2 Pole 20amp Breaker used to power 120v circuit?

Question

In my panel I found a 2 pole 20amp breaker connected to 12/3. When I trace this circuit, it begins to power outlets in my dining room.

At outlet 1, the 12/3 comes in with the Red leg powering the first outlet. The Black leg enters the box, and then leaves that box to the other 4 outlets in the room. All Neutral and Grounds are connected properly.

So here's the question: It appears that this was the start of some kind of a branch circuit that got removed at some point. But since the red leg is powering exactly 1 outlet, and the black legs are powering 4 outlets and nothing else: Would it make sense to not use the red leg, put that outlet on the black leg for a total of 5, then replace the breaker with a single pole 15amp breaker? Wire nut the red in the panel and only connect the black into the 15amp single pole.

Eventually I want to replace that entire wire with 14/2 but I'd have to remove the basement ceiling.

Just seems pointless to power 1 outlet as a dedicated 20amp leg, and then 4 more on a separate leg of the 2 pole breaker. Thoughts?

EDIT: Sorry, the part I didn't mention is that leaving that first box is all 12/2 to the second outlet, and then the other outlets are wired in 14/2. So: 1 outlet on red leg 12/3. 1 outlet on black leg 12/2. 3 outlets on black 14/2 from previous outlet.

My concern with the 'double toaster' theory (which I hadn't considered, thank you!) would be that if you plugged in a heavy load in one of the 3 other outlets, you could cause an issue because 14ga shouldn't have more than 15 amps, but the breaker would be fine with it.

That's my reasoning for removing the wire and dropping down to 15 amp and 14/2. If copper prices come down I could feed that leg in 12/2, but I'll cross that bridge in the future.

Answer 1

What you have there is called a Multi-Wire Branch Circuit or MWBC

It is 1 circuit. However it provides the effect of 2 complete circuits, with only one cable.

There is nothing wrong with an MWBC. They are completely legitimate even though they are "new to you". (though they have fallen out of favor, because they are more difficult to put GFCI and AFCI protection on).

Due to Code requirements, the two breakers are handle-tied, to protect maintainers from only turning off half the circuit and then getting nailed by the other half. Use of a 2-pole 240V breaker is an acceptable way to get a handle-tie; this provides common trip which MWBCs do not need unless they also serve 240V loads. (which they can do!!)

If you are crunched for breaker spaces in your panel, and need to free up a space (a bad situation we don't like to see people in), then yes, you can downgrade it to a simple circuit. There's no need to even re-wire the circuit at all, simply land both the red and black on a 1-pole breaker. (assuming the breaker's labeling authorizes two wires per terminal; if not simply pigtail).

Or, by all means, feel free to upgrade the MWBC to its full glory. Have the red phase power additional outlets, and even attach 240V loads if you desire.

I can see someone having installed a MWBC just like that if that window was a suitable place for an air conditioner. Back in the day when air conditioners had a SEER rating of about 2, it was common to have window air conditioners that either were 240V, or completely maxed out a 120V circuit. That would allow you to go either way: a 240V air conditioner that shared with 120V receptacles, or a 120V air conditioner that used the entire red phase.

Nowadays we see SEER ratings as high as 22, so a lot of BTUs takes only a little power. (SEER = BTU/hr per watt).

Correcting the #14 flaw

You say half your MWBC (the black half) is wired with 12/3 up to the split, and then 14/2 on just the black phase.

MWBCs can do that.

Remember what I said about handle-ties and 2-pole breakers? You can use a 20A breaker for the red, and a 15A breaker for the black, and then use a listed handle-tie to tie the breakers together for common maintenance shutoff.

At that point, you lose common trip which means the circuit can no longer mix 240V and 120V loads.

If you want to do the "downgrade to a simple circuit" strategy, you need to land both hots on a 15A breaker not 20A, because the #14 wire is the lowest common denominator.

It's quite possible the original installer did it correctly, and some goober came along later, saw a 15A breaker on #12 wire and said "that can be 20A" without fully investigating the circuit.

Answer 2

It's a Multi-Wire Branch Circuit or MWBC. Nothing to get excited about. Ripping out the wire is a complete and utter waste of effort.

It's safe as is, leave it alone.

After the post was edited to mention 14Ga wire:

It's safe if you replace the 20A double breaker with a 15A double breaker since there is 14Ga on the circuit. There's no need to remove the 12Ga.