Installing a 100A sub panel off a 200A main panel

15 years ago

I've been reading the forum for 2 days to find an answer,but this doesn't seem to be covered.

Background info--32'x100' Old Brick Firehouse that had the front 1/3 totally renovated & 200A Sq D panel installed. The panel is about 25' from the front of the building.A sub panel with a 70A breaker(#4 AWG wire) is 28' back into the structure from the 200A panel.

Questions?-- We are converting it to a dwelling by adding 3 BR & 2 baths which leaves a 24x32 shop/garage in the back 1/3 of the structure.All the wiring already installed is in flexable conduit,but the 200A panel is up front & I want to rework the 70A sub panel by installing a 100A panel about 40' back from the 200A main and replace the existing 70A breaker with a 100A to feed the new sub panel.

This will allow me to make short runs to feed the additional rooms and have the panel in the shop/garage where I want to have a 220V welder plug close to the 100A sub panel.I'm trying to avoid making all those homeruns from the front of the building to the back as well as having control over the shop feeds right in the shop area.

The existing 70A sub panel is three #4 separate copper (AWG) wires...Will this feed the 100A sub panel? I also have a good amount of #3 copper of the same type,if that will be better.

Would it be better(correct) to run 4 wires (H-H-N-G) or 3 wires? This is all inside the structure.

My main concern (after correct installation) is the shop area.. I can pull big draw items(HW heater/AC,stove) from the 200A box if the 100A sub can't handle the load since it is lightly loaded at this point...ceiling lights and lots of wall plugs,most on seperate circuits and almost nothing else.

I'm just getting things started on this,but these issues will have to either be figured out or find another way to do this.

I've read enough to know that there are some here that know how this needs to be done right.

Thanks for any help...Dave

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Comments (7)

sierraeast
15 years ago
You might run this by the electrical forum, lots of pros over there.
sierraeast
15 years ago
They have your question over at the electrical forum now, kdc49. I copied it over as a link.
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bill_g_web
15 years ago
I installed a 100amp panel off a 200 amp main a couple years back at my house and wrote up a detailed how-to:

A DIY'ers Sub-panel Install

The guys on the Wiring forum here are excellent and they have an FAQ on Sub panel installation, but it's pretty short. I tried to give more supporting information in my document, though I have the disadvantage of NOT being an electrician.

Bill
sierraeast
15 years ago
That's an excellent how-to, Bill. Im definetely bookmarking it as im no electrician either so it could come in handy someday. We are planning an add-on plus garage, so this will probably play right in to that project. Thanks!
bill_g_web
15 years ago
Thanks, SierraEast;
I realized I better do a quick readover to make sure the document was in good shape and I found a missing angle bracket in the html that caused a paragraph to disappear and I also added another note. So, if you saved the file instead of bookmarking it, you might do another save. The doc's been there a couple of years and I expect to remain for at least a couple more - I don't plan to give up my little DIY site anytime soon, so a bookmark is safe.

Bill
kdc49
Original Author
15 years ago
Thanks to sierraeast for straightening my goof-up out & Bill,that's a pretty good detailed work-up about what I've got planned.

I'm going to study it at length,but you confirmed my basic original thoughts about how to accomplish this. Your charts and guidlines will be very helpful as I fine tune this. The point about installing the panel in the garage/interior wall cavity was a great heads-up....I was going to do just that. If I surface mount the sub panel in the garage on top of the firerock(perhaps in a firerock plenum) then I should be OK ? I had planned to run all the garage wiring in conduit anyway.

I should add the additional rooms(12' firewall between the garage/shop & living area) will be 8' ceiling height in an area that is 12' tall(this leaves a 3&1/2' loft/storage area above those rooms). This will also make those areas easier to wire/insulate/& run HIVAC plumbing.

Thanks very much for the information.... back to the drawing board..LOL...Dave
bill_g_web
15 years ago
Hey kdc49;

You might ask the pros over on the wiring forum for advice on installing the panel in a firewall. As I say in my how-to, you might be able to build a box inside a stud cavity with 5/8" drywall, in which the panel will sit. Maybe your AHJ will want 2 layers of drywall. Also, the whole thing about permitting work is an interesting topic, and again, as I describe in my how-to, your local AHJ can be a great resource and is the ultimate authority/resource interested in your project. It might be a good idea to gather a few options for mounting the panel in/on a firewall and then head down to your AHJ and have a talk.

Have fun;

Bill