Bonding Bushing Question

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George Stolz

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Windsor, CO NEC: 2017
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Looking at the diagram below...

View attachment 3114

There is a 480V 100A service disconnect feeding a VFD cabinet with 1.5" LFMC.
At "Point A" there is a concentric knockout.
At "Point B" a hole has been drilled in the cabinet.

There is more room in the VFD cabinet than the service disconnect. The installer puts a bonding bushing in the VFD cabinet on the connector, at point B. He installs a plastic bushing on the service disconnect side, point A.

Legal or illegal?
 

iwire

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The fact that is a service disconnect is not relevant as the run to the VFD is a feeder.

Is there an EGC in the run?

If there is an EGC run with the feeders I think we could argue that the conduit is bonded to the enclosure with the concentric although in a longer route then normal.

If there is no EGC then IMO it is without a doubt a violation.
 

augie47

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I've been wrong so often lately, I hate to post an opinion.
LFMC would not be a permossible EGC here per 250.118 due to the OCP being greater than 60 amps, so a seperate EGC would be needed.
As there is no concentric knockout at one end the LFMC is bonded and there is no need for a bond bushing period.
(has my luck changed, am I right ?)
 

George Stolz

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Windsor, CO NEC: 2017
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I was thinking along the same lines - that, as long as the raceway is bonded, that it didn't matter which side the bonding bushing was located on.

However, looking at the language in question...

Bonding jumpers meeting the other requirements of this article shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground. Standard locknuts or bushings shall not be the sole means for the bonding required by this section.

When read a certain way, it seems to support the inspector's assertion that the bonding bushing is to be installed at the point where the concentric is encountered. The concept of ground fault current arcing at the concentric causing damage to the can itself would explain that reasoning.

Another example of the effect would be the introduction of the infamous Kenny Clamp to combat GECs doing arcing where they enter metallic enclosures.

Is this interpretation invalid?
 

ibew441dc

Senior Member
I was thinking along the same lines - that, as long as the raceway is bonded, that it didn't matter which side the bonding bushing was located on.

However, looking at the language in question...

When read a certain way, it seems to support the inspector's assertion that the bonding bushing is to be installed at the point where the concentric is encountered. The concept of ground fault current arcing at the concentric causing damage to the can itself would explain that reasoning.

My last post was a harmless attempt to take a jab at Pierre C Belarge (I'm pretty confident that he knows the intent of the rule).

But I should of been a little more clear.Bonding is required on both sides!!!!.............The bonding bushing is required on the side where the concentric is encountered (see 250.92)
 

ibew441dc

Senior Member
Another example of the effect would be the introduction of the infamous Kenny Clamp to combat GECs doing arcing where they enter metallic enclosures.

Is this interpretation invalid?

The short answer..........

A metal raceway containing a grounding electrode conductor is required by 250.64(E) to be bonded at both ends .If a GEC is not run in a ferrous metal raceway and terminates to in an enclosure , it is not being used as a raceway, and additional bonding is not required.
 
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George Stolz

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Location
Windsor, CO NEC: 2017
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Service Manager
My last post was a harmless attempt to take a jab at Pierre C Belarge (I'm pretty confident that he knows the intent of the rule).
Yeah, I got it.

Bonding is required on both sides!!!!.............The bonding bushing is required on the side where the concentric is encountered (see 250.92)
Daniel, looking at this the other way,

Electrical continuity at ... equipment, ... raceways, and ... conductor enclosures shall be ensured...
What is the purpose of the bonding bushing? To ensure the bonding of the raceway. If a bonding bushing (or suitable connector) is provided at either end of the raceway, then the raceway is bonded.

The only time I see we must provide bonding on both sides of a raceway is in 250.64(E), which does not apply to this component of the installation.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
OK Here I go getting myself in trouble. The raceway between A and B is not a service raceway (I assume since it goes from 'service' to 'VFD' ) so 250.92 is not directly involved. If a EqGr Cond is employed, and it should be since LFMC is not approved for that current, then the VFD is grounded by the EGC. 250.97 would come into account due to the >250v to ground, but the exception to 250.97 would make it acceptable without bonding bushings since point B has no concentric knock-out.
Am I correct ?
 

George Stolz

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Location
Windsor, CO NEC: 2017
Occupation
Service Manager
The short answer..........

A metal raceway containing a grounding electrode conductor is required by 250.64(E) to be bonded at both ends .If a GEC is not run in a ferrous metal raceway and terminates to in an enclosure , it is not being used as a raceway, and additional bonding is not required.
I'm not quite sure what you're trying to say, but I will clarify on my end.

The only reason I brought up the Kenny Clamp, is that it's principal reason for being created was to counter flashover from an exposed GEC to a metal can, where the exposed GEC would normally just poke through a little hole in the can.

So, the Kenny Clamp is an analogy for what I can envision as to the inspector's reason for wanting to require the bonding bushing at the concentric. Even though the raceway is bonded (at the other end), could there be some potential of arcing at the concentric anyway?

And what is 250.92 asking for? :)
 

George Stolz

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Location
Windsor, CO NEC: 2017
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Service Manager
OK Here I go getting myself in trouble. The raceway between A and B is not a service raceway (I assume since it goes from 'service' to 'VFD' ) so 250.92 is not directly involved.
Correct.

If a EqGr Cond is employed, and it should be since LFMC is not approved for that current, then the VFD is grounded by the EGC. 250.97 would come into account due to the >250v to ground, ...
Correct.

... but the exception to 250.97 would make it acceptable without bonding bushings since point B has no concentric knock-out.
Am I correct ?
I'd say maybe...? :D
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
and why do you say maybe ?
250.97 Bonding for Over 250 Volts.
For circuits of over 250 volts to ground, the electrical continuity of metal raceways and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by one or more of the methods specified for services in 250.92(B), except for (B)(1).
Exception: Where oversized, concentric, or eccentric knockouts are not encountered, or where a box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection, the following methods shall be permitted:
(1) Threadless couplings and connectors for cables with metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate metal conduit, one inside and one outside of boxes and cabinets
(3) Fittings with shoulders that seat firmly against the box or cabinet, such as electrical metallic tubing connectors, flexible metal conduit connectors, and cable connectors, with one locknut on the inside of boxes and cabinets
(4) Listed fittings
 

ibew441dc

Senior Member
You don't know what I mean,How do you think I feel?

You don't know what I mean,How do you think I feel?

George,

My original posts were directly related to your original post..........now I'm confused at our discussion:grin:

I guess to summarize what I was thinking was this.....

Your original post falls under the requirements of 250.97 which since you encountered a concentric ko you had to follow one of the methods in 250.92(B) My statement that a bonding bushing is only required on the side where the concentric was encountered was based on those sections and I did not have the intention of relating it to the requirements for a GEC (the intent and purpose of a GEC and a EGC are separated for good reason) I am not questioning your understanding of the rules......but just want to make it clear that the requirements for the GEC are different because there use is different a GEC is not sized to clear fault current, and the Bonding at both ends is for induced lightning and/or high voltage transients.

Hopefully were not even more confused to the others point:grin:
 

George Stolz

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Location
Windsor, CO NEC: 2017
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Service Manager
Gus: Because I have encountered a concentric at Point A; as long as I am not 100% confident that 250.92(B) says what I think it says, then my answer is "maybe" and my question remains. :)
 

ibew441dc

Senior Member
and why do you say maybe ?
250.97 Bonding for Over 250 Volts.
For circuits of over 250 volts to ground, the electrical continuity of metal raceways and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by one or more of the methods specified for services in 250.92(B), except for (B)(1).
Exception: Where oversized, concentric, or eccentric knockouts are not encountered, or where a box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection, the following methods shall be permitted:
(1) Threadless couplings and connectors for cables with metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate metal conduit, one inside and one outside of boxes and cabinets
(3) Fittings with shoulders that seat firmly against the box or cabinet, such as electrical metallic tubing connectors, flexible metal conduit connectors, and cable connectors, with one locknut on the inside of boxes and cabinets
(4) Listed fittings

There is no maybe............in this application. A bonding bushing is not required where concentric kos are not encountered.
 
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