Grounding, Bonding, confused

[linwue]

I have a quiz question that I just can’t figure out. Here is the information given and can’t find the correct answer of 900 AWG.

Can anyone run me through it and help me see what I’m missing?

What minimum size Aluminum THHW main bonding jumper is required when your ungrounded service entrance conductors are (12), 400 kcmil copper THHW conductors paralleled per phase?

 

[infinity]

Look at Table 250.102(C)(1) Notes #1 & #2 and tell us what you come up with.

 

[linwue]

I’m looking at note #2 because of the different conductor types. Not sure if you still use the 12 1/2% of the ungrounded conductor size because every time I do that I come up with 600 kcmil

 

[infinity]

I’m looking at note #2 because of the different conductor types. Not sure if you still use the 12 1/2% of the ungrounded conductor size because every time I do that I come up with 600 kcmil

The 600 kcmil would be for a copper conductor they're asking for an aluminum MBJ.

 

[linwue]

I’m just not getting the steps to come up with 900AWG. Can you help me out?

 

[suemarkp]

You need to reads notes 1 and 2 under table 250.102. The trickiest thing seem to be the case when the bonding jumper and ungrounded conductors aren't the same material. You have copper ungrounded conductors, but the bonding jumper is to be aluminum. So find what size aluminum conductor would be equivalent in ampacity to 400 KCMIL THHW copper. Look in table 310.15. This shows use size X aluminum.

You have 12 sets, so take the 12X as the combined area. Note 1 says take 12.5% of that to determine jumper size. 12X * .125 gives 900 KCMIL if you chose the correct X from table 310.15.

 

[linwue]

Thanks for the help. I really appreciate it