amps as long as the calculated load is not over 552. Now I calculated the fill and got 3.5"

RHW-2 without an outer jacket
250 KCM = .4596*6= 2.7576
3/0 = .3117*3 = .9351
2/0 + .2624 *1 =.2624

Total is 3.9551 = 4" schedule 40 PVC 5.022... 3.4" pvc is only 3.985
 
Dennis Alwon said:
I think you have a problem. First off if conductors in parallel are installed then you must count each conductor in the pair as a ccc. 310.15(B)(3)(a) last sentence first paragraph.

Now assuming the neutral is not a CCC you have 6 conductors and must derate 80%. 250 MCM RHW-2 is rated 90C both wet and dry so the ampacity is rated 230. 230*3= 690 * .8= 552 so you can go to 600 amps as long as the calculated load is not over 552. Now I calculated the fill and got 3.5"

RHW-2 without an outer jacket
250 KCM = .4596*6= 2.7576
3/0 = .3117*3 = .9351
2/0 + .2624 *1 =.2624

Total is 3.9551 = 4" schedule 40 PVC 5.022... 3.4" pvc is only 3.985
Click to expand...

I am sorry I should have been more clear. I am going to run each set of wires in a seperate conduit. I believe this should be oaky. Direst buried in trench and then into pipe.
Your resposne did make me realize I was using RHW-2 with outer covering. I believe outer covering is something completly over the normal RHW-2 insulation?
 
arnettda said:
I am sorry I should have been more clear. I am going to run each set of wires in a seperate conduit. I believe this should be oaky. Direst buried in trench and then into pipe.
Your resposne did make me realize I was using RHW-2 with outer covering. I believe outer covering is something completley over the normal RHW-2 insulation?
Click to expand...
Then you only need 2.5" PVC. I am not sure what the outer covering is about. If that is what you have then the 2,5"PVC will work
 
My instructor in a code class I took one time talked about it but that is all I remember was him discussing it and saying it was something extra.
 
arnettda said:
I am running a 250-250-3/0 USE cable with a RHW-2 ratting plus ground in a pipe roughly 60ft under a house slab and comming up into the service panel. I am trying to size the PVC pie for it. I used chapter 9 tables 4 and 5 to determine the size. 355.22 tells me to do this. Is there anywhere else in the code that talks about conduit fill and cables? Or just the respective pipe code articles?
Click to expand...

Sorry, I can't resist - Is the pipe buried 60 ft deep?:)

tables 4 and 5 is all that is needed, once actual conductor sizes have been determined, multiple current carrying conductors in same raceway may require adjustments and require larger conductors as has been brought up.

I also come up with 2.5 inch for conductors without outer covering, but you will need 3 inch if there is outer covering - not sure just what outer covering would be as I don't think I have seen such conductor before. you could measure your conductor and see how it compares to dimensions in table 5 if you are not sure what you have. Would need a reasonably accurate caliper or something that will measure to .001 inch increments.
 
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