Bonding the Neutral in an Industrial Control Panel

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Pierre C Belarge

Pierre C Belarge

Senior Member
Location
Westchester County, New York
iwire said:
Picture a large MCC with 30 buckets, each with it's own control transformer.

Each would have be bonded to the EGC, the only difference between this and typical SDSs is the lack of GEC which serves no purpose inside a building.

There can be no 'circulating currents' with only one bond after a transformer.
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So in essence they are bonding control transformers and not grounding them, even though they may be of line voltage?

and
This is because the conductors are not leaving the enclosure in which they originate?
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Pierre C Belarge said:
So in essence they are bonding control transformers and not grounding them, even though they may be of line voltage?

and
This is because the conductors are not leaving the enclosure in which they originate?
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I am not really following you, the transformers are connected to earth via the EGC instead of an EGC and a GEC.

Another example I often see is roof top units equipped with factory installed service receptacles. We will run a 480 volt 3 phase feeder to the unit and internally there will be a 480 x 120 transformer supplying a GFCI receptacle.

One leg of the transformer will be bonded to the EGC and that leg serves as the neutral, those conductors leave the enclosure every time some plugs a cord into the outlet.

Could I do that? No not without a GEC. Can a factory do that? Sure and I bet most homeowner UPS units do that as well. :)
 
H

hardworkingstiff

Senior Member
Location
Wilmington, NC
I don't see that anyone has addressed Pierre's comment.

Control transformer has one side of the output grounded at the transformer, then the grounded conductor is grounded again at the bar (ground bar in the OP) it terminates on.

If the transformer did not have the grounding at the transformer then I think the issue becomes moot, but with 2 points of grounding, I believe Pierre has a valid point that is not being discussed.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
hardworkingstiff said:
I don't see that anyone has addressed Pierre's comment.

Control transformer has one side of the output grounded at the transformer, then the grounded conductor is grounded again at the bar (ground bar in the OP) it terminates on.

If the transformer did not have the grounding at the transformer then I think the issue becomes moot, but with 2 points of grounding, I believe Pierre has a valid point that is not being discussed.
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I don't see it as any different than in a service panel, it is not unusual for the neutral to be bonded or touching the metal enclosure more than once.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
hardworkingstiff said:
Really?

Did it pass inspection? Is it NEC compliant?
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When you bring an SE cable into a service enclosure the bare neutral is likely to be touching the metal enclosure in addition to be bonded to the enclosure.

But again, I don't think the NEC applies to the inside of a listed control panel so it's all a moot point.
 
Pierre C Belarge

Pierre C Belarge

Senior Member
Location
Westchester County, New York
Services and separately derived systems are not treated the same in the NEC.
The NEC permits the bonding of the grounded conductor in more than one location on the line side of the service.
It does not permit the bonding of the grounded conductor in more than one location. There is an exception to the SDS bonding, but it is not nearly as liberal as in services.

I understand the manufacturers do not always follow the same standard as we do, but their work is perused by themselves and the NRTL - hence they can do stuff we in the field cannot.

So, If the field installer is wiring the transformer, I am curious what part of the NEC permits them to do so as the OP stated.
 
M

mdprice55

Member
Location
Tennessee
The 480v comes into the control panel, with a GEC (of Course) that terminates on a ground bar. The neutral coming off of the 480/120 transformer inside the control panel is terminated on this same ground bar. What I am concerned with is the Neutral conductor, would this not allow for current flow on the GEC's upstream?

And yes, we must follow NFPA 70, NFPA 72, and UL508A when building a control panel that will be tied into a premises system.
 
M. D.

M. D.

Senior Member
mdprice55 said:
The 480v comes into the control panel, with a GEC (of Course) that terminates on a ground bar. The neutral coming off of the 480/120 transformer inside the control panel is terminated on this same ground bar. What I am concerned with is the Neutral conductor, would this not allow for current flow on the GEC's upstream?

And yes, we must follow NFPA 70, NFPA 72, and UL508A when building a control panel that will be tied into a premises system.
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I think you mean equipment grounding conductors and ..No ,.. it should not,. it wants to get back home the 120V side,... sort of like when a transfer switch does not switch the neutral the generator neutral is connected to the utility neutral and it is not a problem
 
M

mdprice55

Member
Location
Tennessee
M. D. said:
I think you mean equipment grounding conductors and ..No ,.. it should not,. it wants to get back home the 120V side,... sort of like when a transfer switch does not switch the neutral the generator neutral is connected to the utility neutral and it is not a problem
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I did mean EGC...sorry
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
mdprice55 said:
We are listing it for a UL508A certification. I guess my follow up would be...installed per what instructions? There is nothing in UL508A about this...so if I revert back to NEC, am I ok? QUOTE]

Do I understand correctly that the panel is not listed yet, it is going to be submitted to UL for approval later. You are asking if the internal connections you have discribed are compliant with the NEC or present a possible hazard ?

If the above is true then, the instructions iwire is refering to in this case are or will be written by your company and shipped with the product to your customers.

If I am correct and your employer is designing this product now I think it could stand some improvement. I am with Pierre & you thinking that there is something wrong here. I would like to see it treated ( designed ) the same as if it were under the NEC.
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M

mdprice55

Member
Location
Tennessee
RUWired said:
Attached is a sketch of the proper way in my mind (example 1) to ground the secondary of the control transformer.The bonding jumper can go to the frame or the ground bar.
Rick

View attachment 3957
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That is exactly what I was looking for. Thank you. If possible, could anyone tell me what Art. cites this as a violation?...for reference purposes. By the way RU where did you find this diagram...or did you create it yourself?

Thanks everybody for your wonderful insight on this matter. I am new to this site and am young in the trade. So please forgive me for seeming a bit menial here.
 
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