Supply side bonding jumper

[hhsting]

I have transformer secondary panelboard which has bonding jumper to enclosure. Below exhibit calls that supply side bonding jumper.

Can that supply side bonding jumper be bus, screw or wire? Where is it in the NEC 2017 supply side bonding jumper material?

 

[bwat]

250.30(A)(2) Supply-Side Bonding Jumper. If the source of a separately
derived system and the first disconnecting means are located in
separate enclosures, a supply-side bonding jumper shall be
installed with the circuit conductors from the source enclosure
to the first disconnecting means. A supply-side bonding jumper
shall not be required to be larger than the derived ungrounded
conductors. The supply-side bonding jumper shall be permitted
to be of nonflexible metal raceway type or of the wire or
bus type as follows:

I was half tempted to not answer. Please start making the effort to look it up yourself.

 

[augie47]

If you are talking about the one noted in red, it might often just be a screw depending on the size of the panel.
(I'm not sure I would call that particular item a supply side bond jumper but I'm not saying the terminology is wrong)

 

[petersonra]

I don't get why the SSBJ is not just an EGC. It seems to serve the same purpose.

 

[augie47]

I don't get why the SSBJ is not just an EGC. It seems to serve the same purpose.

Perhaps because it's required to be sized by 250.102 and not 250.122

 

[wwhitney]

Per the (2017) definition, bonding jumpers are EGCs. I guess when there is not an OCPD "ahead" of the bonding jumper, then it's just that different sizing rules apply:

Grounding Conductor, Equipment (EGC). The conductive path(s) that provides a ground-fault current path and connects normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.

Cheers, Wayne

 

[Fred B]

I don't get why the SSBJ is not just an EGC. It seems to serve the same purpose.

1. because it's in relationship to the separately derived system before or with first overcurrent device.

 

[petersonra]

1. because it's in relationship to the separately derived system before or with first overcurrent device.

But how is it any different than an EGC? Just because we call the EGC from the transformer to the PB something else to confuse things does not make it really any different.

 

[wwhitney]

But how is it any different than an EGC?

It is an EGC, it just can't be sized via Table 250.122 as there is no OCPD "ahead" of it. So it gets an additional name and a different table for sizing.

Cheers, Wayne

 

[petersonra]

Perhaps because it's required to be sized by 250.102 and not 250.122

Is that a reason? Or is that just what we ended up with because of the mess article 250 has become.

since the only thing protecting the conductors is the main CB in the downstream PB, why shouldn't it be sized according to the rating of that main instead?

On top of that, does anyone believe the SSBJ that is the mounting screw for the ground bar is even close to the same size as what T250.102 requires?

 

[augie47]

Is that a reason? Or is that just what we ended up with because of the mess article 250 has become.

since the only thing protecting the conductors is the main CB in the downstream PB, why shouldn't it be sized according to the rating of that main instead?

On top of that, does anyone believe the SSBJ that is the mounting screw for the ground bar is even close to the same size as what T250.102 requires?

As pointed out, that is the key. There is nothing protecting the secondary conductors on the line side of that breaker if there is a fault so they have to be treated like service conductors with appropriately sized bond.

As far as the screw, I think many of us have questioned that for all service and SDS bonding

 

[wwhitney]

Seems to me that in principle if 250.122 were replaced with a table that referenced the size of the circuit conductors, then the term SSBJ could be retired. But I haven't studied 250 closely enough to see if I'm missing something.

Cheers, Wayne

 

[petersonra]

As pointed out, that is the key. There is nothing protecting the secondary conductors on the line side of that breaker if there is a fault so they have to be treated like service conductors with appropriately sized bond.

As far as the screw, I think many of us have questioned that for all service and SDS bonding

How does making the ssbj bigger protect those conductors?

 

[Fred B]

But how is it any different than an EGC? Just because we call the EGC from the transformer to the PB something else to confuse things does not make it really any different.

The difference is the conductor sizes related the transformer, it's not an EGC (Equipment Grounding Conductor) because it is the direct bonding connection to the Ground Electrode, and it bonds the equipment to the SDS. It is also why the sizing is very similar between the GEC (Grounding Electrode Conductor) and a SSBJ, and not the EGC that is related to the overcurrent/short circuit protection that is not present between the SDS and equipment.
It's not done to try to confuse things, but to clarify, 250.66 there are exceptions with regards to the GEC that allow undersized conductors in relationship to the ungrounded conductors under certain circumstances like residential to ground rods vs water pipe that exception are not present for the SSBJ, 250.102.

 

[wwhitney]

it's not an EGC (Equipment Grounding Conductor) because it is the direct bonding connection to the Ground Electrode

In the diagram in the OP, the conductor bonding to the GES is called a GEC, not an SSBJ. The conductors labeled SSBJ are indeed EGCs, per the definition, as they are part of a potential ground fault clearing path.

Cheers, Wayne

 

[petersonra]

I think you guys are missing the point I am trying to make. How does making the SSBJ larger protect the unprotected conductors?

The only thing protecting them is the primary side OCPD regardless of the size of the SSBJ.

Suppose you get a ground fault at X1. There is no fault path that clears that fault.

 

[augie47]

I see your point and don't know the answer. I've just always done it due to Code requirement. Sizing by 250.102 does provide you with a stronger fault return path perhaps increasing the possibility of opening the primary protective device but the actual reasoning is beyond my reach.

 

[hhsting]

I see your point and don't know the answer. I've just always done it due to Code requirement. Sizing by 250.102 does provide you with a stronger fault return path perhaps increasing the possibility of opening the primary protective device but the actual reasoning is beyond my reach.

I wish NEC or some supplemental resource to NEC would explain the “why” part of code. If that is provided then AHJ can actually determine other acceptable means of equivalent safety. Otherwise it makes it difficult to speculate if something provides equivalent safety or not

 

[xptpcrewx]

I think you guys are missing the point I am trying to make. How does making the SSBJ larger protect the unprotected conductors?

The only thing protecting them is the primary side OCPD regardless of the size of the SSBJ.

I don't believe it affords any protection per se, other than ensuring the OCPD can trip fast enough at a high current.

Suppose you get a ground fault at X1. There is no fault path that clears that fault.

You got me there.

 

[don_resqcapt19]

Seems to me that in principle if 250.122 were replaced with a table that referenced the size of the circuit conductors, then the term SSBJ could be retired. But I haven't studied 250 closely enough to see if I'm missing something.

Cheers, Wayne

They did that in the first draft of the 2020 code, but there were so many issues that were created and not accounted for that it was changed back in the second draft.