Open Bottom Switchboard Raceway Bonding Jumper

[infinity]

Someone asked me this today, open bottom switchboard with 7-4" EMT stubs into the bottom parallel 750's. 1600 amp feeder. What size single bonding jumper for all 7 raceways?

 

[Tulsa Electrician]

Cu or Al on the 750

 

[don_resqcapt19]

Interesting question, and one I am not sure that the code directly addresses.

I believe you are compliant with a single 4/0 copper bonding jumper, but could understand someone applying the parallel rule and require a full sized bonding conductor to each raceway.

 

[Tulsa Electrician]

Here is what I come up with
If you use Al at 75 c for phase and neutral conductors
2.5 ratio 750,000/300,00
4/0 cu (211,600) per 250. 122 feeder single bonding jumper.
Useing a single cu jumper, 600 mcm cu 211,600 x2.5= 529,000
What size did he run in each raceway?

 

[texie]

This would be 4/0 CU. Same as if you had a junction box in the run you could tie all the EGCs together and run 1 4/0 jumper to the box. I know in practice with conductors this big you would like just attach each EGC directly to the box but it seems compliant to me. Am I not getting something here?

 

[infinity]

Interesting question, and one I am not sure that the code directly addresses.

I believe you are compliant with a single 4/0 copper bonding jumper, but could understand someone applying the parallel rule and require a full sized bonding conductor to each raceway.

Sorry Aluminum 750's. Yes that is the question. The EGC's if used would be required to be up sized since 7 sets of 750 Al conductors are way oversized but I couldn't think of a code section that said explicitly that a raceway bonding jumper would also need to be up sized. There are no EGC's within the EMT.

 

[Tulsa Electrician]

I see your point. No egc installed. I see nothing saying larger than 4/0 is required.
Maybe a consideration of 250.4 (A) 5
Also Soares book on allowable length of 4" EMT with 750 which is around 280'
Not code so just a consideration.

Thank for pointing out that up size not required on bonding jumper for feeder.

 

[infinity]

A brief update. Guys on the job have been kicking this one around now for weeks. One said #3/0 is the largest required. Another said size the jumper to 25% of the OCPD. Someone else said 250 kcmil is the largest ever required. I just said buy a codebook and look at 250.122.