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I've come across some existing wiring in my home and I wonder if it needs to be changed.

In my basement, there is a 4" box with a 20a GFCI receptacle. A conduit runs into this box. The conduit contains 2 #12 wires (hot & neutral) for circuit 3, which is a 20a circuit. The receptacle is connected to this circuit.

Also running in the same conduit is 1 #14 wire (hot) for circuit 4, a 15a circuit. This circuit is just passing through and exits the box through another conduit. However, this circuit's #14 neutral comes back into the box through this conduit and connects to circuit 3's 20a neutral. Specifically, circuit 4's neutral, circuit 3's neutral, and a pigtail #12 are wire nutted together, with the pigtail connecting to the GFCI.

So 15a circuit 4 is sharing 20a circuit 3's neutral. Is this a problem and should a new #14 neutral for circuit 4 be run from the panel into the box in the incoming conduit, and connected to the circuit 4 neutral which runs back into the box from the second conduit?

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That's fine, but you'll need to apply a factory handle-tie to those two breakers. When someone shuts them off, they must shut off together for safety reasons.

The handle-tie also assures they are phased correctly (on opposite phases), which they must be to avoid overloading neutral.

This type of arrangement is called a multi-wire branch circuit or MWBC. There's nothing wrong with it, and nothing wrong with asymmetrical ampacity as long as the neutral is sized for the higher ampacity. MWBCs have fallen out of favor because of the rise in AFCI and GFCI applications, which they don't always play well with.

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  • I know what you mean by breakers that are tied together, I've seen that before. But how do you tell which phase (leg?) a breaker is on? Or, is it such that because the handle-tie connects 2 breakers that are side-by-side, it is guaranteed that any breaker above or below another will be on opposite phases? That is, top to bottom in the panel, every other breaker is on the same phase?
    – Darryl
    May 8, 2020 at 3:57
  • @Darryl Not alternating breakers. Alternating spaces. It's possible to get duplex/twin double-stuff breakers that put 2 breakers in 1 space, and things get confusing fast, and that's where people blow it with MWBCs. If you're using a factory-supplied handle tie by the maker of the breaker, it will generally force the breakers into the proper position so the two tied breakers are in alternating spaces which means alternating phases. Beyond that you don't really care which phase is which. Though it's obvious if you look at the bus stabs.
    – Harper - Reinstate Monica
    May 8, 2020 at 5:26
  • Aha got it, thanks. In my case, the two breakers in question (circuit 3 & 4) are unfortunately from differing manufacturers and the switches (handles) don't align and are of differing form factors as well. So I will need to replace both with breakers of the same brand and get that manufacturer's handle-tie as well.
    – Darryl
    May 8, 2020 at 6:22
  • @Darryl Holy smoke, yeah, get rid of all those alien breakers! All breakers in your panel must be either from the panel's maker or successor... or UL-Classified for your panel (which basically means Eaton CL). CL is not BR; BR go in nothing but BRyant->Cutler->Eaton panels.
    – Harper - Reinstate Monica
    May 8, 2020 at 6:29
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To add onto Harper’s answer: the only reason this would be safe and legal is if these two circuits are on opposite legs and the breakers are handle tied.

If these are on the same leg, the neutral is overloaded (potentially 35 amps on a 20 amp rated wire). This is very bad!

Also, do not tie in the neutrals for other circuits. This would result in the total current being shared between multiple wires. If one of these connections were to fail, instead of one circuit not working, the shared current could again overload the remaining neutral(s).

Finally, if you are using GFCI breakers, you can never interconnect neutrals or the breakers will trip. To share a neutral on opposite leg circuits (Multi-Wire Branch Circuit), you must use a 240 volt (dual) GFCI breaker.

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