Odd 240v outlet discovery

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  #1  
Old 04-25-13, 07:31 PM
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Odd 240v outlet discovery

Hello everyone - I have a 100 year old house partially wired with knob and tube. In the upstairs I uncovered a painted-over single outlet on a bedroom wall, which to my surprise turned out to be 240v. All the other similar single outlets I've found, mostly on the baseboards, have been 120v. The strange thing is, I can't find where this outlet is wired into the breaker box. I have three 240v double pole breakers for the two AC units and the dryer in the house. But when I shut them all down, the outlet stays hot. Where do I look next to find where this outlet is wired? I want to convert it back to 120 so it's usable but don't quite know where to start. Unfortunately, previous owners never labeled the breakers, so I have no idea what breaker goes where... and there are a lot of breakers to track down.

Thank you!

Gary
 
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Old 04-25-13, 07:37 PM
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Welcome to the forums! First determine if you have a subpanel anywhere in the house. Next, although very rare, this could be accidentally serviced by two single breakers on the same side of the panel, placing them opposite as far as "phase" goes, thereby rendering 240 volts if tested across the two leads normally hot and neutral. I would try shutting down single breakers one at a time to see what controls it.
 
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Old 04-25-13, 08:23 PM
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Is the single outlet you uncovered an actual 240 type receptacle or is it a single 120v receptacle with 240v on it ?

It would be pretty rare to see the actual knob and tube setup for a 240v receptacle.
As Chandler mentioned.... you will probably find two unrelated breakers controlling it.

My guess is that you may end up capping it off as it may be almost impossible to reconfigure it to 120v.
 
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Old 04-26-13, 08:57 AM
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Hello, and thank for the welcome and the replies. Chandler, the notion of a sub panel somewhere in the house crossed my mind, especially with the mix of standard and knob-and-tube wiring. So far, I haven't found anything in the basement, but since a lot of the old wiring is strung in the attic, and this outlet happens to be on the second floor, I guess it's possible that there's something up there. One of the A/C air handlers is up there, so I guess it's possible.

If not, then I'll have to pull the cover on the box in the basement and see if I can see anything tying two breakers together that shouldn't be, or worst case, do as PJMax suggests, and start going through every breaker until I find the right one. Probably should do that anyway for the entire house so I can label the breakers.

BTW, the outlet was so heavily coated in paint that I couldn't tell what type of outlet it was. I just assumed it was a standard 120v outlet because all the rest that found were, and I actually replaced it with a standard outlet... yes without turning it off at the breaker. Not advisable, I know, especially since it was a 240V outlet, but good heavy electrician's gloves and careful handling of the the each wire allowed me to change out the outlet. It was discovered when a lamp was plugged in that practically blinded the family. My multi-meter confirmed the issue. I''m thinking it was used for a window A/C unit sometime before central was put in the house.

Again, thank you for the help!
 
  #5  
Old 04-26-13, 12:05 PM
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I uncovered a painted-over single outlet on a bedroom wall, which... turned out to be 240v. The strange thing is, I have three 240v double pole breakers... But when I shut them all down, the outlet stays hot.
Then you almost certainly don't have a subpanel somewhere. If you do, then it is likely fed from two vertically adjacent breakers that do not have their handles locked or tied together. It is likely that that arrangement exists in your panel, but that it only feeds this surprising outlet.

What positions are the three 2-pole breakers in?

I actually replaced it with a standard outlet...
When you replaced that receptacle, was it wired with one black wire and one white wire? If so, you can look in your panel for any single-pole breaker that has a white wire connected to it. If you find one of those, turn it off, along with the one just above or below it. If that didn't kill the power to that outlet, leave the breaker with the white wire off, turn the one just above or below it back on, and turn the one on the other side of it off. One of those combinations may kill it.

Unfortunately, previous owners never labeled the breakers, so I have no idea what breaker goes where... and there are a lot of breakers to track down.
A circuit tracer can make that job easier. Just remember that these only work when the receptacle you plug the transmitter into has power. In your case, I would also test the receptacle to make sure it had 120V, and not 240V, before plugging the transmitter in!
 
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Old 04-27-13, 07:09 AM
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In a 100 year old house, you might have old hubbel/edison combo plug with "T" slots.
IIRC, those are rated 110v 10amp.

Curious about that old outlet... Does it have a ground prong?
Is it a "normal" outlet, with the slots parallel (=) or are the slots "T" shaped.
Look like any of these?


Those old "T" slots, just happen to match 1950s-60s style ungrounded 120v 20amp plugs
which generally had one blade vertical and one blade horizontal.
Oddly, ungrounded 240v 20amp plugs have the same configuration of one blade vertical, one blade horizontal.

Just last week I head a story about how, in the late 1960s an uncle of mine bought a 240v A/C unit from a neighbor. The neighbor won a 240v A/C/ unit, but lived in a house with 60 amp single phase service.

My likely scenario-
Your prior owner gets new 1960s A/C unit, plugs into old 10 amp outlet, keeps blowing fuses
Prior owner switches to 240v A/C unit which uses fewer amps, finds it won't work, cross-connects wires to get it to work.
 
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