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sizing new subpanels

Posted by paredown (My Page) on
Wed, Mar 24, 10 at 6:43

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.


Follow-Up Postings:

RE: sizing new subpanels

If you can run a separate ground wire then you can run a whole new feed. Which is what needs to be done. 2/2/2/4 aluminum cable should be acceptable or 4/4/4/6 copper. Not too bad to pull. As long as one of the 100 amp panels doesn't have way too many loads (like oven, dryer, and A/C) the loading may be fine. Don't get caught up on adding up the breaker amounts, it is based off of calculations. If you can run some of the major loads to the main panel instead (such as A/C, dryer, oven, etc) You may not need 100 amps at your sub panels anyway. Just run 6/3 with ground romex fed with a 60 amp breaker. 2/2/2/4 aluminum is not much more than the copper romex 6/3 however. The 6/3 would just be easier to pull and make the connections.

RE: sizing new subpanels

Thanks for the suggestions. I really hate the idea of stripping out the old feeds and upgrading since it was a tidy job & I love the BX style cable since the rodents can't eat it...

I have been reading another thread regarding sub-panels, & understand that the controlling breaker & wire size must be matched, while the ostensible capacity for the subpanel could be higher.

I've already relocated laundry to the new main; I have the wire for the wall oven below the first sub panel, but I could easily put in a j-box, and continue the run to the main as well.

A/C would likely be at the farthest point from the main panel--a good 140' away so easiest attachment would be into subpanel 2.

So main has/would have:

-220v dryer
-inside/outside lighting (15a)
-inside lighting (office/workroom) (15a)
-office wall plugs (20a)
-GFCI washer / laundry counter 20a
-GFCI-garage 20a
-GFCI-powder room 20a
-GFCI-master bath
-garage (power tools) 220v 15a
?-wall oven 220v 40a

1st sub:
3-lighting circuit (15a)
2-mixed lighting/plugs (15a)
2-plug circuits (20a)
-Furnace 1 110v 15a

2-GFCI kitchen (20a)
1-light/plug circuit kitchen (15a)

2nd sub:

-well pump-220v (20a)
-fridge (15a)
-dishwasher (20a)
-furnace 2 (15a)
2-plugs (20a)
6-light/plugs (15a)

-GFCI kitchen (#3) (20a)
-range hood/microwave (20a)
-GFCI bathroom (20a)
-underfloor heat bathroom (15a)

? A/C system 220v (??a)

The first sub looks like it would likely be fine with a 60a.

Do you think the load on the second sub is high enough to warrant a 100a? Would it take a typical A/C and the rest with a 100a capacity?


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