I have a two-phase 20A breaker in my garage panel with the rest of the house, which is run to my outdoor patio. It's a 10GA 3-wire red/black/white that used to power an old Jenn-Air electric cooktop.

Instead of running anything new - can I just use either the red or black along with the white (neutral) and the bare copper ground with a new run of 14/2 romex to power a standard 110v GFCI? Will the two-phase 20A breaker still trip properly if anything happens on the 110v line? Will a 15A GFCI work fine as it normally does on a single pole 15A breaker?

The 220v 10ga run will never, ever, again be used to power any 220v devices and maybe two GFCI outlets maximum when the new outdoor area is built.

Also, as I understand it, I 'should' break the tab and feed each top/bottom receptacle separately in the duplex unit from each 110v leg, but since this is a GFCI can I just cap off one (either red or black) and feed the GFCI with a single 110v leg for both receptacles in the GFCI outlet?

Yes. Treat it as a multi-wire branch circuit.

I just double checked and it's a 30a dual pole breaker.

I'm assuming 12 or 24 gauge wire wouldn't work here so I need to replace the 30a breaker with a 20a dual pole in order for my plan above to work to code?

Or since I am using 110v on one leg only can I leave in the 30a dual pole and use the smaller Romex still? I know that each side is not 15a but are both 30a 120v legs

Isd is 30 amps on each leg of the breaker.

Make sure your fire insurance is paid up.

I highly doubt you have a two phase panel and instead likely have a split phase panel. There is a difference in the two and you really need to know which it is.

If you are set on using 14-2 romex, you need a 15 amp single pole breaker.

I would use 12-2 romex and a 20 amp single pole breaker.  Just leave the red 10 ga wire safed off in the panel, put a spacer in place of the vacant opening from removing the 2 pole 30.

Alternatively, you could put the old range wire into a very small sub panel on the patio, leave it on the 30 amp 2 pole, and feed your new outlet from there, and have some extra circuits.

Solved.  All good advice, esp. using 12-2 and single pole breaker.  There are other ways, but this is the simplest and best.

Is the 10ga wire too large for a single slot 20a or 15a breaker? Or is it ok to use it kind of like using 12/2 on a 15a breaker.

The 10 Ga wire is great as long as the terminals will fit it, which they should, especially on a 20A breaker.  Check the ratings if it seems too tight.

The amperage issue has to do with the 30A bkr with #14 wire downstream. A 30A bkr will not protect #14 adequately + it is not code-legal to do so. #14 can be on a max 15A ckt while #12 can be on a 20A ckt. The receptacle that you will likely use is also not rated for 30A either...


...As said above you have options:

1) Remove the 2-pole 30A bkr & install a 1-pole 20A and use #12 wire (1-pole 15A and use #14). Regardless, use a receptacle rated for the amperage of the breaker. Blank off the extra bkr space and cap off the red.

2) Keep the 2-pole 30A bkr and install a small load center (sub-panel). This will allow you to install a couple individual bkrs. Your total amperage will still be the 30A feeding that sub-panel.

Yeah I'm trying to avoid turning off the whole main-sub-panel to do a breaker switch but it's looking like that's going to happen.

I can just install two 15A single pole breakers in place of the 30 and feed each of them with the red/black in the current run so I'm just going to do that so I have two 15A going to the outside for compressor-driven items like ice makers and beer fridges.

You can, as in they will fit just fine, but you shouldn't and it's against code unless you install a breaker handle tie. It is a safety issue because with single-pole breakers, you could leave one leg of the multi-wire branch circuit hot and electrocute yourself when working in the box that contains the first splice off of the 10-3 cable.

What do you mean main-sub-panel? Is it the main disconnect panel, or is it a sub-panel?

Why do you need to turn the panel off to swap breakers?

It sounds like you want a multi-wire branch circuit. That is what you're describing by using the red and the black on separate breakers for 120V instead of 240V. In that case, code requires that both breaker simultaneously disconnect, which in almost all cases will be a two-pole breaker.
Here is what I would do:
Replace the 30A two-pole breaker with a 20A two-pole breaker.
Use 12-2 w/ ground to feed the 2 new circuits.

Of course, you could do it with a 15A two-pole breaker and 14-2, but if you can I'd go 20A.

'main-sub-panel' means my 'service panel' is on the side of the house with only a few breakers in it - and that is feeding another large sub panel in my garage that is full up with 15a/20a breakers for the whole home.

Yeah I forgot that two separate breakers will leave one hot wire within the cable if both dont trip or for servicing, oops.

And I would feel more comfortable turning off the main breaker feeding the whole-house main sub panel as described above just in case, it's no big deal, really.