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Old 03-24-2010, 08:22 AM   #1
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sizing subpanels


Our house was wired with a 200a main panel, that contains 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.

Last edited by paredown; 03-24-2010 at 08:25 AM.
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Old 03-24-2010, 08:31 AM   #2
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You mentioned armored cable. Is that what feeds the subs or is armored cable used just for the branch circuits?
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Old 03-24-2010, 09:55 AM   #3
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All original wire is armored cable through the crawl space, including the runs feeding the subpanels. It's the old style BX with the separate flat aluminum grounding "ribbon" that is wrapped back around the spiral of the cable when making connections. (Not sure if the subpanel feeds have this separate aluminum)
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Old 03-24-2010, 11:08 AM   #4
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That "ribbon" is an internal bond for the armor. It is not intended to be terminated to devices or panels. Sounds as if your panels are already conected by the armor. Checks for continuity could affirm this, in which case separation of neutrals and grounding conductors could follow.
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Old 03-24-2010, 03:14 PM   #5
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Understood--I followed what I saw when I have done any work; the previous electrician would pull the "ribbon" clear, & wrap it back around the outside of the armor and trap it in his cable clamps--don't know if this is "belts and suspenders" but I copied what I saw.

As far as I can tell, all of the circuits are grounded correctly--all three prong plugs and all test OK. (Guess that that doesn't tell you much about separation though...) I'm assuming all terminated armor cable in the subpanels are grounded onto the frame of the subpanels.

How would I go about separating?
The subpanels are old Pushmatics so no ground bar, & I don't know if the neutral is even isolated from that
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