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

13 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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