*note: I am fairly experienced at remodeling level electrical work.
So I am replacing a 1970s era 2 prong wall receptacle (no ground). First thing I notice is that the top and bottom outlets seem to be on different circuits (positions next to one another on the 110V panel) or so my potential detector tells me- if I kill both circuits all wires appear to have no potential. The connection tabs between top and bottom are broken on both sides of the receptacle.
There are four wires connected- both left ones are white, and the right top is black and the bottom right is red. Inside the junction box is just a mess of wires.
If I measure between the various possibilities, I get:
(U)pper (L)eft to U ®ight = 240 V!
(B)ottom L to BR = 120 V
UR to BR = 240 V
BL to UR = 240 V
Okay, clearly something is really messed up here- is it possible that somehow the previous owner tapped into one of the “other” lines? The drier is only a few feet away on the other side of the wall. There is no way possible to read 240V correct? Or is this a bad sign of horrible grounding?
Is it an immediate electrician call? (I am worried as I have already replaced a three-way switch which wasn’t working properly and found a “jumper” line that connected the two poles to override the switch so I wouldn’t put it past the previous owner to have done something silly.)
Other concerning tidbit, I leaving the wires free and with the lines powered on, my TV on the next outlet tries to power on but fails.
I’ve seen 240 V in the same sort of socket that I plug my normal 120 V stuff into. Seems like it’s asking for trouble. I’ve seen different sorts of plugs coming out of appliances that require 240 V, but never a plug that looks like 120 V.
Provided you wire the outlet correctly(as it was) it’s a non-issue. It was wired so the outlets duplex has 2 different circuits.
I wouldn’t wire a box like that today but it was done in the past.
If you want to make the box ‘safer’ you could locate the breaker one of your colored wires is on and move it up or down one slot. That would put both circuits on the same potential so measuring between the 2 coloreds you’d get 0 and measuring between each colored and ground you’d get 120. ie no potential for 240 in the box.
that situation is unsafe and against the current electrical code.
both legs would give 240V between them (could be coded B and R)
the receptacle that powers your tv seems to be fed by one of these legs.
it may have been wired like that to provide two circuits worth of current at one point. you could wire only one leg to the receptacle and cap the other leg or wire it to pass through.
if you interconnect things like audio visual equipment then it should either be on grounded circuits or at least all the devices on the leg if not the same circuit.
You’ve said that in a previous post as well concerning a similar situation. Do you happen to know where in the NEC it covers this. To my knowledge 2 separate circuits in a residential box is a legal configuration.
Wired correctly it is safe. Someone opening said boxes without a good understanding of electricity is what creates a safety issue.
Does it even make sense what I am observing though? Only one ground on the four wires?
To make even more of a puzzle, with power restored but no receptacle connected (only 4 bare wires). I measured the TV’s outlets. One is 0V across the outlet and the other is 240V. Conveniently, I fried the Wii power supply and the Tivo.
I think I have some phone calls to the inspector, the realtor, and the insurance company.
Your typical residential service these days is a single split phase. You have 240 volts from one line to the other, and 120 volts from either line to neutral. If you need 240 volts (like for an oven or a dryer) you run both lines, one with a red wire and the other with a black wire, and the white neutral wire to the outlet.
For all of the 120 volt outlets, you run them with just a black wire (hot) and a white wire (neutral). You want to split up your outlets so that half of them are on one of the lines and half are on the other. You always use black and white wires even though the outlets are on two different lines, but for 120 volt outlets it’s not an issue. You don’t really care which line the outlet is on.
For whatever reason, they’ve wired both lines to this box and put one outlet on line 1 and the other outlet on line 2. Red to black voltage (line to line) should be 240 volts, and red to white or black to white (either line to neutral) should be 120 volts. If I’m reading your description right this is what was done, so it’s not a problem. Both outlets are 120 volts, they just happen to be on different lines. In a normal house half of your outlets will be on one line and half on the other, so this isn’t a big deal, though it is a bit unusual to have both outlets in one box on different lines.
The advantage of doing it this way is that if you have a couple of heavy duty appliances of some sort plugged into the outlet they won’t load down the same circuit. The disadvantage of doing it this way is that it costs a lot more to wire up your house if you do this to a lot of outlets.
The scary thing is sometimes this was done so that people could get 240 volts out of the outlet by making up a plug to go into both outlets. This is downright scary IMHO and you should never do this.
Don’t do that unless you also take the red wires and wrap the ends of them with black electrical tape. If you see red and black wires in the same box an electrician is going to assume that they are the two separate lines.
I’m not sure what you are saying here. What exactly do you mean by “leaving the wires free”? With all of the wires connected the next outlet should work ok, unless there is some other type of wiring problem. If you’ve got wires disconnected, you could be disconnecting the hot or the neutral for the outlet that powers the TV.
Something else is mis-wired. I’m guessing someone wired one connection backwards (hot where neutral should go, and vice-versa) somewhere.
ETA: Actually, it may not be mis-wired. You may have just disconnected the neutral so the voltages are feeding through some other device on the same circuit.
What you’ve got there is something pretending to be a Multiwire Branch Circuit. I’d suspect that someone didn’t know what they were doing, based on the broken-off tab on the neutral side. The presence of 2 white wires is also suspicious. Note that not all of the possible ways to create a multiwire branch circuit are legal.
My guess (which is only a guess, since I have not examined the wiring in question in person) is that there is at least a problem with one of the neutral wires. It could either be a poor connection (which would cause it to have potential referenced to ground), or perhaps incorrectly connected to a hot leg, either in the mess in the back of the box or somewhere else in the house.
That is what I would suggest. If you insist on doing something yourself, you would want to re-measure the outlet across all 6 of the straight and diagonal lines:
o---o
|\ /|
|/ \|
o---o
You might also use something that pulls a little current, as opposed to a voltmeter or a neon tester. A 60W light bulb with some probes would work. Of course, once you find out how messed-up this is, you’ll probably need the electrician anyway…
Are you sure about this? Those measurements should not be possible in a normal residential system.
(B)ottom L to BR = 120 V => one of these has to be the neutral
BL to UR = 240 V => it’s not the BL
UR to BR = 240 V => it’s not the BR either
Something’s FUBAR. At the least, I think you’ll need to check every connection on both circuits.
On the outlet you removed, was either socket shorted?
Given the broken neutral-side tab and what I understand as two separately pigtailed white wires (emerging from the “mess”), it appears to be pretending to be two branch circuits (what Terry’s link calls “completely electrically-independent split-wired receptacles”), not one multiwire circuit. The OP seems to have identified the breakers involved and cuts off two single-pole breakers to deenergize the box.
So far, not necessarily any problem, though it is an unusual thing to see in a general-purpose outlet. I’ve heard of kitchen counter circuits run something like this.
But if it were that, you should read only 120V between either white wire and either hot wire, 240V between the two hots, nothing between the two whites. Moreover, there really ought to have been some marking on the wires to show which neutral went with which hot, else you risk getting more than one circuit’s share of loads on one white back to the panel. (The other kind of split-wired receptacle described in the link above intentionally combines both loads onto one neutral; I’ll just say I find this bad practice for a situation involving unknown future loads.)
But this,
…is an unmistakable sign of somebody mucking around in electrical boxes who had no goddamn business doing so.
So any number of strange and bad things might have been done in various places. There’s probably a number of electrician calls in the future of this house.
For any kinds of loads you’ll see outside of heavy industry or a research lab, the current in the common neutral will never exceed the current of one full leg (100% load on that leg, 0% on the other leg). As current begins to flow in the second leg, that is current that doesn’t pass through the shared neutral. You can extend this to 3-phase as well - 3 hots and one neutral. Normally 120V from neutral to any hot, 208 across any 2 hots. Also done at 277/480 in business for lighting.
Sounds like the time to map out the wiring in your house. You’ll have to shut off all the breakers except one, then look for outlets that could be wired to it. In your case it sounds like you’ll have to try both top and bottom on each unit. A non-contact voltage tester will make it easier. Once you have it mapped out you can assess whether you have 1 or 2 miswired outlets, or you need to consider major electrical work. Good luck, my previous house was like that.
It sounds like you’ve either got a “loose” neutral or one of the white wires masquerading as a neutral in that box is tied to a line somewhere. Or, you’re chasing a ghost. More about that below.
If you want to explore further, get a “Wiggy” type tester that puts some load on the circuits. Testing with a digital meter is pointless as its high impedance will let you see ghost voltages that don’t actually exist - for example, a DMM can pick up capacitively coupled voltage from parallel wires and let you think a disconnected wire has power on it., but a Wiggy will show zero volts on the same wire.
In that your TV tries to turn on but won’t, and you’ve fried some other things, I’d say there’s no ghosts in the wires, but you’ve definitely got some real problems that need to be resolved quckly before anything else is damaged. It’s one thing for a do-it-yourselfer to cut a breaker and swap out a bad receptacle or switch, but when things go wrong like they have here, that’s when you need an experienced pro.