Bonding the Neutral in an Industrial Control Panel

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mdprice55

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I've got an Industrial Control Panel with a 480vac/3 phase/60hz feed coming from a power distribution panel. There is a power transformer inside the panel that drops the voltage to 120vac. The neutral and ground are bonded on the transformer, and the neutral is terminated at the ground bar on the backplate of the panel; which is in turn connected to the incoming grounding conductor. Is this permissable? My interpretation of Art. 250.30 (A) is telling me that it is not, but I need a definitive explanation.
 
I seldom if ever disagree with Pierre, so I must be overlooking something.
For a control panel and control transformer, this seems like a "normal" install. Other than those transformers in 250.30(A)(3) exception, a grounded-grounding bonding jumper is required. This may be a bit of a "shortcut" but I have often seen using one bar as a common termination point in control applications.
I have not questioned it as the cabinets are normally a NRTL listed assembly and outside of the scope of my jurisdiction.

Apparently I am overlooking something. Pierre ???
 
iwire said:
If this is a listed control panel installed per instructions I see no issue at all.
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This is to be a listed Industrial Control Panel per UL508A. This is how we typically wire these panels, but I wanted to make sure that we were not in violation of any codes here. I guess my follow up question would be...what instructions do you refer to? UL? NEC? If so, can you cite.
 
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iwire said:
If this is a listed control panel installed per instructions I see no issue at all.
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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?

When I read Art. 250.30(A) of the NEC it says that "Except as otherwise permitted in this article, a grounded conductor shall not be connected to normally non-current carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the point of grounding of a seperately derived system"

So, do I have to issolate my nuetral on the load side of the transformer?
 
mdprice55 said:
When I read Art. 250.30(A) of the NEC it says that "Except as otherwise permitted in this article, a grounded conductor shall not be connected to normally non-current carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the point of grounding of a seperately derived system"
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250.30(A) is saying no regrounding the neutral conductor past the point where you ground the neutral on the control transformer.

250.20(B) requires the ac system grounding, 250.20(D) refers you to 250.30 if the transformer requires grounding. The system bonding jumper is sized per 250.30(A)1 exception # 3.

A grounding electrode conductor is not required per 250.30(A)3 exception # 3 providing the bonding jumper is connected to the equipment grounding conductor( in so many words or less).

It is quite normal to connect the incoming and outgoing equipment ground conductor to the SBJ.
mdprice55 said:
So, do I have to issolate my nuetral on the load side of the transformer?
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No.
 
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?

When I read Art. 250.30(A) of the NEC it says that "Except as otherwise permitted in this article, a grounded conductor shall not be connected to normally non-current carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the point of grounding of a seperately derived system"

So, do I have to issolate my nuetral on the load side of the transformer?
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The NEC covers premises wiring. If the panel is going to be UL listed, it is not under the purview of the NEC. However, that said, all wiring which enter and leave the manufactured unit are under NEC purview... so the unit must be configured such that the wiring entering and leaving can comply with NEC requirements.

I don't recall the number, but the NFPA has a codified publication which covers wiring of industrial control panels... perhaps someone more familiar remembers the number...
 
The simple way to remember what is allowed or not is to understand the fault current path, if a transformer is isolating then for there to be a fault current path the load side of a transformer has to have a conductor bonded to ground, without it there would not be a fault current path. but once it is bonded you can not bond it again. also in certain systems the bonding may be omitted for the purpose of safety of equipment shutting down all at once.
 
mdprice55 said:
I've got an Industrial Control Panel with a 480vac/3 phase/60hz feed coming from a power distribution panel. There is a power transformer inside the panel that drops the voltage to 120vac. The neutral and ground are bonded on the transformer, and the neutral is terminated at the ground bar on the backplate of the panel; which is in turn connected to the incoming grounding conductor. Is this permissable? My interpretation of Art. 250.30 (A) is telling me that it is not, but I need a definitive explanation.
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Explain to me how this is considered a safe installation? How will there not be circulating currents not introduced to the metallic enclosure?

I am always one for learning something.:cool:
 
mdprice55 said:
I've got an Industrial Control Panel with a 480vac/3 phase/60hz feed coming from a power distribution panel. There is a power transformer inside the panel that drops the voltage to 120vac. The neutral and ground are bonded on the transformer, and the neutral is terminated at the ground bar on the backplate of the panel; which is in turn connected to the incoming grounding conductor. Is this permissable? My interpretation of Art. 250.30 (A) is telling me that it is not, but I need a definitive explanation.
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and in his OP (in red)

Which since it doesn't leave the control panel is ok by the NEC, it just can not be re-grounded once it leaves, this is no different from a service panel cabinet is it?
 
Pierre C Belarge said:
Do you have a code section for this?
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well as close as I can get would be 250.24(A)(4)

what I was thinking is if the bonding point at the transformer to the enclosure is a screw, I would think if there is a equipment grounding bar that is also attached by screws, the above could be interpreted to require a wire to the equipment grounding bar, but in reality this requirement is only for the GEC, not the EGC, but what if this bar was used for the grounded conductors in this control panel? would there be a requirement for the bonding to be a wire and not a screw? I think it would be better to have a wire, I just don't think its a code violation to run an extra wire to the equipment grounding bar because its all in the same enclosure, but that is only my opinion.

I have done this, where the control transformer had the X2 factory bonded to the mounting hardware, and I just ran the X2 to the grounding bar and treated it just like a neutral bar in a service panel.
 
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hurk27 said:
well as close as I can get would be 250.24(A)(4)

what I was thinking is if the bonding point at the transformer to the enclosure is a screw, I would think if there is a equipment grounding bar that is also attached by screws, the above could be interpreted to require a wire to the equipment grounding bar, but in reality this requirement is only for the GEC, not the EGC, but what if this bar was used for the grounded conductors in this control panel? would there be a requirement for the bonding to be a wire and not a screw? I think it would be better to have a wire, I just don't think its a code violation to run an extra wire to the equipment grounding bar because its all in the same enclosure, but that is only my opinion.

I have done this, where the control transformer had the X2 factory bonded to the mounting hardware, and I just ran the X2 to the grounding bar and treated it just like a neutral bar in a service panel.
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That is typically where grounding at the first disconnect or panel is permitted.

The grounding (not bonding) of the transformer is what I am discussing. [480v/120v] To me, this is not a low voltage transformer.


To better understand this, I need some information that I can wrap my gray cells around. ;)
 
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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