Ring KO's remaining and 250.97

Status
Not open for further replies.

Carultch

Senior Member
Location
Massachusetts
In a situation over 250V to ground, where you have ring KO's remaining, I understand that you need additional bonding means for raceway continuity to the enclosure. Bonding bushing, bonding locknut, or similar.

What if you don't need raceway continuity in your application, because of a wire EGC?

Example: it connects with no ring KO's to metal enclosure 1, and it connects with ring KO's to enclosure 2. There is an EGC from the ground bar of enclosure 1, that connects to enclosure 2, bypassing the need to rely on the raceway continuity for equipment bonding of the circuit. The raceway is already bonded to enclosure 1.
 

Carultch

Senior Member
Location
Massachusetts
The way I read it means you still have to bond the end with ring knockouts.

Maybe a bonding bushing and a jumper.


If Enclosure 2 is plastic, I don't need to bond conduit because it is already bonded to metal enclosure 1. The EGC alone bonds the two pieces of equipment, and the conduit is separately bonded to enclosure 1.

However, if Enclosure 2 is metallic with ring KO's remaining, and I don't use additional grounding fittings for >250V-g, I understand that the connection is not considered "electrically continuous enough". Do I need to additional bonding fittings/jumpers, EVEN IF I do not need them for the overall electrical continuity of the system?
 

Johnnybob

Senior Member
Location
Colville, WA
I don't know, maybe it's just one of those habits I've picked up, but I always use bonding/grounding bushings on eccentrics, no mater what the voltage.
 

Carultch

Senior Member
Location
Massachusetts
The way I read it means you still have to bond the end with ring knockouts.

Maybe a bonding bushing and a jumper.

Please see this picture, and tell me if you think the picture on the right complies or does not comply.

In both cases, there is a wire EGC connected between grounding bars of each enclosure. This is not directly bonded to any conduit fitting. I know that the picture on the left complies, but is the picture on the right electrically the same as the picture on the left?
 

Attachments

  • ringKOsRemaining.jpg
    ringKOsRemaining.jpg
    75.3 KB · Views: 1

iwire

Moderator
Staff member
Location
Massachusetts
Please see this picture, and tell me if you think the picture on the right complies or does not comply.

In both cases, there is a wire EGC connected between grounding bars of each enclosure. This is not directly bonded to any conduit fitting. I know that the picture on the left complies, but is the picture on the right electrically the same as the picture on the left?

In both cases addtinal bonding is required. The raceway itself still needs proper bonding even with plastic enclosures at the far end
 

Carultch

Senior Member
Location
Massachusetts
In both cases addtinal bonding is required. The raceway itself still needs proper bonding even with plastic enclosures at the far end

Does the raceway itself NOT already have proper bonding, by connecting on the other end to the already bonded metal enclosure that doesn't have KO's?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Does the raceway itself NOT already have proper bonding, by connecting on the other end to the already bonded metal enclosure that doesn't have KO's?

I say that is does but the NEC is not very clear on this one. If you use the plastic vs. metallic enclosure example then bonding on one end of the raceway is obviously sufficient.
 

Carultch

Senior Member
Location
Massachusetts
I say that is does but the NEC is not very clear on this one. If you use the plastic vs. metallic enclosure example then bonding on one end of the raceway is obviously sufficient.

That's exactly why I used that example.

I'm trying to understand whether there is a difference between zero electrical continuity as there would be for a plastic enclosure and the insufficient electrical continuity existing because of the metal ring knockouts.

For this example, there would still be an effective ground fault current path, i.e. through the EGC. And there also would be one through the connection to the enclosure without KOs, should the fault involve the metal raceway. However, the setup also creates an ineffective ground fault current path in parallel with the effective ground fault current path. And I'm inquiring to see if this is a problem.

The reason why I'm pushing so hard to get an accurate answer, is that I have "close quarters" inside the enclosure with the ring KOs, and it may not be practical to connect the proper bonding fittings/jumpers due to the physical space.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
That's exactly why I used that example.

I'm trying to understand whether there is a difference between zero electrical continuity as there would be for a plastic enclosure and the insufficient electrical continuity existing because of the metal ring knockouts.

For this example, there would still be an effective ground fault current path, i.e. through the EGC. And there also would be one through the connection to the enclosure without KOs, should the fault involve the metal raceway. However, the setup also creates an ineffective ground fault current path in parallel with the effective ground fault current path. And I'm inquiring to see if this is a problem.

The reason why I'm pushing so hard to get an accurate answer, is that I have "close quarters" inside the enclosure with the ring KOs, and it may not be practical to connect the proper bonding fittings/jumpers due to the physical space.

I agree with you. The EGC provides grounding for the equipment, the metallic raceway is grounded at one end, why would you need anything more?
 

Carultch

Senior Member
Location
Massachusetts
I agree with you. The EGC provides grounding for the equipment, the metallic raceway is grounded at one end, why would you need anything more?

Well, if a significant fraction of the fault current goes through the ring KO, instead of through the EGC, there could be a heat concentration at it. Probably not a problem, but that's why I found it questionable.

Thanks for the clarification.
 

Smart $

Esteemed Member
Location
Ohio
Well, if a significant fraction of the fault current goes through the ring KO, instead of through the EGC, there could be a heat concentration at it. Probably not a problem, but that's why I found it questionable.

Thanks for the clarification.
Code is not explicit regarding this issue, i.e. dead-end vs. pass-through bonding. I believe 250.4 requires pass-through bonding. Others do not.
 

kwired

Electron manager
Location
NE Nebraska
Well, if a significant fraction of the fault current goes through the ring KO, instead of through the EGC, there could be a heat concentration at it. Probably not a problem, but that's why I found it questionable.

Thanks for the clarification.

Why is that any different then if you had some non electrical component that also is incidentally bonded - yet has a weak point (continuity wise) somewhere in a potential current path? Similar situation will be imposed on this item during a ground fault. Could be something like metal railing, sheet metal panels used as building finish, chain link fence...

You may or may not get some arcing at such weak points, kind of depends on fault current levels, and actual resistance of such weak points, and how low of impedance the intentional EGC actually has.
 
Status
Not open for further replies.
Top