Multiple main bonding jumper?

akheilman said:

Imagine a house with a properly installed main panel. I have to add a sub panel because the main panel is full. If all the ground bars were full would it be code compliant to add a double lug to the panel case and land both the ground and neutral of the sub panel to that lug? Would that cause the neutrals and grounds of the sub to be joined at both the double pig and the original main bonding jumper?

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I'd never do it this way! What happens if the screw holding your double lug comes loose? You don't want to go there. Install a separate ground bar in the main panel. Bond it to the existing ground bar with a piece of #8. Move some of the existing grounding conductors from the original ground bar to the new one, freeing up space on the existing one. Install your neutral and your grounding conductors feeding the new sub-panel to the existing ground bar. Clear?

akheilman said:

Clear. Good point. Thank you, I'll do as you said. So technically it's not illegal then?

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I usually install separate ground bars in a main panel and land the grounding conductors on them. Even though the mounting screw bonds them to the existing ones through the panel frame, I still install jumpers. Just my practice. I had a local tell me that the mounting screw was no longer sufficient. Maybe someone has a code or maybe it's a local thing.

pharon said:

the sub-panel should have separate neutral and ground buses that do not connect. The neutral and ground wires in the sub-panel feeder should go back separately to the main panel and get connected into those neutral and ground buses, respectively. See nec 250.30(a).

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250.32 (b) (1) is another one.