PVC sizing

[aelectricalman]

I was hoping someone had a free minute and might help me figure out an issue. Looking at conductor ampacity per T310.16, I found the allowable ampacity of a THWN conductor (75deg) 350KCMIL to be 310 A. However there are 3 ungrounded and 1 grounded conductors to be placed in the PVC conduit (4 current carrying conductors). When I use the quick check adjustment factors of Table 310.15(B)(2)(a) , it states we must use 80% which is quite a bit difference. By those tables we should have to use 4- 500 KCMIL for 300 amps. How? Thats not right. Does anyone know what im doint wrong? Thanks

Basically, Im trying to run 300 A in PVC and want to use the minimal size conduit. I have to use THWN. Thanks again.

 

[charlie b]

However there are 3 ungrounded and 1 grounded conductors to be placed in the PVC conduit (4 current carrying conductors).

Why are you counting the grounded conductor? What is the load? If it is not a highly non-linear load (i.e., not a lot of harmonics), then it need not be counted.

 

[raider1]

I agree with Charlie, most likely the neutral will not be a current carrying conductor unless there is a large non-linear load on the conductors.

Take a look at 310.15(B)(4) for when a neutral counts as a current carrying conductor for ampacity adjustment purposes.

Chris

 

[infinity]

I agree with Charlie, for most feeders the neutral would not be a current carrying conductor therefore you wouldn't need to derate for more than 3 CCC's in the raceway. Definition of when the neutral is a CCC is in 310.15(B)(4).

 

[aelectricalman]

Thanks for the help. Your right.