grounding bushings question

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rhamblin

rhamblin

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Kaukauna, Wisconsin
I've got a few questions regarding what is necessary per the NEC regarding bonding. My recent project was to run power to a new piece of equipment, When I got the conduit to the disconnect I noticed that I would be entering a eccentric knockout. I used a grounding bushing, because I would not be removing to the largest hole. Was this overkill?

When I pulled out the codebook, the only article that mentions eccentric and concentric supplemental bonding is at Services. Does that mean that it isn't necessary feeding into a disconnect? The run feeds off of a 480V Bus, there are no transformers involved. So it isn't a service.

Following on, are we required to use a bonding bushing every time we run a 1/2" conduit through 1/2" eccentriced hole with a 3/4" k.o.? That can't be right...

Next question, if in my situation the eccentric k.o. is not considered a legitimate bonding connection and I don't use a grounding bushing or anything, but do pull a grounding conductor in and terminate it at both ends, would this suffice? Essentially even thought enclosure isn't bonded via the conduit, because of the eccentric ko, does the grounding conductor make up for it?
 
In 250.97 you will find wording that advises voltages over 250 to ground require bonding equal to 250.92 for services.
There is, importantly, an exception.
Answering the 2nd part of your post 1st, most 4" sq boxes are listed for grounding with the concentric and eccentric knockouts so no additional bonding is needed.
You disconnect is lkely not so listed so a bond bushing might be needed.
(continued)
 
In regard to the disconnect, you need to look at the install in detail. Was a wire type equipment grounding conductor ?
If so, and the disconnect is bonded to the EGC and the conduit is properly bonded on the "supply" end, you may not need the bond bushing as both the switch and conduit are bonded.
If the disconnect or conduit depend on the conduit termination the you need the bushing to assure a compliant bond.
 
rhamblin said:
1)When I pulled out the codebook, the only article that mentions eccentric and concentric supplemental bonding is at Services. Does that mean that it isn't necessary feeding into a disconnect? The run feeds off of a 480V Bus, there are no transformers involved. So it isn't a service.

2)Following on, are we required to use a bonding bushing every time we run a 1/2" conduit through 1/2" eccentriced hole with a 3/4" k.o.? That can't be right...

Next question, if in my situation the eccentric k.o. is not considered a legitimate bonding connection and I don't use a grounding bushing or anything, but do pull a grounding conductor in and terminate it at both ends, would this suffice? Essentially even thought enclosure isn't bonded via the conduit, because of the eccentric ko, does the grounding conductor make up for it?
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1) Not sure what you mean by a service but service raceways have different bonding requirements.

2) Most boxes are listed for bonding over 250 volts so nothing other than the standard locknut is required.

3) A disconnect switch with eccentric KO's (probably applies to equipment too) is not listed for bonding for over 250 volts so if you had eccentrics on both ends you would need to bond the raceway with a bonding bushing. The NEC is kind of fuzzy as to whether or not you need bonding bushings on both ends.
 
Here is Carultch's excellent flow chart:


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infinity said:
Here is Carultch's excellent flow chart:


attachment.php
Click to expand...

I think I disagree with the very end of the flow chart. Lets say I have two metal cabinets/boxes/switches, both >250 volts, ringed KO's remaining, not listed for grounding and bonding. Say it is EMT between. I am not buying that I need a BB at only one end. IF that were the case, then a fault on the further cabinet would have to pass through the impaired EMT connection. I could see it differently if there were a Wire EGC bonded to that box with the fault, but I dont see any code language that allows me to use that reasoning.
 
electrofelon said:
I think I disagree with the very end of the flow chart. Lets say I have two metal cabinets/boxes/switches, both >250 volts, ringed KO's remaining, not listed for grounding and bonding. Say it is EMT between. I am not buying that I need a BB at only one end. IF that were the case, then a fault on the further cabinet would have to pass through the impaired EMT connection. I could see it differently if there were a Wire EGC bonded to that box with the fault, but I dont see any code language that allows me to use that reasoning.
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I mentioned that in post #5 but if I'm guessing that the flow chart assumes that there is a wire type EGC in the raceway, if not I agree with you.
 
infinity said:
I mentioned that in post #5 but if I'm guessing that the flow chart assumes that there is a wire type EGC in the raceway, if not I agree with you.
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And what code section would give relief from the requirement if there is a wire EGC? I know intutively it makes sense as the fault would not have to travel over the impaired connection. It gets tricky though because, of course, we often pull the EGC through a pull box without landing it on that box.
 
electrofelon said:
And what code section would give relief from the requirement if there is a wire EGC? I know intutively it makes sense as the fault would not have to travel over the impaired connection. It gets tricky though because, of course, we often pull the EGC through a pull box without landing it on that box.
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I honestly don't know that there is and if an inspector required one on both ends even though I had a wire type EGC, I would be hard-pressed to talk him out of it. Personally I have not required it but if you are trying to maintain redundant grounding for whatever reason one would be required on both ends.
 
electrofelon said:
And what code section would give relief from the requirement if there is a wire EGC? I know intutively it makes sense as the fault would not have to travel over the impaired connection. It gets tricky though because, of course, we often pull the EGC through a pull box without landing it on that box.
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As I stated before the NEC isn't very clear on this.

For example let's say you have a run of raceway between two panels and the run contains an EGC. 1/2 of the run is EMT and the other is PVC. The EMT is only bonded on one end but both panels are bonded by the EGC which is code complaint. Same thing would apply to a run of EMT with a piece of FMC at the end that does not qualify as an EGC.

So why then would a run of EMT require bonding bushings on both ends if the raceway has a wire type EGC?
 
If the enclosures are bonded with EGCs, the concentric KOs are already bypassed, and the bushing is only there to make sure the conduit itself is bonded.
 
LarryFine said:
If the enclosures are bonded with EGCs, the concentric KOs are already bypassed, and the bushing is only there to make sure the conduit itself is bonded.
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Right. I think that we all agree on that, the question is does the metal raceway still require a bonding bushing on both ends if it's already bonded on one end.
 
To me it's similar to have a metallic conduit ground then pulling an undersized wire type equipment ground.
Most inspector won't accept that.
If you have a conduit as an equipment ground regardless of the presence of a wire type, an argument could be made that you need to take whatever steps necessary to maintain the conduit ground.
 
Metallic boxes/enclosures must be bonded.

Metal raceways must be bonded.

Bonding any those items can be established at just one point.

If you have underground PVC but emerge with RMC, do you bond both ends of the RMC? One point is sufficient.
 
I actually see 3 code sections referring to grounding bushing requirements: 250.92(A) (service panel) and 250.97 (> 250V) as mentioned, but also 250.64(E)(1) for GEC in ferrous metal raceways.

Here's a new question related to these sections: If NEC says the raceways shall be electrically continuous and bonded at each end, as in 250.64, or electrical continuity shall be ensured as in 250.97, is it necessary to use one of the methods in 250.92(B)(2-4) between each section of conduit, or just at the ends?
 
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