480V MCC main feeder

[peterwire]

I have a 480V switchgear feeding a 480V MCC. Originally it was closed coupled, so there was a bus connecting the SWGR with the MCC. However the client wanted the MCC separated from the SWGR and using cables instead. The MCC bus and main breaker are 1600Amps.
I chose 3-1/C#350kcmil as main feeder. is that ok for the 1600A main CB?
Also, the distance between the SWGR and MCC is barely 3ft so I'm concerned that the bending radius of those cables will be hard to work with. Any ideas?

 

[electrofelon]

I have a 480V switchgear feeding a 480V MCC. Originally it was closed coupled, so there was a bus connecting the SWGR with the MCC. However the client wanted the MCC separated from the SWGR and using cables instead. The MCC bus and main breaker are 1600Amps.
I chose 3-1/C#350kcmil as main feeder. is that ok for the 1600A main CB?
Also, the distance between the SWGR and MCC is barely 3ft so I'm concerned that the bending radius of those cables will be hard to work with. Any ideas?

3 x 350 copper is no where near 1600. Is that a typo?

 

[petersonra]

3 x 350 copper is no where near 1600. Is that a typo?

What he said.

 

[peterwire]

3-350 (570Amp per NEC 310.17 XHHW-2).
that was one thing i was not sure. 3 x 570A is not correct.
but then i cant get to 1600A with feeders unless i use for example 6-1/c # 750kcmil (885Amps) . and then i will have real problems with the bend radius. can i have bus duct instead?

 

[charlie b]

First of all, you can't use the 90C column unless you have 90C rated terminations (and that is very much unlikely).

Next, you appear to be saying that if a single conductor (for example, Phase A) has an ampacity of 570, then a set of three conductors (i.e., Phases A, B, and C) will have an ampacity of three times that. Not true. The current leaving the source on one phase returns to the source on the other two phases. It is the same current. So the notion of "total current equals 3 times phase current" is nonsense.

Next, you almost certainly need to be using table 310.15(B)(16), not ...(17). You are running all three phases, so table ...(16) applies.

Finally, what you need to do is to run parallel sets of conductors. One example would be five conduits, each 3.5 inches, each containing three 400 KCMIL phase conductors, one 400 KCMIL neutral, and one 4/0 ground wire.

 

[peterwire]

thank you for your replies. 400kcmill is not on stock so i ended up specifying 4-3/c #750kcmil (475Ax4=1900Amp). Cable will be laid on tray inside the bldg.

 

[charlie b]

Someone else will have to comment on the possible need to derate for multiple conductors in the same tray. I never deal with tray, so I don't know whether the derating factor applies.

 

[don_resqcapt19]

There are different ampacity rules that apply to conductors installed in tray. You could make three sets of 750 kcmil copper work for this installation in tray. See 392.80 for the general cable tray ampacity rules. See 392.80(A)(2)(a) for the set up that would let you use three 750 kcmil. 392.80(A)(2)(c) would let you use three sets of 500 kcmil copper.

 

[electrofelon]

3-350 (570Amp per NEC 310.17 XHHW-2).
that was one thing i was not sure. 3 x 570A is not correct.
but then i cant get to 1600A with feeders unless i use for example 6-1/c # 750kcmil (885Amps) . and then i will have real problems with the bend radius. can i have bus duct instead?

As the conductors get larger, you get less "bang for the buck" in ampacity per unit of cross sectional area. Just look at the table value for 250 vs 750 and you will see how inefficient it gets when you get up around 750 sizes. I personally wont use 750 unless there are some special circumstances. So in general try to use more sets of smaller conductors. OF course with smaller conductors you either have more smaller raceways or derating if you pack a bunch of sets in a single larger raceway, so there is a balancing point. I dont know off the top of my head the cable tray rules so I cant really comment on how it works out in tray.