bonding additional grounding buss

[amp-dude]

What do you do to bond a new ground/neutral buss?

Let's say you're adding three new circuits to an old service entrance panel (125A), and the ground/neutral buss bar has only one accessible open space.

You install another terminal bar by using two
8-32 screws (in tapped holes in the panel can). The grounded conductors of three 20A branch circuits and the EGCs terminate on the new buss.

I believe that the two mounting screws are not enough to handle the possible fault current, and you don't want the panel can to be part of the return path anyway (i.e., to serve as the grounded conductor between the new and old buss). So, you connect the new buss to the old one with a bonding jumper.

But what gage? Should the bonding jumper be the same gage as the MBJ (AWG8)? I checked the 2002 Code and Soares, and couldn't find anything that was I think is specific to this situation.

Thanks for any info.

Cliff

 

[websparky]

Re: bonding additional grounding buss

Hi Cliff,

I believe you will find your answer in 250.28 for the Main Bonding Jumper. I will be sized according to 250.66 based on service conductor size.

 

[tonyi]

Re: bonding additional grounding buss

In the situation described, you COULD just splice the 3 gnds together with another short length of #12 (or use a greenie and cut one longer) and just put that under the one available screw and call it a day and return the new bar for a refund

 

[pierre]

Re: bonding additional grounding buss

This is an interesting thread, because of the very nature of installation practices and how common this situation is.
As far as clearing fault current and bonding to the existing bus, it is very important to be properly installed.

I would venture to say (as I have in the past installed in this fashion) that using a selftaping screw into the enclosure is probably the most common type of installation procedure. I wonder now if that is really capable of handling a larger fault current? Any thoughts to this out there?????

Pierre

 

[don_resqcapt19]

Re: bonding additional grounding buss

In my opinion the add on terminal bar that is fastened to the enclosure with machine screws is suitable only for use with equipment grounding conductors and not for use with grounded conductors. Most panel manufacturers have both equipment grounding and grounded conductor treminal bars availabe for this type of applicaiton. If there are EGCs on the existing bar, I'd move them to the new grounding bar and land only the grounded conductors on the existing terminal bar. If there is not enough space for all of the grounded conductors on the existing bar I would add an additional neutral bar and land both the grounded and EGCs on this new bar. An additional neutral bar will require a jumper from the original bar. In my opinion the enclosure itself should not be used to carry neutral current.
Don

 

[rasmithircgov.com]

Re: bonding additional grounding buss

I agree with Don. You could only use the new term. bar for equipment grounds. You could arange the neutrals and equip. grounds putting all equipment grounds on new term bars. That may make space for the neutrals on the factory term. bar. The encloser isn't rated to carry current. 408.21 states all neutrals shall be landed seperately under one lug.Make sure the MBJ is in and as far as I'm concerned you could land many equip. grounds in the encloser.not to sure about slicing the grounded condutors in the encloser. 408.21 Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor. So I would say you couldn't slice them in the encloser

 

[pierre]

Re: bonding additional grounding buss

I agree with Don.
My thoughts are on the EGC bus and the connection and current flow through the enclosure. If the subpanel is supplied using the metallic raceway as the EGC, is it a good enough connection using the screws installed to the enclosure to ensure enough ground fault current to flow, eventually opening the OCPD? Would a conductor run with the circuit conductors, connected to the bus, be that much better?

Pierre

 

[amp-dude]

Re: bonding additional grounding buss

Thank you all for the comments and information.

This confirms my opinion that it's appropriate to connect the two busses with a conductor sized as the MBJ.

If you've seen some of these older service entrance panels (where grounding and grounded conductors are landed on the same termial bar), you'd see that it's not practical to shuffle around the EGCs to the new buss and land all the grounded conductors on the original buss.

Cliff