I am replacing my complete electrical service entrance (weatherhead, meter box, breaker panel in the house). The meter housing I have chosen is a meter/main combo that will have a 200 AMP breaker. So the meter/main combo is essentially my main panel. The neutral and ground will be bonded in this meter/main combo main panel. Then I will run 4 wires (hot, hot, neutral, ground) to the interior breaker panel which will in essesnce be a sub-panel. I will not bond the neutral and ground in this interior sub-panel.
Now I am able to install my entire new service entrance while the old service entrance is still working. Then the power company will come and switch the service drop from the old service entrance to the new. So I won't be out of power very long at all. Now when the service drop is switched, all my house circuits will still be connected to the old fuse box, which is now dead. I would like to then TEMPORARILY make this old box a sub-panel of the my new sub-panel to give me time to move the circuits from the old box to the new when I am able to do so. But the problem is the neutrals and equipment grounds are bonded in this old fusebox and they share the same bus bar. The fuse box is so old, and is rusty inside, so I would very much not like to try to separate the grounds and neutrals unless I really need to do so. Also, this box does not have an earth ground of any sort. I found the wire, but it went to a plastic water pipe. I know the grounds and neutrals should always be separated in a sub-panel. But I don't even know if I can do so due to the age of the box. So with all of that said, what risk is there to making this old fuse box a subpanel for a short time even though the neutrals and grounds are bonded?