Next Size up with Unknown Load

[Moore Power]

I am bidding on some landlord work. Installing a 400 amp 277/480 panel, 75kva transformer and a 200 amp 120/208 panel. I am considering going with 750 AL (385A) because there is a nice empty 4'' going half way to the new location. I am wondering if I need to size this for the full 400 amps without knowing what the actual future tenant load will be. I don't mind doing that, but I don't want to get beat out by somebody who is going to give the landlord 385 amps for cheaper.

 

[infinity]

I would go with the 750 aluminum. Since the future load is unknown chances are it won't exceed 385 amps anyway so why waste the money on different conductors.

 

[Moore Power]

I would go with the 750 aluminum. Since the future load is unknown chances are it won't exceed 385 amps anyway so why waste the money on different conductors.

Thank you.

 

[infinity]

Thank you.

You're welcome.

 

[electrofelon]

Another option is you could use parallel 300's, which would give you the full 400 and prolly cheaper than the single 750's

 

[JoeStillman]

Another option is you could use parallel 300's, which would give you the full 400 and prolly cheaper than the single 750's

Certainly less actual aluminum, but he'd have to run two conduits. He's already got one 4" going halfway there and if he runs the parallel 300's in that the derating puts him right back to the same ampacity.

 

[augie47]

Certainly less actual aluminum, but he'd have to run two conduits. He's already got one 4" going halfway there and if he runs the parallel 300's in that the derating puts him right back to the same ampacity.

I may have misread but in checking the Tables it appeared he would have no problem paralleling 300s in a 4"

 

[JoeStillman]

I may have misread but in checking the Tables it appeared he would have no problem paralleling 300s in a 4"

Right, and therefore, derating {per 310.15(B)(3)(a)} from 230A x 2 = 460A to 230A x 2 x 0.8 = 368A.

 

[david luchini]

Right, and therefore, derating {per 310.15(B)(3)(a)} from 230A x 2 = 460A to 230A x 2 x 0.8 = 368A.

Or 260A x 2 x 0.8 = 416A

 

[JoeStillman]

Or 260A x 2 x 0.8 = 416A

I've made a very common error. You correctly derate using the 90°C column.