How would you fix this bad circuit? 14 gauge on a 20 amp

Question

I've got an old house with issues. This one is a circuit that starts at two 20A breakers, then maybe 60 feet of shared neutral 12/3, but ends with two 14/2 branches. The actual demand on the circuits is pretty low (just basic bedroom, but who knows what someone will do in the future). The breakers are not connected. My question is, would you suggest:

1. downsize the breakers to 15A and replace the long run of (shared) 12/3
2. downsize the breakers to 15A and put in two long runs of 12/2
3. keep the upstream and replace all the 14/2 and outlets

This is USA.

Diagram:

Answer 1

What you have is a multiwire branch circuit. Two breakers, hopefully handle tied, on different legs and sharing a neutral. They are very common. The change in wire sizes from #12 to #14 is not good since the breakers are 20 amp. To correct this, just replace the two 20 amp breakers with two 15 amp breakers and handle tie them. You could also use a double pole breaker but they will be more expensive than two single's. You need to make sure the breakers are on different legs in the panel. It would be a good idea to leave a note in the breaker panel as to why there are 15 amp breakers on the #12 AMC wire, because of the change to #14 AWC.

Answer 2

So it's an MWBC (Multiwire branch circuit) which then splits into two branches.

Replace 20A breakers with one double-pole 15A breaker (not strictly required, but a minimum of a handle tie for MWBC breakers is required by current code. Common sense suggests doing so even if grandfathered on older code) is the quick and easy solution since no replacing wires is required.

If you want 20A you need to replace all the 14AWG with 12 AWG, but you say the loads are low. And replacing wiring is generally a huge pain.

There's no problem at all with having 15A breakers on 12, 10, 8 gauge copper wire. Around 6 gauge there would be a slight problem (well, inconvenience) on my panel of needing an 8 gauge pigtail to the breaker, as that's the largest wire size my 15A breakers are listed to take in their clamps. There is a problem with having 14AWG wiring anywhere on a 20A circuit (other than the special case of fixture wiring, and that's between the fixture manufacturer and UL - any wiring installed as wiring in the house needs to be 12 AWG copper or larger if protected by a 20A breaker.)

Answer 3

Another option is to make the split point a subpanel with 2 15A breakers.

Then the configuration is that you have a 240V 20A circuit feeding a subpanel which in turn feeds 2 120V 15A circuits down to the outlets.

That way every wire is properly protected and you don't have to replace any run of wire.

Answer 4

I would argue that it depends which version of the code is enforced or desired for compliance. Since this circuit is not up to code now, you could do the "right" thing and invest in a 2-pole 15 Amp AFCI breaker to protect the bedrooms. That wasn't required when "old" houses were built, but it has been code since 2002. Some local authorities have made exceptions to that rule though.

This also assumes the system isn't so old that it can't accommodate AFCI breakers.

Answer 5

I am going to disagree with the accepted answer for 1 reason. Future work.

Your house has wires of different capacity that feed into eachother. This isn't a bad thing if you have a breaker that is fitted to your lowest capacity wire (the 14 gauge). But as you have noticed this isn't always the case.

It is possible that some diyer down the road will look at your 15 amp circuit breaker that is feeding a 12 gauge wire ask "Why did they do that?", and swap it out for a 20 amp circuit breaker assuming that all the wiring from that point is the same size. And then burn the house down. (I am willing to bet this is why your 14 gauge MWB is fed by a 20 amp circuit breaker).

They shouldn't do it that way and should inspect the branch to see what size of wires they are dealing with, but people are lazy, and that takes a lot of work.