Old house, what is going on in this junction box?

[WallyWest]

I need you to separate the black wire nut and the red so I can see exactly what is going on

Ok, hopefully this works.

Additional info. The lights in the basement are switched up at the top of the stairs. Two of those light sockets are the kind that have a single outlet on the side of the socket. I know you can wire that so the outlet is still live even when the lights are switched off. Mine are not like that, with the lights off the outlets are dead too. And I checked those outlets with a plug in circuit tester, they both were fine. Had ground, wired correctly, at the outlet anyway.

 

[Megawatt]

Ok, hopefully this works.

Additional info. The lights in the basement are switched up at the top of the stairs. Two of those light sockets are the kind that have a single outlet on the side of the socket. I know you can wire that so the outlet is still live even when the lights are switched off. Mine are not like that, with the lights off the outlets are dead too. And I checked those outlets with a plug in circuit tester, they both were fine. Had ground, wired correctly, at the outlet anyway.

It appears that the red wire is being used as a switch leg

Ok, hopefully this works.

Additional info. The lights in the basement are switched up at the top of the stairs. Two of those light sockets are the kind that have a single outlet on the side of the socket. I know you can wire that so the outlet is still live even when the lights are switched off. Mine are not like that, with the lights off the outlets are dead too. And I checked those outlets with a plug in circuit tester, they both were fine. Had ground, wired correctly, at the outlet anyway.

That is a code violation for sure, as you have stated that the 2 top cables are coming from your panel on separate breakers which if there different phases like 240 vac measured between the 2 live wires really could cause a catastrophic event. I can’t wrap my head around that there are 2 separate live circuits coming in the box. It also appears that the top right cable is being used only as an extra live wire only. You say your a decent electrician so if I was you I would get rid of the cable on the top right because it don’t matter what you are trying to achieve you don’t need but 1 live wire in that box. Like said above you need to take the light or lights apart and figure out the purpose of the red wire and black wire on the left side cable. As stated above it is a violation to share neutrals from 2 separate circuits. It is about impossible for us to be able to tell you what’s going on in the box without taking the first device loose to investigate because it seems the cable on the left is the problem. Get rid of that top right cable

 

[Megawatt]

It appears that the red wire is being used as a switch leg
That is a code violation for sure, as you have stated that the 2 top cables are coming from your panel on separate breakers which if there different phases like 240 vac measured between the 2 live wires really could cause a catastrophic event. I can’t wrap my head around that there are 2 separate live circuits coming in the box. It also appears that the top right cable is being used only as an extra live wire only. You say your a decent electrician so if I was you I would get rid of the cable on the top right because it don’t matter what you are trying to achieve you don’t need but 1 live wire in that box. Like said above you need to take the light or lights apart and figure out the purpose of the red wire and black wire on the left side cable. As stated above it is a violation to share neutrals from 2 separate circuits. It is about impossible for us to be able to tell you what’s going on in the box without taking the first device loose to investigate because it seems the cable on the left is the problem. Get rid of that top right cable

Just replace the old black cable and if your not having problems with the other wiring don’t worry about it. Caution since the neutral obviously shares 2 separate circuits and you turn off just the breaker for the outlet and take the neutrals loose it will spark since the other circuit will still be live. Hell just cut both breakers off and good luck

 

[Lucien Nunes]

Hey MW have a look at my attempt at unravelling it in post #7. I do not see any evidence that the neutral is being shared, because the switched hot (from the 'other' breaker) flies through this J-Box without picking up the neutral, so the light nearer the panel (cable top right) has its own neutral back to the panel. But the light between this box and the switch might be using the wrong neutral, we don't know yet.

If I am correct, can you say whether it is acceptable to use the white and black wires of the left cable as hot and neutral of one circuit (outlet) but the red as the switched hot of the other circuit (lights), provided the two circuits use the proper neutrals?

 

[Megawatt]

Hey MW have a look at my attempt at unravelling it in post #7. I do not see any evidence that the neutral is being shared, because the switched hot (from the 'other' breaker) flies through this J-Box without picking up the neutral, so the light nearer the panel (cable top right) has its own neutral back to the panel. But the light between this box and the switch might be using the wrong neutral, we don't know yet.

If I am correct, can you say whether it is acceptable to use the white and black wires of the left cable as hot and neutral of one circuit (outlet) but the red as the switched hot of the other circuit (lights), provided the two circuits use the proper neutrals?

@Lucien I think that would be bad practice and help me understand that what you are saying the cable on the left is being used for one 120vac circuit and the red is carrying the other live on the top right cable which just has the neutral just floating and not hooked up

 

[Megawatt]

@Lucien I think that would be bad practice and help me understand that what you are saying the cable on the left is being used for one 120vac circuit and the red is carrying the other live on the top right cable which just has the neutral just floating and not hooked up

@Lucien I cannot go back to post # 7 it stops at #13

 

[Mike Johnson]

Page 1 should be available at the top or bottom of the thread.

 

[Megawatt]

Page 1 should be available at the top or bottom of the thread.

Thanks my friend for the info ?

 

[Megawatt]

The hot and neutral of your outlet circuit are both present in the cable to the left (black and white) along with the red switched hot returning to the light nearer the panel via the 3rd wirenut. But the switch is not taking its power from that black hot, because when you kill that breaker, the lights stay on. Another hot from the second breaker must arrive at the switch via a different route to power the lights.

If you ignore the red switched hot passing through the J-box to the light, everything seems normal, it's just power arriving straight from the panel and splitting to your outlet and the cable to the left. The light has its own neutral back to the panel, it's not sharing the neutral in this box as you point out the white wire is not connected. So far, there is no evidence that the two circuits are cross-connected in a way that would violate code. The pecularity is that the 4-wire cable contains hot and neutral of the outlet circuit heading away from the box, and a switched hot for the light circuit heading towards it. I am not sure if that is code-compliant if they are not part of one multi-wire branch circuit.

What would be interesting though, would be to look in the light between this box and the switch, and find out where it gets its neutral from. If it uses the neutral in this cable (which belongs to the outlet circuit) it's a cross-connection (aka borrowed neutral) and probably a violation. If a neutral belonging to the light circuit arrives from the direction of the switch or panel, again it could be OK.

Some pics of the light and switch would be revealing.

I have to agree with your post #7 and yes it is a violation of the NEC. The person that wired this was smoking some good stuff LOL. I’m like you I would like to see pictures of the OP’s light and or outlet

 

[WallyWest]

I think you guys are on the right track.

My confusion now is this. If we assume the light circuit is wired at least sort of ok, and the red wire is a switch leg for said lights, then what is the black/white pair going out the left of the box for? As far as I can tell it doesn't power anything. It certainly doesn't power the lights, because I can shut power to those wires off and the lights stay on.

I'm going to pull the light socket on the other end of that left cable and see what is going on there.