Multiple MBJs OK?

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suemarkp

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Kent, WA
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Retired Engineer
Since the 2020 code is going to require outside disconnects on dwellings, I want to setup a new main service so it is easy to convert it to a subpanel instead of a main panel (it is being installed under 2017 code so it can still be a Service). I want to add a separate equipment ground bar in the panel and run all equipment grounds to it. Just bolting the bar to the panel is sufficient for equipment grounding with the main bonding jumper connecting the neutral bar to the panel enclosure. But I've never liked relying on screws for equipment ground bars and would like to run a #6 copper jumper from the neutral bar to that equipment grounding bar.

Is that legal? This #6 cu jumper is effectively paralleling the main bonding jumper. Can you have more than one main bonding jumper? Even if it is legal, do you think it is a bad idea? I think the factory one needs to stay since it is a green screw. I know the ground electrode conductor must go the grounded neutral bar and cannot go to the equipment ground bar.
 
Electrically it's all the same. As a matter of conscience, it's all the same, you are relying on a screw to bond the neutral to the case or if the neutrals are isolated you are relying on a screw to bond the EGC buss to the case. Code wise, all you are accomplishing by adding an extra wire jumper is making it legal to land neutrals on either buss bar. I'd have to think a little harder about what size the jumper would need to be.
 
ActionDave said:
Electrically it's all the same. As a matter of conscience, it's all the same, you are relying on a screw to bond the neutral to the case or if the neutrals are isolated you are relying on a screw to bond the EGC buss to the case. Code wise, all you are accomplishing by adding an extra wire jumper is making it legal to land neutrals on either buss bar. I'd have to think a little harder about what size the jumper would need to be.
Click to expand...

I would say it needs to be as large as the little green screw!:)

But seriously, I would say at least the size of the GEC.
 
Didn't think about the size issue, whether to use 250.66 or 250.122. I suppose if the screw is still there, I could size my jumper per 250.122 since that is all an equipment ground bar needs. My main concern was having 2 paths if it was somewhere prohibited. It should be obvious to any electrician seeing a fat green or bare jumper on the neutral when converting to a non-service panel.

I don't want to land neutrals on the ground bar, and I'm not so sure ground bars are rated to be used as a normal current carrying ocnductor. They don't seem as beefy as neutral bars.
 
If you install your new service under 2017 code, and that's what's accepted in your area at the time of the install,there should be no reason you should have to change it in the future.

JAP>
 
You shouldn't be landing any (Neutral or Grounded Conductors) on the equipment ground bar to begin with, and, no, equipment Ground Bars are not intended to be used for normal current carrying conductors.

If it were me, and, I planned on turning this Service Panel into a Subpanel in the future for some reason, I'd leave a little extra slack in my EGC's, go ahead and land them accordingly on the Neutral bar for now in the Service Panel and deal with the whole Subpanel thing later.

You're going to have to wait until the future date to install the 4th wire anyway. I you plan ahead and do it now, you would actually be creating a parallel return path which is a code violation.

Maybe best not to plan too far ahead and just leave it as a Service Panel for now.

JAP>
 
suemarkp said:
Since the 2020 code is going to require outside disconnects on dwellings, I want to setup a new main service so it is easy to convert it to a subpanel instead of a main panel (it is being installed under 2017 code so it can still be a Service). I want to add a separate equipment ground bar in the panel and run all equipment grounds to it. Just bolting the bar to the panel is sufficient for equipment grounding with the main bonding jumper connecting the neutral bar to the panel enclosure. But I've never liked relying on screws for equipment ground bars and would like to run a #6 copper jumper from the neutral bar to that equipment grounding bar.

Is that legal? This #6 cu jumper is effectively paralleling the main bonding jumper. Can you have more than one main bonding jumper? Even if it is legal, do you think it is a bad idea? I think the factory one needs to stay since it is a green screw.
Click to expand...
suemarkp said:
Didn't think about the size issue, whether to use 250.66 or 250.122. I suppose if the screw is still there, I could size my jumper per 250.122 since that is all an equipment ground bar needs. My main concern was having 2 paths if it was somewhere prohibited.
Click to expand...

Technically I think the main bonding jumper size is now based on table 250.102(C)(1) but the size would be the same as 250.66 in your case.

If you wire a bonding jumper from the ground bar to neutral bar you are connecting the screws that attach the ground bar in parallel with the green bonding screw. Then if there's a fault from service conductors to the panel housing (and not necessarily a bolted type fault), then some of the fault current will flow through the ground bar screws. Depending on the magnitude and duration of the fault, that could potentially blow out the ground bar screws whether or not the green screw also blows out. And if your concern about the potential inadequacy of the ground bar screws is valid, this scenario is even more likely.

There will be some current through the neutral bar that would be diverted through the parallel path of the jumper you are proposing. But I think it would be quite small because of the very low resistance across the neutral bar from one tap to another..

So I don't think a jumper from the neutral bar to a standard ground bar is wise. Alternatively, you could use isolated ground bars similar to those at the links below. Of course, with these bars the wire bonding jumper you proposed would be required. Whether this is feasible in your situation is not known. And whether it would be worth the cost and effort is something that you have to decide.

www.homedepot.com

Siemens Insulated Ground Bar Kit 14 Position ECINSGB14 - The Home Depot

Visit The Home Depot to buy 14 Position Insulated GRD Bar Kit ECINSGB14
www.homedepot.com

 
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synchro said:
If you wire a bonding jumper from the ground bar to neutral bar you are connecting the screws that attach the ground bar in parallel with the green bonding screw. Then if there's a fault from service conductors to the panel housing (and not necessarily a bolted type fault), then some of the fault current will flow through the ground bar screws. Depending on the magnitude and duration of the fault, that could potentially blow out the ground bar screws whether or not the green screw also blows out. And if your concern about the potential inadequacy of the ground bar screws is valid, this scenario is even more likely.

There will be some current through the neutral bar that would be diverted through the parallel path of the jumper you are proposing. But I think it would be quite small because of the very low resistance across the neutral bar from one tap to another..

So I don't think a jumper from the neutral bar to a standard ground bar is wise.
Click to expand...

A ground bar listed to be installed in a panel with the associated screws in factory screw holes is sufficient as a ground fault return path.

As stated earlier, the only reason there would be a need to install a wire type jumper from the Neutral to the Ground Bar in addition to that would be if he was going to utilize the newly added ground bar also as a neutral bar since you cannot use the enclosure itself as a return path for a Neutral conductor.

Other than that, In a Service Panel , to add an additional jumper wire from a neutral bar to a ground bar that doesn't require a jumper in the first place, to me, is nothing more than assured bonding.


JAP>
 
synchro said:
.....Depending on the magnitude and duration of the fault, that could potentially blow out the ground bar screws whether or not the green screw also blows out. And if your concern about the potential inadequacy of the ground bar screws is valid, this scenario is even more likely...
Click to expand...
Pretty sure the folks that manufacture these things for a living have that under control.
 
My concern is more with the panel MBJ. There is one 10-32 screw bonding the neutral bar to the chassis. The ground bars have two 10-32 screws, so they are more assuredly bonded than the MBJ. The older Eaton panels had a ground bar and a neutral bar, and the MBJ tied them and the panel chassis together via a bus bar. That made things easier if you chose to make the panel non-service and put all the grounds on the correct bar, and I had confidence in that MBJ's connection since it landed on the connecting bar and screwed into the chassis.

This newer Eaton panel has 2 neutral bars with a solid connection between them (insulated from the panel chassis). The MBJ connects one of the bars to the panel chassis. The two bars cannot be separated, so converting to a non-service panel requires adding the ground bars and moving grounding wires around. On the left side side, the ground and neutral bar are quite a bit apart, so wire lengths would be an issue. On the right side, I think the ground bar is in a bad location and may need to use the MBJ hole for one of its two screws. So I can't install that one at this time (but it really isn't needed).

Maybe I should just leave it be and let the next person deal with it if it ever needs to change... And maybe leave the equipment grounds on the left side long enough to reach the bar should it need to ever move there.
 
Don't tell anybody but I've seperated the 2 neutral bars in an Eaton panel to do exactly your describing by unbolting the main and removing it. I'll probably burn in you know where for doing it. :)

I thought the inboard neutrals on the new BR load centers would be the ticket but they are not set up to remove the jumper and turn it into a sub panel by doing that either.

Siemens seemed to be the easiest panel to do what your describing by removing the jumper between the 2 bars.
They seemed to have figured it out first.

The other manufacturers wil catch on before long.

Jap>
 
Little Bill said:
I would say it needs to be as large as the little green screw!:)

But seriously, I would say at least the size of the GEC.
Click to expand...
or SSBJ, but I agree - not per 250.122.

suemarkp said:
Didn't think about the size issue, whether to use 250.66 or 250.122. I suppose if the screw is still there, I could size my jumper per 250.122 since that is all an equipment ground bar needs. My main concern was having 2 paths if it was somewhere prohibited. It should be obvious to any electrician seeing a fat green or bare jumper on the neutral when converting to a non-service panel.

I don't want to land neutrals on the ground bar, and I'm not so sure ground bars are rated to be used as a normal current carrying ocnductor. They don't seem as beefy as neutral bars.
Click to expand...

Square D QO loadcenters they are often the exact same cross section, just vary in length depending on how many terminals they have. Lot of NF and NQ and even I-line panelboards use same bar stock for at least portions of the neutral assembly as they use for EGC bars for small conductors.
 
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