Splicing & bonding in Metal Trough

[pkelectrical]

Here is the setup

300A generac disconnect service rated feed by 350 mcm ALU

1. (2) 200A subpanels in the basement & 200A fusible disconnect with 110A fuses for Steam unit
Running 250 mcm copper from the 300A disconnect and using 3/0 copper taps for the sub panels and 1/0 Alu tap for the Steam disconnect

Table 250.102(C)1 calls for #2 copper bonding. The 300A Generac has 1/0 Copper bonding jumper

Q1. Why does the 300A generac has 1/0 bonding jumper? Is it oversized or I need to run 1/0 bonding to the subpanels/trough/disconnect etc as well?
Q2. I noticed 3/0 SER Copper (not being used in this installation) has 1/0 ground as well. Is there a reason why they make it with 1/0 where smaller size would be sufficient?
Q3. Can I run for example 1/0 Bonding jumper from the 300A ATS and bond the trough and then run all my other bonding to it? Or I have to run the back to the ATS? (what I would normally do, but that is 50' run)
In my opinion if I ran 1/0 Copper bonding from the ATS to the trough and run the other bonding to it, it would be a better system then running #2 Copper bonding jumper from the ATS and then loop bond all the equipment

What are your thoughts?

 

[augie47]

The first thing that comes to mind is the 350AL is undersized for a 300 amp service rated T/S.
Generac may have installed a 1/0 bonding jumper to cover the possibility of someone installing a conductor larger than 350AL
Anything on the load side of the 300 amp ATS will only require a #4 Cu or #2 AL bond/equipment ground.

 

[pkelectrical]

The first thing that comes to mind is the 350AL is undersized for a 300 amp service rated T/S.
Generac may have installed a 1/0 bonding jumper to cover the possibility of someone installing a conductor larger than 350AL
Anything on the load side of the 300 amp ATS will only require a #4 Cu or #2 AL bond/equipment ground.

350 mcm works for 300 A. 75 degree column rated at 250A/.83= 301A
Per 250.102(C)1 over 3/0 through 350 requires #2 copper or 1/0 Alu

 

[augie47]

You are correct.. I forgot the .83 residential... too accustomed to commercial.
On the LOAD side of the 30 amp breaker Table 250.122 would apply, not 250.102(C).