sizing new subpanels
We have a house that was wired with a 200a main panel, and from that 2 100a breakers that control their respective subpanels in other parts of the house. Not sure of wire size running between them, but it is older style armored cable.
Service was switched from overhead to underground (? 80s) and new shut-off/main installed on the back of the meter, so old main effectively became a subpanel.
Because of when it was wired (1962-3) the ground and neutral were bonded at each of the 100a subpanels, and are still bonded at the old main (although a separate ground was run back from the old main to the grounding bar of the new main/meter).
My first problem is--how to convert this to a modern 4 wire/ with unbonded subpanels. (This all started when I wanted to hook up my new 220v oven and dryer...)
I have a new panel to replace the old main--since it is going to be moved anyway. Easy enough to unbond here...
As soon as I start looking at the two downstream subpanels, my heart sinks.
Can I run an external supplementary ground wire outside the armored cable from each subpanel to new main subpanel, or should I just replace the works?
(These are extremely long runs through crawl space and would be expensive and hard to replace...)
If they were to be replaced, how does one size a subpanel & wire size appropriately--since the initial set up--200a split between 2 100a subpanels does not seem to match the load currently assigned to each.
I have had a really good electrician look at the current set up but I can't afford to have him do the work, so it is likely to be DIY.
All thoughts and suggestions appreciated.