2 Circuits from a 3-Wire?

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  #1  
Old 01-20-07, 11:02 AM
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2 Circuits from a 3-Wire?

I have a 220V receptacle leftover from a bygone window AC unit that the previous owners had on the 1st floor of my house. It shares a double-gang box with a 120V receptacle.

I'm in the process of cleaning up and reorganizing much of the 70-year-old wiring in this house, including running a dedicated circuit to the gas furnace and to a newly installed sump pump. Both the furnace and the sump pump are located in unfinished basement, directly below the room with the 220 receptacle.

It occurred to me that I already have two 12-gauge conductors right there in the cable that feeds the receptacle. It would be really convenient to cut the cable, install a junction box, and split out my two desired circuits from there. Of course, I would also replace the double breaker with two individual breakers.

One concern is that it might not be kosher for the single common to serve both circuits, but it was considered large enough for the 220 circuit, right? Another concern is that it might confuse a future owner to have two circuits in the same junction box (say they only turn off one and assume the box is no longer hot), but this couldn't be much worse than having a 220 and a 120 receptacle in the same box.

Thoughts?
 
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  #2  
Old 01-20-07, 11:31 AM
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You are proposing a multi-wire circuit. These are not common in residential situations, but do exist anre legal, if done properly.

However, they must be done properly. This means that you need three insulated conductors plus a ground. A 240 volt air conditioner does not generally require a neutral wire, so there may not be one present.

If there is a neutral and a ground and then two conductors for the hot wires then you can turn this into a multi-wire circuit. However, I would leave the 240 volt breaker in place. The 240 volt breaker will ensure that the entire circuit (both halves) will be turned off at the same time, and will also ensure that you don't incorrectly connect the individual breakers to the same leg of the incoming 240 volts, which would not be good.

One further comment. If you are able to put in a multi-wire circuit, and do so, make sure that you pigtail the neutral connection at the box where you make the split. Do not connect the neutral directly to a receptacle if you happen to put one in that box.
 
  #3  
Old 01-20-07, 11:42 AM
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Actually multi-wire circuits are very common in residential installations. I use them all the time. If done properly they are perfectly safe. For a DIY application the use of a two-pole breaker is recommended IMO.

You definitely do need three insulated conductors plus a ground. Typically this is black, red and white.
 
  #4  
Old 01-25-07, 08:28 PM
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Wow...racraft was right: the 240V circuit did not have a common. I saw the two conductors that ran to the breakers and assumed that the 3rd conductor would be there when I went to look for it. Sure enough, the cable was just a 12/2.

So, even though I had to run fresh cables (I used a 12/2 for the furnace and 14/2 for the sump pump), your posts prepared me for what I later discovered. I probably would have been pretty shocked to find that the circuit lacked a neutral.

You mentioned not connecting both breakers in a multi-wired circuit to the same leg of the incoming service? Why would that cause problems? Do the two circuits need to be out of phase with one another?
 
  #5  
Old 01-25-07, 09:39 PM
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The two circuits need to be from separate legs of the service so that the current do not add together, but rather subtract.

If both halves of the circuit come from the same leg than the neutral current is the sum of the current on each circuit, meaning it could reach 40 amps on a dual 20 amp circuit.

Coming from each leg, the current on the neutral is the difference between the current on each leg. Meaning it could be anywhere from 0 to 20 amps for your dual 20 amp circuit.
 
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Old 01-25-07, 10:21 PM
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I see...that makes sense. So the current is kept in check, even though voltage between the two conductors (and therefore across an accidental short) is 240V, rather than 0V as it would be if both were from the same leg. I suppose this, however, is a lot less likely to occur than a homeowner overloading the circuits and drawing too much current.
 
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