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Here's the question from kdc49. I advised them to go here for pro advise.

Here is a link that might be useful: question crossed over from remodel forum

Thank You sierraeast for that help... I'm not sure what happened to cause the blackout.

Dave

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.

Whether you can protect the #4 individual conductors (I'm assuming they're THHN) with a 100A breaker is "iffy" because it may depend on local code interpretations. In some jurisdictions you can use #4 for 100 amps for indoor subpanels. In others you'd need to upsize to #3 copper. (The latter would be true where I live, BTW.) Best to check with your AHJ (authority having jurisdiction--the local permit/inspections office) to be sure.

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.

Makes sense to me.

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.

If I had sufficient #3 on hand, that's what I'd use.

(If I'm interpreting your info correctly, you'll be moving the subpanel further to the rear of the building. Thus, I'm also factoring in the advantage of not having to splice the old wires in an junction box located in the middle of the run.

However, I'm not certain I understand your plan. When say "rework", does that mean you'd leave the existing 70A subpanel in place and have it feed a second subpanel? Or do you plan to move it altogether? Just want to be sure...)

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

Yes, it would be better (correct) to use 4 wires with separate neutral and grounding conductors. So called 3-wire feeds are legal only in certain circumstances for detached buildings. But even in that context, they are being phased out by the 2008 National Electric Code.

The grounding conductor can be smaller than the hots and neutral. In this case you should be able to use #8 bare or green, although that's also worth verifying with your AHJ.

Whether you reuse the old subpanel or buy a new one, when wiring a 4-wire feed, you need to fully separate the neutral (grounded) conductor from the grounding conductor. The neutral feed and all branch neutrals should land on the neutral bar. The grounding conductor and all branch equipment grounds should connect to a separate ground bar. (You may have to purchase a ground bar separately--usually under $10.) You also need to remove any bonding connector from the neutral bar to the box and/or...

I intend to eliminate the existing 70A(one 40A breaker) sub panel entirely and I'll replace the existing #4 with #3 copper all the way from the 200A main box. The #4 will probably get used on some smaller draw circuit from the 100A sub panel.

It's my understanding the three seperate #3 wires & say the #8 ground all need to be in conduit from box to box..correct? I've got some large flexable conduit I can use or should the conduit be rigid type? I've been collecting stuff for years when I found a deal,like rolls of flex conduit at 250' for $5 in all sizes..time to use it.

Thanks for the information...I'll have this project under control in no time at this rate.I lean towards over-kill when at all possible so it looks like I'm in the ballpark with my original assumptions,but now with better direction I'm doing the fine tuning of the plan correctly instead of flying by the seat of my pants.

Dave