I'm in the middle of a kitchen remodel which originally had a separate cooktop circuit (220V, 40A) and a wall oven circuit(220V,30A) residing in a shared 1" EMT conduit. I'm switching to just a single range (cooktops and oven combined in one unit), so I won't need the old wall oven circuit anymore. However, I will need a new circuit for a built-in microwave I'm installing. So I was thinking, could I use the old wall oven wiring (#10 (2 blacks, 1 white)) for the microwave circuit? I would swap out the existing double pole 30A circuit with a new single pole 20 A breaker, and use just one of the black wires with the white. It seems obvious that this is OK, but just wanted to check for sure. Also, could I make two 120V circuits from that single 220V circuit by splitting that single #10 white into two whites? Basically, the two circuits would share a single neutral back to the breaker box for about 30 feet. I'd like two 20A circuits ideally, but it seems the wire would be undersized. Perhaps one 20A, and one 15A?? The reason I ask about this is that running new wire back to the panel is very nasty (tiny crawlspace), and would have to be fished through an outside wall, so I hesitate to run new cable. Thanks for your time!

 

Yes you can convert the circuit to two 20amp 120v circuits. Sharing a neutral for both circuits is acceptable and commonplace in my region. you need to be sure though that the new breakers are above and below eachother (not directly across).Post back if you need more info.

 

Thanks... that's what I was hoping to hear. But wouldn't the single #10 neutral be too small for two 20A circuits (i.e. 40 A total)? I thought a #10 wire was rated for 30 A circuits, max? Or am I missing something? Just wondering...

The existing 30A double pole breakers are mounted side-by-side, and the breakers flip on and off vertically. I was just going to remove them and replace with two new 20A breakers. Is that OK? I guess I'm not fully clear what you're getting at with your last comment about the breaker locations. Thanks again for your help!!

 

No, a 10 gauge neutral is more then enough for two 20 amp circuts. Changing to two 20 amp breakers is fine, and thats whats needed to be done. Just make sure they are two serperate breakers. Not a 20 amp 220 double pole breaker.
The formula for hot wire and amperage is different from the current carrying loads on neutral wires, relax, you're more then safe.

 

You can share the neutral between two 120V circuits if the two circuits are on opposite phases. Replace the 30A double pole with a 20A double pole and you should be fine. One black to white will be one circuit, the other black to white will be the other.

Shared neutral circuits have certain peculiarities, especially with regard to GFCIs. Search this forum for info, there have been many threads on the subject. Two primary requirements are (1) opposite phases for each leg of the shared netural circuit and (2) the breakers controlling the two sides of the shared neutral circuit should have their handles tied so that when one trips they both trip. By replacing the existing 30A double pole with a 20A double pole you take care of both issues.

Another potential probles is that cheaper outlets don't always accept #10 wire. Resist the temptation to "thin out" the strands in the #10 wire to make it fit - just spring for a more expensive outlet (maybe $2.00 vs. $0.50).

 

This is not true - the ampacity of a hot and a neutral wire is the same. The reason the neutral can be shared is that the return currents from the two circuits which share the neutral are out of phase and will cancel out in the neutral - the neutral only carries the difference in the load currents.

Perhaps you are thinking of the sizing rule for equipment grounding conductors (rather than the neutral aka "grounded conductor")?

 

Ouch! That is what I was thinking. Thanks.

 

I would suggest just the opposite.

If and only if wired to separate legs.

 

I too recommend a double pole breaker.

Using a double pole breaker will have the annoyance that two circuits will be out when (if) the breaker trips, but it will prevent one breaker from being left on when working on any device on either breaker.

 

Wow... great info, guys. It makes a lot of sense. Your info also explains (I think) why the 220V, 40A service to my range has #8 wire for the two hots, and a #10 for the neutral. And I thought the original electricians for the house must have had their head up their %@#, when actually it was me!! Funny, none of my DIY electrical books address this issue.]

BTW, there won't be an issue with #10 wire going to a receptacle. My plan is to make the junction between the two new 20A circuits within the aforementioned range receptacle box, and branch out to the two new circuits from there with 12/2 cable. I presume I won't have trouble with a 20A breaker accepting #10 wire though?

 

Just a post to let you all know I went ahead and converted the old 220V, 30A wall oven circuit into two 120V, 20A cicuits. Everything worked great, and am so relieved I don't have to lay additional cable in my crawlspace or attic. Great trick to remember for the future! Thanks again for all the input!!