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I am about to replace the main service breaker panel in my house. The old panel (a 200amp Siemens 40-circuit) had bus bars that extended the entire vertical length of the box. The new panel I would like to install (A Square D QO) has doubled up bus bars that only come down about 30% of the length of the box.

The problem is, this box feeds a 100-amp sub-panel, whose wire comes in from the bottom of the box. For the old install, the large (#2 I believe) ground wire feeding the sub-panel is connected at the very bottom of the bus bar. Unfortunately, in the new panel, it will not reach the shorter bus bars.

Will the code allow me to add a ground bus bar to connect the sub-panel ground to? There are threaded bus bar holes near the bottom of the breaker box that would be within reach of the large ground wire. If so, what do I use to bond the additional ground bus bar to the main ground bus bar?

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Yes, you can add an insulated ground bar in the can (you should be able to get one specifically designed for your particular can, but here's what they look like):

insulated ground / neutral bar

.. and then connect it to the other ground bar using a piece of #4 AWG (solid bare copper, or stranded THHN insulated) assuming you're still at 200 amps. This guarantees that the green bonding screw that's actually bonding your neutral on the other bar will affect both bars if it is removed. It's common to add these for isolated grounds, where they're connected to a different ground altogether instead of the other bar.

Note - the one you buy may come with its own bonding screw, and the appropriate tapped hole to receive it - so check for that prior to connecting the two together (IOW, read the installation notes that came with it).

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Adding onto a bus bar sounds like a really bad idea if you don't have the exact hardware it was designed to be done with. Wouldn't it be easier to replace the wire that feeds the pony panel with a longer run, running the new ground all the way up to the existing bus bar? You'd probably want a piece of 2/3 with an insulated ground.

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    The plan was to add in a Square D QO approved bus bar (there are existing holes at the bottom of the breaker box to add an additional ground bus). In the end, there were other problems with the QO box, so I returned it, and we used a Siemens with a similar bus bar layout to the original box. The problem with simply replacing the sub-panel wire is that the sub panel is a story above, and it would necessitate ripping up drywall in multiple rooms which I wanted to avoid.
    – MarkD
    Mar 25, 2013 at 14:55
  • Well... that makes sense then. Good that you found a simple solution.
    – user12228
    Mar 25, 2013 at 15:22

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