I have a question about conductor sizing.
I have a 1200 amp main breaker on a 120/208Y service feeding a grocery store.
I came up with (4) 500 MCM per phase.
380A x4= 1520amps.
1520A x .80(T310.15(b)(2)(a)4-6 conductors in a raceway.=1216 A
1216A Round down to next standard= 1200A.

Have I gone astray somewhere? With all the lights, computers, etc. I would consider the neutral a current carrying conductor. I have seen other topics on here and none had mentioned the wire size I came up with.

As always thanks for the help gentlemen!
Gary

I would run 4 conduits, each containing 4 400MCM...A,B,C,N

For derating purposes, you use the 90deg column...so 400MCM is 380A x 4 = 1520A * 0.8 = 1216A

However, whether the Neutral is actually a CCC can be disputed. If not, then you can get away with 350MCM

I thought you could only use the column that corresponded to the temperature rating or the terminations?

for sizing...for derating you can use the 90deg column..

I dont understand what you mean. I thought you sized the conductor according to ampacity required, then adjusted the selected conductor according to ambient temp, # of conductors in conduit, etc. I am not trying to sound obtuse, but I guess I was tuaght differently.

nope...you size the conductors based on the terminal temp rating...but for derating purposes only, you can use the 90deg column. because you are worried about the insulation rating, not the connection rating.

without derating, 4-350's or 4-400's have enough ampacity in the the 75deg table to carry the load. but for derating only, you can use the 90deg table.

maybe someone else can explain it better.

but wouldnt that result in a conductor sized below the ampacity of the 75 degree column?

i am going to look at this tommorrow morning and laugh im sure!

nope...not in this case...

I will say this though...find out if the inspector wants you to consider the neutral as a CCC...if not, no derating necessary...then you can run 350's...

I am thinking the same as Mahlere. I was taught you size your conductors to the terminal ratings of the equipment you are hooking up, which most of the time is the 75 deg C colum. You use that colum for your basic calculation for ampacity, then if you derate due to conduit fill or a change in ambient temperature..you look right to the 90deg colum. So rather than changing wire gauge you change insulation type by looking at the 90 deg colum only for conduit fill or ambient temp derating. This was explained to me in the Mike Holt exam book. Good luck, Brian

I have always wondered why the Code panels decided on 800 amps as the cut off point for going to the next larger Over Current device instead of 1200 amps. NEC 240.4(B)
In section 230.90 Exception 3 the two to six disconnects can have a combined rating greater than the conductors if the calculated load is less than the conductors.
If the calculated load is less than 1200 amps but, has a single disconnect then using three 500 mcm (1140 amps) is prohibited.
I don't think I have ever seen a 1,200 amp service loaded to its maximum. Most have less than 1,000 amps of load at the extreme.
Always seemed like a waste of copper to me.