Why is there a fourth wire in this outlet?
July 6, 2019 12:42 PM   Subscribe

Please help me figure out this weird home electricity set-up so my house doesn't burn down.

I've checked three other outlets in my house. They all have one white neutral wire, one black hot wire, and one bare ground wire. The top plug in the mystery outlet has not worked since we moved in, so I got a new outlet to replace it. It's the same kind and it looks identical. However, this one specific outlet has a bare wire, a black wire, a white wire...and a red wire. They were all plugged in when I took the old outlet out. I put the new outlet in with the same configuration in the hopes that this won't burn my house down while I figure out what's going on. Why would this one outlet have four wires instead of every other outlet in my house, which only has three?

Here is a picture of the mystery outlet. I apologize for the picture quality since it was taken while the lights were out and the electricity was turned off.
posted by zeusianfog to Home & Garden (14 answers total) 6 users marked this as a favorite
 
The red wire likely goes to a switch somewhere, which is why the top plug doesn't appear to work; it's not switched on.
posted by Aleyn at 12:44 PM on July 6 [2 favorites]


One piece of information I forgot to mention: installing the new outlet worked and now I can actually run electricity through it. No switches were turned in the interim between the non-working outlet and the working one. There's no switch that I can see that would go to the outlet unless there's one hiding in the crawlspace, which I can check.
posted by zeusianfog at 1:00 PM on July 6 [1 favorite]


Receptacles have a link between the hot screws. If it is left in there is power to both receptacles. If the link is broken out the receptacles are separated electrically. This was usually done to supply the tops and bottoms in a room separately, so that when a fuse blows, you don't lose all the power to that room. It was also done to allow the top to be switched and the bottom to stay on.

So, look at the old receptacle to see if the link has been removed. If you didn't remove the link in the new receptacle, this would explain why both top and bottom work.

1. Leave it like it is, it will act like a normal outlet.
2. Cut the link, look for the switch.
posted by H21 at 1:03 PM on July 6 [2 favorites]


Good pictures of the tab that has to be broken.

So, the red wire comes from a switch. If you want the red wire outlet to be operated from the switch you want to break off the tab. That way, the black wire outlet is always on, and the red wire outlet is switched. If you don't care about the switch, you can just disconnect the red wire and leave it hang. If you do this, put a wire nut over the end of the red wire, so it can't short out when the switch is flipped.
posted by Marky at 1:20 PM on July 6 [2 favorites]


1. Leave it like it is, it will act like a normal outlet.

Don’t do this. If you want it to work like a normal unswitched outlet, disconnect the red wire and cap it. (On preview: yep, that.)
posted by jon1270 at 1:23 PM on July 6 [3 favorites]


Yeah just to triple confirm, if you connect the red and black wires and don't remove the knockout between them you will be back-feeding the switch, wherever it is. When you eventually find it and innocently turn it on you will be in the market for a truly impressive short circuit (basically based on a coin toss -- if the other feeder circuit is from the other AC phase you'll have a 240V short, which is quite a bit more memorable than a normal 120V short). Either remove the red wire and cap it or yank out the removable section between them. Or have an electrician trace everything and figure out wtf is going on for sure.

(The presence of a single neutral return implies they're coming in off the same breaker but that assumes an awful lot about the competence of the electrician that set it up. Two breakers feeding a single neutral happens in the wild more than you'd like...)
posted by range at 2:24 PM on July 6 [3 favorites]


Erm, is that all coming out of a random dusty hole in the wall that's just normally hidden behind a plate? I ask because there's supposed to be a plastic or metal box segregating the wires from more flammable things, and it's supposed to extend all the way forward through the plaster/drywall. There are little extender pieces one can very easily add if there's a box an inch or so back there, but if there's no box at all, it's wall-surgery time.
posted by teremala at 2:38 PM on July 6 [1 favorite]


The term you are looking for is "multi-wire branch circuit." They are typically used, as others have said, for switched receptacles and also for circuits controlled by multiple switches.
posted by wierdo at 4:39 PM on July 6 [1 favorite]


Your wires should be coming out a box like this one.

The red wire is either controlled by a switch, or if there is no switch, it is a separate circuit. I have an outlet like this in my garage. The top and bottom outlets of the receptacle are from different circuits and are each the only outlet in the their respective circuits. The purpose is to be able to plug in more than one higher wattage appliance without blowing the circuit.

I only have one neutral in mine as well. Now I’m concerned about that.
posted by GliblyKronor at 5:52 PM on July 6


Range, what happens when two breakers feed into a single neutral?

I have one receptable like that in my house. The top and bottom outlets of the receptacle are on different circuits and are each the only outlet in their respective circuits.
posted by GliblyKronor at 5:58 PM on July 6


The term you are looking for is "multi-wire branch circuit." They are typically used, as others have said, for switched receptacles and also for circuits controlled by multiple switches.

A multi-wire branch circuit is NOT the same thing as a split switched outlet.

What the OP seems to have is a split switched outlet. The outlet is split into two separate pieces by breaking off the tab. One half of the outlet is always active and the other is active only when the wall switch is turned on. Both of these are on the same panel leg and breaker. So if you leave the tab, it doesn't matter if the switch is on or off. Both halves of the outlet are fed by the same leg so there is no "impressive short." You just have two parallel paths to the same outlet.

A multi-wire branch circuit is different. It was commonly used in older homes to save a little bit of copper wire for kitchen outlets. The code requires that kitchens have at least two separate breaker circuits to prevent appliance overloading. This can be done with just three wires to the kitchen instead of four by putting two hot wires on separate panel legs and separate breakers and sharing the same neutral wire. Since the two legs are 180 degrees out of phase, the currents in the two legs cancel out in the neutral wire. Since the currents cancel, the neutral wire can never carry more than a single breaker amperage load and less if the currents balance. The three-wire (multi-wire) runs from the panel to the kitchen. From there, in an outlet box, two separate two-wire circuits split to various outlets in the around the kitchen.
posted by JackFlash at 7:38 PM on July 6 [2 favorites]


Range, what happens when two breakers feed into a single neutral?

I have one receptable like that in my house. The top and bottom outlets of the receptacle are on different circuits and are each the only outlet in their respective circuits.


[note: not Range.]

Having two outlets on the same receptacle fed from different circuits doesn’t necessarily mean you’ve only got one neutral, but if you do, it might be fine or it might not.

If both breakers are on the same phase/panel leg then the single neutral carries the total current for whatever is plugged into both outlets. If the neutral is sized up to handle that combined current then all is fine, but if it’s all wired with 12 or 14-gauge then that single neutral wouldn’t be protected by either breaker and could overheat and start a fire if both outlets are used for high-amperage loads.

If the two circuits are on different phases/panel legs then you’ve got the multi-wire branch circuit that JackFlash just described, and the currents cancel out in the neutral so the wire size isn’t such a problem.

Either way, when top and bottom outlets in a single receptacle are fed by different wires it’s important to break off those tabs to separate them electrically. Otherwise you’re risking a number of dangerous situations, from a 240v short to an outlet being live when you expect it to be switched off.

The thing that makes wiring dangerous to mess with is that it’s very easy to set things up in a way that works fine initially and for a long time afterwards, but creates a major problem sometime later when the planets align in just the wrong way.
posted by jon1270 at 4:59 AM on July 7


I only have one neutral in mine as well. Now I’m concerned about that.

Like JackFlash said this is very normal (for example my entire wood shop is done this way). Assuming it was done properly in the first place and not messed with in a specific way you arrangement is perfectly safe and saves quite a bit of money where numerous circuits are required in a concentrated area.
posted by Mitheral at 9:18 AM on July 7


Yeah, to be clear there are lots of legit ways for two circuits or two branches of the same circuit to feed a single neutral. The issue is when it's a new-to-you house and a "mystery box" situation -- when you're looking to modify what's there you have a certain amount of due diligence required in the form of tracing the existing work, especially because after the fact you have no idea who did the work, and assuming competence can be dangerous.

(In my case this presented as me pulling the neutral out of a breaker panel for a circuit where I had already removed the hot wire, and suddenly all the basement lights (on a different breaker) go out and I'm standing in the dark in front of an open breaker panel. Turned out that somebody, sometime, got mixed up in one of the boxes and ended up merging two circuits onto a single return -- in theory loading a 15A wire with up to 30A, so at least it's good I found it.)
posted by range at 10:51 AM on July 8


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