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We are replacing some older outlets in our home with newer, more modern outlets. In one room, we found that two of the old outlets have two hots: a black and a red wire. The tab has been removed on the hot side to make these "split-tab" outlets.

I'm used to seeing this configuration to make a "half-hot" outlet, where one half is switched with a light switch, etc. But in this case we can't find a switch that controls the red wire.

The red hot wire loses current when we cut power to the breaker that the black wire is on, so I believe this means they are on the same breaker. However opening the breaker box is a bit beyond my comfort zone.

picture of outlet

What else would an hot red wire be for in a split-tab outlet? If we're not using any switch for these outlets, should we just cap the red hot wire off, or should we replace with the same configuration even though we can't verify the source of the red wire?

This is 120v, in the USA.

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    You certainly must know the origin of the red and black hots, since you are switching the relevant circuit breaker(s) off prior to doing the work, right? – Harper - Reinstate Monica Nov 20 at 0:58
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    OK... so yes, I know the origin is the same breaker as the black @Harper because I did cut the breaker. This is likely relevant and I'll add it to the question (because I could look for a red wire matching this in the breaker box, but this exceeds my comfort level) – Josh Nov 20 at 1:00
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    admittedly, MWBCs can be a tough customer. You plug the vacuum cleaner into one socket, and cut breakers til it goes silent... But that may not shut off the other socket. Who thinks to check both? That is why MWBCs are supposed to have handle ties. – Harper - Reinstate Monica Nov 20 at 1:02
  • Can you post photos of the inside of the box in question please? – ThreePhaseEel Nov 20 at 1:15
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    Is the breaker single pole or two pole? If the former, probably they were switched outlets and the switch has been removed at some point (possibly in a buried junction in the wall). If the latter, it's a MWBC. – J... Nov 20 at 12:59
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Fundamentally, the color codes are

  • Ground -- green, yellow/green or bare only and ever
  • Neutral -- white or gray (can be re-marked to be hot if in cable)
  • Hot -- every other color including orange
  • 240V 3-phase wild-leg phase - if exists must be orange.

That is it. That is the whole of NEC color coding.

Therefore, black and red are functionally equivalent. Both are hots, and can be anything.

Sometimes it is are precisely nothing - just another hot. It might be always-hot while black is being used for switched-hot for instance (ugly, but legal).

It might be a former switched-hot that has been deprecated.

It might be a switched-hot actively in use. Red is a preferred wire color for switched-hot.

It could be half a multi-wire branch circuit, as discusse in another answer.

In practice, in cable it will tend to be either an MWBC or a switched hot.

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    Nit: "the color codes in the USA are..." In Europe neutral is blue, and grey is a phase colour - at 240V! ... unless you are dealing with an old installation, in which case neutral is black (UK), or maybe grey (Germany), or maybe blue (Sweden). – Martin Bonner supports Monica Nov 20 at 14:07
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    @MartinBonner this is an answer. Answers, by their very definition, operate in the context and scope of questions. Question states USA. Answer also states NEC. – Harper - Reinstate Monica Nov 20 at 14:13
  • Oops! I missed that the question stated "USA". – Martin Bonner supports Monica Nov 20 at 14:17
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It could be a Multi-Wire Branch Circuit (MWBC). You can test this with a multimeter -- if there's 240V between the red and the black hots, it's an MWBC.

These used to be common for kitchen outlets, since often heat making devices need the whole capacity of a circuit themselves, so this technique allows double the circuit capacity with just one more wire. More recently, it's fallen out of favor, since GFCI and AFCI protection, required by modern code, are more difficult to add to MWBCs.

  • Thanks Nate, this seems likely, and may be more than we want to mess with. It's actually a living room so it's odd that a MWBC would be used here, raising concerns about what other odd things were done with the wiring. We may call a pro to check things out. – Josh Nov 19 at 22:52
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    It may be slightly odd to have it in a living room but it's not illegal or unsafe in any way. If you want to test it, you can do so safely yourself if you have a multimeter -- you don't even need to have it opened up. Just measure the voltage between the hot slots of the top and bottom outlet. – Nate S - Reinstate Monica Nov 19 at 22:56
  • @Josh I've considered doing similar in my own house, in my living room, for electronics. Desktop computer, NAS, printer, router, monitor, etc... one circuit simply isn't sufficient. Computers and electronics usually use a powerstrip w/ surge-protector anyway, so one outlet with two circuits is practically ideal. Note: You can return two separate circuits along one neutral wire if they are on different phases, as the phases partially cancel each other out. – Jamin Grey Nov 21 at 5:45
  • @Josh it's not that unlikely.. a 240V circuit in a living room might have been put there for an A/C unit or a mobile heater.. – WooShell Nov 21 at 7:03
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raising concerns about what other odd things were done with the wiring.

You are right to have such concerns. My experience in rewiring older homes is that everyone who lived there before you was an idiot when it comes to wiring, and did crazy stuff for you to discover.

As others have said, it could be a two-hots-shared-neutral circuit. You should carefully determine if that is the case. If it is, then you should understand the consequences of having such a circuit in your house, particularly the consequences of making changes to your wiring. I say this from experience; every house I have helped someone rewire (including my own) that had such a circuit had it wired in the unsafe configuration. Moreover, upon doing some archaeological analysis, it appears that in each case the circuit was originally wired safely, and then rewired by homeowners into an unsafe configuration later.

The important thing to understand about such a circuit is that you are required to have the two hots on opposite phases in the circuit breaker box. The reason for that is: the current on a circuit is by definition the same at every point; that's what makes it a circuit. If you have two hots sharing a neutral, then the current on the neutral is the sum of the current on the two hots if the hots are on the same phase, and the difference of the currents if they are on different phases.

Consider the consequences of that. If you have two 15 amp breakers and a 10 amp current on both hots, then the current on the neutral is 10 + 10 = 20 if the hots are on the same phase, and 10 - 10 = 0 if on opposite phases. That is, in the unsafe configuration there is a 20 amp current on a neutral wire that is rated for 15 amps, but neutrals are not protected by the breaker, so the breaker will not trip and the neutral will get warm, possibly very warm.

If the person who installed the circuit did so correctly, the two hots will be on opposite phases, and ideally will be forced to be on opposite phases by the use of a double-wide breaker. But if they are on two single breakers, what then happens is that homeowners do not realize that the phase of each is an invariant that must be maintained, and in the course of some change to their wiring, move one of the breakers to the same phase without understanding the consequences and produce an unsafe situation.

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    +1 for noting the importance of a "dual-hot" circuit requiring the two hots to be on opposite phases, i.e. registering 240v between them. That point cannot be emphasized too strongly. I was taught at a very young age that we have an electrical code, and it is there for a very good reason. It ensures that wiring that adheres to code is safe. – dgnuff Nov 21 at 0:24
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Are there any switch boxes in the room? Most rooms require some sort of ability to switch a light or fixture.

If not, it probably is a multi wired branch circuit, two hots sharing a neutral. Check your main panel to see if it's labeled. I'd try to trace the circuit, remove a few outlet covers and determine where the red goes. It's your house, good to know where everything goes...

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    Thanks. Unfortunately the house was built over many years and nothing is labeled :( but now we have a fun project to trace it back and see what it is. MWBC is looking pretty likely. – Josh Nov 19 at 22:49
  • Is there a switch that controls a ceiling light? – JACK Nov 19 at 22:51
  • There is. It does not control this outlet – Josh Nov 19 at 22:52
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    Check to see if the red wire is in there. Maybe the red wire is there in case someone wanted to install a double switch, or maybe the switch originally controlled the outlet and the ceiling light was added. – JACK Nov 19 at 23:55

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