Supply-Side Bonding Jumper

[But-a-mear-splice]

Hello all.

I am currently studying for my Oregon General Journeyman license. I am using Mike's "Understanding the National Electrical Code" books, and I'm having a problem figuring out one of the questions in the workbook. The question is from Vol. 1 workbook, page 96, question 150.

150. What is the minimum size copper supply-side jumper for a service raceway containing 4/0, THHN aluminum conductors?

(a) 6AWG AL

(b) 4AWG AL

(c) 4AWG Cu

(d) 3AWG Cu

The Key says that the answer is, (c) 4AWG Cu.

But when I look at Table 250.102(C)(1), I read that the answer should be 2AWG Cu.

And as I type this up I suddenly see the error that I made. Which was, I was reading from the copper column and not the aluminum column. So instead of "wasting" the time I took wracking my brain, and writing this up, I thought this would be a good opportunity to remind all of those in my position to make sure you are using your tables correctly! And also to say thank you to all the seasoned code jockeys here, who are dispensing good information.

 

[charlie b]

Thank you for the lesson learned. Now I have a question for the membership. Was I wrong in believing that an SSBJ is not installed with a SERVICE? I had thought that, for example, if you have a 480V service and a 480-208Y/120V step-down transformer, you run an SSBJ with that transformer's secondary conductors.

 

[augie47]

As charlieb points out the words supply side bonding jumber and service don't mix but a SDS SSBJ and service SBJ would both be sized the same. With that in mind:

Look closely at 250.102... a 4/0 AL is in the 3rd row down (4/0-250) Following that line across It would allow a #4Cu or #2 AL for the SSBJ. The #4 Cu is only choice give for the two.

 

[don_resqcapt19]

A supply side bonding jumper is required to bond metallic service raceways that stub up in the bottom of open bottomed service equipment.

 

[augie47]

Thanks, don. I forgot the term was applicable there also

 

[mikeames]

The "SBJ" or system bonding jumper is at the transformer secondary neutral. The "SSBJ" is the "supply-side bonding jumper" that goes from the secondary neutral to the first disconnect on the secondary. Although I think you can bond both at the first disconnect on the secondary then it just becomes the "SSJ".

Correct me if I am wrong.

 

[don_resqcapt19]

The "SBJ" or system bonding jumper is at the transformer secondary neutral. The "SSBJ" is the "supply-side bonding jumper" that goes from the secondary neutral to the first disconnect on the secondary. Although I think you can bond both at the first disconnect on the secondary then it just becomes the "SSJ".

Correct me if I am wrong.

The bonding conductor between the secondary and the first OCPD, is always a supply side bonding jumper. It is required no matter where the system bonding jumper is installed. Where the SBJ is at the first OCDP, the SSBJ is the fault clearing path for a secondary to ground winding fault at the transformer itself.

 

[Dsg319]

I wonder how often the SSBJ is sized as an EGC after a step down transformer to the first OCPD. Example 480delta—208/120wye.

 

[augie47]

I wonder how often the SSBJ is sized as an EGC after a step down transformer to the first OCPD. Example 480delta—208/120wye.

here, often about 1/3 of my inspections it's either left out or undersized