Is it safe to have two circuits passing through one junction box?

Question

So I found the following circuit diagram in my home. After receiving a fun little shock, I found that someone was using 14/3 cabling to carry 2 different circuits to this 1 gang box (line on left, load on right). Since the load for Circuit 2 doesn't have a matching neutral, it's connected to Circuit 1's neutral line.

And before you ask, Circuit 1 and 2 are not MWBC circuits. Each one can be turned on/off independently. I'm presuming that this only works because the hot wires are on different phases, but this level of electrical work is a little over my head.

1. This doesn't seem to be very safe, though it hasn't caused a fire for (presumably) a number of years.

2. How do I go about fixing this so that it is safe?

I realize this can be and maybe should be fixed using a double pole breaker, which is certainly possible given they are neighbors in the breaker box, however I would like to keep them separate so I don't need to shut off power to half the house to work on an outlet (e.g. the circuits in the house are already on the large side of what I'd like).

Answer 1

As an answer to the last part of your updated question: If you want two separate breakers instead of a double-pole breaker, you will need to rewire from the panel to the point at which the circuits currently diverge from the 14/3 wiring so that there are truly two separate circuits without a shared neutral.

I would recommend an electrician for this type of work unless you are prepared to invest some serious time learning the NEC and acquiring the necessary permits.

Answer 2

That's what's known as a Multi-wire Branch Circuit, and it's perfectly acceptable if done properly. Though it sounds like yours is not.

First, the ungrounded (hot) conductors must be on separate legs of the service. Secondly, the breaker(s) protecting the circuit must have the handles tied together, or a double pole breaker should be used.

To fix the problem, install an appropriately sized double pole breaker. The red wire will connect to one pole, and the black to the other. That way both circuits will always be turned on/off at the same time.

Answer 3

That is precisely a Multi-Wire Branch Circuit (or MWBC). Those are defined as exactly what you drew: 2 hots that share a neutral up to a point.

Your setup is fine, but with one problem. It's not the breakers.

When you removed that outlet, you interrupted the neutral for the other half of the circuit. Around 2002, they changed Code to prohibit MWBC neutrals from going through any device. You must "pigtail" neutrals - bring them all together in a splice, and add a short wire to the outlet. That way you can remove the outlet without breaking the neutral. This is important - Do that ASAP. Here's the right way:

You only need to pigtail the neutral. (and ground of course.)

The breakers are fine if your work is pre-2008. In 2008, they realized electricians were not doing the pigtailing properly, so they added another rule - that there must be a manual disconnect switch, and it must shut off both legs of the MWBC together. Typically you use an approved "handle tie" - a $2 piece of plastic or metal that fits between two breakers. A 2-pole breaker is an expensive way to do that same thing. This rule does not apply to you if your work is prior to your state adopting 2008 NEC.

It is vital that these two "hots" absolutely must be on opposite poles. That means the 2 breakers occupy the same 2 spaces a 2-pole breaker would. An approved handle-tie (for that breaker type) will enforce that for you. Properly set up, they only carry the difference in current flows, which is quite efficient use of copper!

Also - next time you work on a circuit, make sure it isn't also a MWBC. This guy loved them, clearly, but he didn't love pigtailing.