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Re: ground wires from different circuits in same box [Re: EV607797] #178118 05/23/08 07:36 PM
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BPHgravity Offline
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The reason the code requires / permits the EGC's of different circuits to be connected together is due to the fact that EGC's serve as the primary effective fualt path for circuits.

With the EGC's of different circuits connectred together at junction boxes and other enclosures, a ground fault on the load side (downstream) of the box or at the utilization equipment will return all paths back to the source. With the fault current splitting among 2 or more EGC's, the total resistance of the effective fault path is reduced thus resulting in less stress on the insulated ungrounded conductor and reducing the trip time of the branch circuit overcurrent device.

The "other" persons argument makes no sense in that a metal junction box may contain several circuits that are brought to the box in metal raceways thus bonding all metal parts and EGC's together through mechanical connection. The same concept need not be changed for non-metallic wiring methods.

Be certain the person is refering to grounding conductors and not grounded conductors. Grounded conductors of separate circuits would NOT be permitted to be connected together.


Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter
2017 / 2014 NEC & Related Books and Study Guides
Re: ground wires from different circuits in same box [Re: BPHgravity] #178132 05/24/08 04:13 AM
Joined: Oct 2000
Posts: 2,721
Scott35 Online Happy
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Standing On My Soapbox!

< SOAPBOX MODE = 1 >
< PATH = SOAPBOX.EXE >
< MODULE: SOAPBOX = ENABLED >

I agree with Bryan (BPHgravity).

If there are Circuits from separate Systems, not bonding all the EGCs (Equipment Grounding Conductors) together + to a Metallic Enclosure / Box would result in an odd Ground Fault path, as described below:

One (1) 480Y/277V 4 Wire Multiwire Circuit (20 Amp) + One (1) 208Y/120V 4 Wire Multiwire Circuit (20 Amp) in same Box.

Branch Circuits run underground via PVC to stubs at finish slab level, EMT terminates to the PVC between the slab and Panelboard or Box, each raceway includes One (1) EGC, Outlet Box is 5s (Metallic).

Ground Fault path for an L-G 277V fault with only the EGC from the 208Y/120V "side" would have to flow back to the SDS Transformer, through the case to the Primary Circuit's EGC (bonded to the case), then back to "Some" Panelboard on the 480Y/277V System side.

Not necessarily the best fault path, and may have issues if the Transformer's Primary Circuit EGC is inadequately bonded, or not bonded at all.

For an opposite Ground fault (L-G 120V Fault, only bond is the EGC for the 480Y/277V "side"), the Fault Current would have to flow back to the 480Y/277 Panelboard, "find" the EGC which is included with the Transformer's Primary Feeder Circuit, which:
* Might be fed from another Panelboard,
* Might be fed from the Service Equipment,
* Might be fed from a Distribution Section ("Switchgear"), that is different than the one feeding the Panelboard, where the Transformer is derived from.
Then, flow through that EGC to the Transformer, through the Transformer case, and to the System Bonding Jumper between the case and "X0" Terminal.

Again, this fault path relies on solid and correct terminations for a low Impedance Circuit.
Being that the total loop length may be very long, there might be enough loop Impedance to keep the fault level down, thereby not tripping the OCPD on the Faulted Circuit.

If this occurs, the Metallic Equipment / Enclosures connected to the fault Path will have > 40 Volts (something between 1 Volt and 120 Volts) Potential to Ground (Earth), and a significant level of Current behind it!

With all the EGCs bonded together, the hazards from the above scenarios are greatly reduced (never say "completely removed"! - Poo-Poo does occur!!!).

Besides, why would anyone NOT want to bond all the EGCs together?

Is there some reason why separate terminations would be preferred (normal conditions only)?

Is there a benefit to separate bonding, or reduction in a Circuit's merit if all EGCs are bonded together (they eventually are bonded at the Transformer)?

Would someone prefer to identify ALL EGCs between separate systems, and apply additional labor for separate makeup?
(Project Manager's worst-case scenario: counter-productive labor!).

As mentioned by another Member, IG Conductors is an exception to this view, as these "Dedicated" EGCs are intended to be "Isolated" from raceways, boxes and enclosures, between the Outlet and the Panelboard of origin.

In this case, there is a merit for the circuit to separate the bonding conductors.

Lastly, Bryan's final statement is something that should be noted.

All GROUNDED CONDUCTORS (AKA "Neutrals") between two different systems (or even 2 different Panelboards of the same system) SHOULD NOT BE TERMINATED TOGETHER!!!

Also, none of them should be bonded to an outlet box, or other enclosure - unless:

* Bonded at the same point where the System is bonded to the Grounding Electrode System + Metallic Enclosures (Service or SDS Transformer / 1st Panelboard if bonded there),

* "Older" Multiwire Circuit, which does not include an EGC, instead the Grounded Conductor is bonded to the load equipment's Metallic Enclosures.

Bonding the Grounded Conductor to Metallic Equipment / Raceways / Outlet Boxes at multiple points results in "Unbalanced Current" flowing on both the Grounded Conductor(s) and the EGC / Metallic items.

This not only creates an L-G shock hazard, but also a Fire Hazard when loose or inadequate Conduit Terminations are encountered.

Leaving the Soapbox!!!

< SOAPBOX MODE = 0 >
< MODULE: SOAPBOX = DISABLED >

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: ground wires from different circuits in same box [Re: Scott35] #178148 05/24/08 11:41 AM
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Posts: 1,335
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sparkyinak Offline
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By bonding all the grounding conductors will only increase the reliabiliy of the grounding system. If the electrical system is properly wired, then it should have no ill-effect on any of the systems. By tying each ground together in a sense gives you a larger ground wire between the connection and the breaker because you are paralleling them and it gives the fault an aternat path incase the shortest path is corrupted or open.


"Live Awesome!" - Kevin Carosa
Re: ground wires from different circuits in same box [Re: sparkyinak] #178151 05/24/08 12:16 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
Let's break this up into separate issues.

If the box contains splices, and the box is metal (where the raceway or cable is not), then the ground needs to be tied to the box. Makes sense; if another splice comes loose, you want it to trip the breaker, not keep the box energized.

If you're using pipe, or AC, then you need not do that, as the raceway is the ground path. In this example, you're NOT tying all the ground paths together in the box.

If you're having another circuit pass through the box, without splices ... I see no need to tie that circuits' ground wire to the box. You've already got that box bonded. Whether you have transformers, generators, multiple service drops, whatever ... the grounds of all the systems are supposed to be tied together at the sources. If they're not ... I'm not sure you want to do that at some remote, unknown box, using tiny wire and a little wire nut.

Now, let's assume that you have two separate systems in the box, both with splices. One word comes to mind: PARTITION. Isn't that box supposed to be partitioned? Even when the one system is 277, and the other 120, it's possible for there to be more than 300v. difference between the two (phase differences). With the box partitioned, I can see where you would make separate bonds to the box.

Why have more than one wire nut tying the ground wires together? Well, wire nuts are limited in their capacity, and it can be a challenge making a good connection of five or six wires. You've got to start thinking about box fill at this point.

Scott's right to emphasize the distinction between groundingING (green) and groundED (white) wires. The white wires ought to be kept separate in the box, joining only at the panel. While the code might allow some wiggle room on this point, you're introducing an opportunity for all manner of bad things to happen.

Re: ground wires from different circuits in same box [Re: renosteinke] #178156 05/24/08 01:57 PM
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sparkyinak Offline
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Originally Posted by renosteinke
If you're having another circuit pass through the box, without splices ... I see no need to tie that circuits' ground wire to the box. You've already got that box bonded. Whether you have transformers, generators, multiple service drops, whatever ... the grounds of all the systems are supposed to be tied together at the sources. If they're not ... I'm not sure you want to do that at some remote, unknown box, using tiny wire and a little wire nut.
Even if there is no splices in the box does not mean the wire will not short out downline from the box. Although the ground from the seperate system may not be comprimised, by tying them all together will only improve the grounding system, not hurt it.

Quote
Now, let's assume that you have two separate systems in the box, both with splices. One word comes to mind: PARTITION. Isn't that box supposed to be partitioned? Even when the one system is 277, and the other 120, it's possible for there to be more than 300v. difference between the two (phase differences). With the box partitioned, I can see where you would make separate bonds to the box.
I do believe that the intent of the partition is to insulate the exposed screws that are in close proximity to one another in the box. depending on the phase configuaration between the 120 and the 277 circuits, there is potential for the voltage to be over 300 volts that could in theory if the conditions are right to allow arcing between the two. It has nothing to do with the grounding.

Quote
Why have more than one wire nut tying the ground wires together? Well, wire nuts are limited in their capacity, and it can be a challenge making a good connection of five or six wires. You've got to start thinking about box fill at this point.
I start thinking about wire fill when laying out the work. I use the 6 P's. Prior planning prevents piss poor performance. typically in new construction, the few wirenuts I have to use there are never more the four wires in them and my boxes only have the bare minimum wire count. Anything else is a waste especically with the cost of copper and plastic.

Quote
The white wires ought to be kept separate in the box, joining only at the panel. While the code might allow some wiggle room on this point, you're introducing an opportunity for all manner of bad things to happen.
The neutrals from seperates services are rarely tided together even at the service. A building that have multi-voltages are typically fed with one voltage at the service. The lower voltages comes from transformers on-site that typically derive their own neutral at the transformer. Granted if the neutral from both the primary and secondary are tied together, then the neutrals is not seperate. I do not ever recall seeing the neutrals tide together both on the primary and secondary on power transformers. I have on buck and boosts, control, and auto transformers but that this is out of scope of the OP.

As for the neutrals on the seperate systems having a little wiggle room in the code, can you clearify what you mean on that?


"Live Awesome!" - Kevin Carosa
Re: ground wires from different circuits in same b [Re: sparkyinak] #178179 05/24/08 09:33 PM
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renosteinke Offline
Cat Servant
Member
The neutrals of different systems HAVE to remain separate. One might argue, however, that the code allows some different ways to run neutrals of the same systems.

While one might be able to find ways to combine neutrals in a junction box, and thus having only one big wire (rather than several small wires) go back to the panel, I advise against this practice. There's just too much potential for a loose wire to cause an over-voltage condition in some of the combined circuits.

Re: ground wires from different circuits in same b [Re: renosteinke] #178181 05/24/08 11:00 PM
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sparkyinak Offline
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I think I understand what you were saying now. Thanx


"Live Awesome!" - Kevin Carosa
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