Transformer Bonding Strap and SSBJ

[charlie b]

I continue to be confused by the notion of a Supply Side Bonding Jumper (SSBJ). Here's my current conundrum: Do I have any of this right?

• Utility transformer.
• Main panel (MDP) in main electrical room has N-G bond.
• Secondary conduits are PVC.
• SSBJ, sized per 250.102(C) is run with the secondary A, B, C, N.
• SSBJ is bonded to both the transformer enclosure and the MDP enclosure (not sure exactly where).
• In order to prevent parallel current flowing in both the secondary N and the SSBJ, the bonding strap between the transformer's N point and its enclosure must be removed.

Am I also right in understanding that the SSBJ is never, never part of a fault current path, as there is no upstream breaker (on the secondary side) that would trip, if current were flowing in the SSBJ?

 

[augie47]

As I age I question some of understanding of the NEC but, as far as I know, a SSBJ is not used on a utility service transformer but only of SDS systems,.

 

[texie]

In the case you describe there would be no SSBJ. The POCO transformer has it's neutral bonded and the service disconnect will be bonded. A SSBJ would only be needed if you have some metallic items in the path along the way and a SSBJ is one way of bonding those items per 250.92.
YES, the SSBJ is part of the fault return path regardless of upstream OC protection. That is why they are sized larger (250.102) than EGCs (250.122).

 

[augie47]

For a SDS

 

[charlie b]

I bringing this thread back to life, because the topic has come up again on another project. I will start here:

. . . but, as far as I know, a SSBJ is not used on a utility service transformer but only of SDS systems,

If that were true, then why does the definition of SSBJ in 250.2 include mention of "service"? Also, the image in post #4 is primarily addressing the system bonding jumper (the N-G bond), not the SSBJ.

The SSBJ appears in both halves of the image, but is labeled only in the top image. The N-G bond is shown at the transformer in the top image and at the main panel in the bottom image. It is not shown at both places in either image. If it were made at both places, and if there is an SSBJ connecting the transformer enclosure to the main panel enclosure, then the SSBJ would carry half the neutral current at all times.

So how do utilities do this? Do they typically put an N-G bond at the transformer? I will typically shown the N-G bond at the main panel. Here again, if an N-G bond is installed at both places, then I don't want a metal-to-metal connection between the two enclosures. Instead, I would use PVC conduit for the secondary conductors and only include the A, B, C, and N.

 

[oldsparky52]

I've never seen a utility bring a bond wire in with their conductors, so they would be like the bottom image.

 

[oldsparky52]

I will typically shown the N-G bond at the main panel. Here again, if an N-G bond is installed at both places, then I don't want a metal-to-metal connection between the two enclosures. Instead, I would use PVC conduit for the secondary conductors and only include the A, B, C, and N.

I believe everyone will agree with this.

 

[augie47]

250.30 (A)(2) Exception which refers back to 250.30(A)(1) Exzception2 seems to apply to your install thus a SSBJ is not required. (IMO)

Note aslo in this case it is a UTILITY transformer and the definition of a SDS is "other than serice" so I don't know that250.30 would apply.

 

[don_resqcapt19]

The use of a supply side bonding jumper on a service is rare as you are permitted to use the grounded conductor for all required bonding and grounding on the line side of the service equipment. I guess if you had a metallic enclosure that was not connected by metallic raceways on the line side of the service equipment you would be permitted to use a supply side bonding jumper to bond that metallic enclosure. Not sure why you would do that as you can use the grounded conductor for that purpose.
You could also have one where there is line side solar or wind disconnects where there is no grounded conductor and a supply side bonding jumper is used to bond those disconnects.
I can't think of other cases where you would have on on the line side of the service disconnect.

 

[don_resqcapt19]

I believe everyone will agree with this.

In our town, there is always rigid conduit between the meter and the service equipment per local code. That means that the service raceway between the meter and the service equipment is always a parallel path for the neutral conductor current.

 

[electrofelon]

So how do utilities do this? Do they typically put an N-G bond at the transformer? I will typically shown the N-G bond at the main panel.

POCO will typically bond primary neutral (if its an MGN distribution), secondary neutral and a ground rod(s) all together. But I dont think you have to think about what the POCO does with their transformer, it is none of your business

Here again, if an N-G bond is installed at both places, then I don't want a metal-to-metal connection between the two enclosures. Instead, I would use PVC conduit for the secondary conductors and only include the A, B, C, and N.

Don kinda just beat me to this, but note for services we only have the vague term "objectionable current". There is really no specific requirement to avoid a parallel neutral path and it happens all the time and I have never known anyone who cares. You may be able to avoid these by choosing non metallic raceways in key places and/or using a SSBJ instead of bonding directly with the grounded conductor, but no one seems to go too far out of their way to do this.

Finally, a bit of a curiosity, note that SDS's DO HAVE more restrictive wording, and 250.30(A)(1) exception #2 does use the term "Parallel path for the grounded conductor"

 

[oldsparky52]

In our town, there is always rigid conduit between the meter and the service equipment per local code. That means that the service raceway between the meter and the service equipment is always a parallel path for the neutral conductor current.

From your description I'm assuming the metal raceway is bonded at both ends?

Easy at the meter base but do they do it at the pole or pad mount and how do they do it? Do they install grounding bushings? What size conductor do they use since it will be a parallel path with the neutral? Sounds like a total disregard for what we electricians are taught.

 

[don_resqcapt19]

From your description I'm assuming the metal raceway is bonded at both ends?

Easy at the meter base but do they do it at the pole or pad mount and how do they do it? Do they install grounding bushings? What size conductor do they use since it will be a parallel path with the neutral? Sounds like a total disregard for what we electricians are taught.

The neutral is factory bonded to the meter can in all of the meter cans that are acceptable to our utility, and the main bonding jumper is installed at the service equipment. The local code requirement that all service conductors be in rigid conduit, places the conduit in parallel with the neutral between the meter can and the service equipment. There are no bonding conductors other than what the code requires.

The rule does not require that the conduit be made into a parallel path, it is just the result of the rule where the conduit is installed between two metal enclosures, so risers or raceways that originate at a pad mount, are not parallel paths.