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Electrician told me a problem I was having was caused by a new arc fault breaker they installed.

I was told you, "can't mix neutrals"

Can you describe what that means? How would these neutrals get mixed?

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    This question is a little unclear as currently phrased. The electrician is the one who told you the new AFCI they installed was mixing neutrals? If not, how are your first sentence and second sentence related?
    – TylerH
    Aug 12 at 14:04
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    If they installed it, and there's a problem with it, then they'll fix it, right?
    – RonJohn
    Aug 13 at 6:21
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The problem

Somewhere in your house (or possibly multiple somewheres) there's a box where this is happening:

Neutrals being combined in a box

There are two cables coming in from the circuit panel. Each cable has a neutral and hot. When the loads were connected to those cables, the neutral from one cable was inadvertently connected to the hot from the other cable (through the load, that is -- if it was directly connected, that'd be a different problem).

Finding it

This is going to be the challenge. One possible method to find the offending box(es):

  1. Turn off all your circuits and open your electric panel.
  2. Turn one circuit on and make sure there's a reasonably steady-state load on it.
  3. Visibly trace where the hot from that breaker leaves then panel, and place a clamp meter around the corresponding neutral. If there's no current on that neutral, then you've found the offending circuit. For an MWBC or two pole circuit, you'll have to check both hot wires against the same neutral; the current on the neutral should be the sum of the current on the two hots.
  4. Make note of which lights/outlets have power, turn the circuit off, then open up those boxes and physically inspect for mismatched neutrals. Run the test again if you find an fix any.
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Neutral is not ground

Ground wires are all connected to each other in a big web. That is fine, since grounds do not flow current except during (momentary) fault conditions.

Many people think "Oh, neutral wires are the same, whatever". No. Neutral is not ground. Neutral is the normal return current path, and it carries the same current as the hot(s).

Neutrals don't have breakers!

So being careless with neutral is a pretty big deal. A crossed neutral could end up carrying 30A of current on a 15A wire. Not good!

The only way to avoid overloading neutral is to make sure it ONLY returns current for its partner hot(s).

Wait, what, hots plural? Yeah, because of how split-phase power works. Like the power to your house - if you have 43A on one hot wire, and 41A on the other hot wire, the neutral is only flowing 2 amps (the difference). Even if one was 100A and the other was 0A, the neutral would still only be 100A. So, the same deal works with multi-wire branch circuits (MWBC) if they are phased correctly.

So neutral must be monogamous

(Or bigamous in the case of a MWBC). The neutral must serve ONLY loads served by its partner hot wire(s).

ONLY.

So far, everything I've talked about is simply required for safety. We haven't gotten into GFCIs yet (Ground Fault Current Interruptors)

GFCIs need neutral monogamy to even work

GFCIs operate by comparing current on the hot(s) to current on the neutral. As long as those currents are equal and opposite, everything is OK. An imbalance of current means current could be leaking out via a third path e.g. by shocking a human!

However, if a promiscuous neutral is taking hot current from another circuit, currents will not be equal and opposite, and the GFCI will trip! So, GFCI devices enforce neutral monogamy - i.e. force you to do what you should've been doing all along.

Wait, aren't we talking about AFCIs? (ARC Fault Current Interruptors)

Most AFCIs use "GFCI style detection"

AFCIs must detect hot-ground and neutral-ground arc faults. As it happens, those are also ground faults. So the cheapest way to detect them is have a weak GFCI inside the AFCI.

Thus, most AFCIs behave like GFCIs when it comes to ground faults and promiscuous neutrals. You can identify AFCIs that work like this because they require you to attach the neutral wire to the AFCI.

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    With Electricity, there is a big difference between "It works" and "It is safe". Mixed neutral "works", but it can kill you and you wouldn't even know what happened. You shut off the breaker thinking it is a monogamous neutral. It isn't and you die.
    – Nelson
    Aug 12 at 7:30
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    Tiny nitpick re: "[...] AFCIs? (ARC Fault Current Interruptors) [...]" -- "Arc" should not be in all caps; it's just a word, not an acronym.
    – TylerH
    Aug 12 at 14:07
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    @Marinaio they found the issue by installing an AFCI breaker. The AFCI made the problem impossible to ignore. If your house is wired using cables (most are), you avoid crossing neutrals by using hot and neutral from the same cable. Any device should only take neutral from a cable where it also takes a hot. Aug 12 at 17:20
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    Nelson - My 1966 house has a "shared neutral", also called a multi-wire branch circuit (MWBC). I wasn't aware of the concept of shared neutrals and got shocked replacing a receptacle. It scared the c**p out of me because the breaker was off. Fortunately the neutral wire only had 50V. It took me several hours of research to learn a shared neutral was why I got shocked by the neutral. They are very common in older homes and I no longer consider them dangerous because I understand I must also turn off the "sister" breaker and know to also always test the neutral wire for current. Aug 12 at 19:23
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    @Julius ...and that's why Code now requires the "sister" breakers be handle-tied. So anyone trying to maintain the circuit will be forced to turn off the whole circuit. I would advise retrofitting handle-ties on all your MWBCs, or it also works to put them on 2-pole breakers. Aug 12 at 19:30
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Mixed neutrals are when the neutral wires on two different circuits are tied together somewhere.

For example, a bedroom can have outlets on one circuit, and ceiling lights on a different circuit. The switch by the door can have a switch for the outlet, and a switch for the light. If the neutrals in that box are connected together, that's a shared neutral.

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    While Harper's excellent answer is 100% correct, this is the answer for the layman.
    – FreeMan
    Aug 12 at 12:11
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AFCI and GFCI breakers to operate they need to have the exact current flowing out the hot and returning on the neutral or in the case of a two pole breaker it needs for the neutral to only carry the imbalance.

Your likely problem is that in some junction box you have two separate circuits, like in a kitchen a box has a receptacle and light circuit. Somebody crossed the neutrals or joined the neutrals together.

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The most common way to get neturals mixed is to connect them all together in an outlet box or something similar. The breaker monitors the current in only one circuit. It measures the current in both the hot and neutral and if there is an imbalance above a given amount it trips. GFCIs also do this. If current from another circuit goes through this it will cause it to unbalance and trip or if current from the circuit is shunted away it will cause an unbalance and cause it to trip. It also monitors the noise on the line and determines if there is an arc on the circuit. This is a complicated process originally done with digital signal processors. Brush type motors (brushes spark) would cause them to trip especially on older designs. For these to operate properly only the circuit being monitored can go through the breaker, any other will cause an imbalance creating immediate and or nuisance tripping.

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