Shared Neutral

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  #1  
Old 11-27-04, 01:35 PM
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Shared Neutral

Can I share a neutral wire in the following scenario.

I putting double gang boxes in a room. The duplex outlet on the left will be always on and the duplex on the right will be swithed. I have a double pole, 20 amp breaker in the panel. I want to run 12-3 from the panel to the outlet boxes. I want to share the white neutral among the switched and non-switched outlets. The black will go to one pole of the breaker for the always on side and the red will go - via a switch to the second pole of the breaker.

Thanks,

David
 
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  #2  
Old 11-27-04, 02:53 PM
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Any particular reason you want to use up a douple pole on just two receptacles? A single pole breaker will do the job and still leave you with another position in the panel.
 
  #3  
Old 11-27-04, 05:07 PM
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There is nothing wrong with your plan. Just make sure you use a double pole breaker not a tandem breaker and that it is installed in the correct slot. There must be 240 volts between the red and black wires. In some panels it is possible to install a double pole breaker to get them both on the same leg and thus zero volts between the red and black.
 
  #4  
Old 11-27-04, 07:00 PM
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Thanks for the help - clarification and additional question

Thanks for the posts.

In reply to Roger's question about why use up a double pole breaker for just one receptacle - there will actually be several receptacles in the room, half will be switched, half always on. As for the double pole breaker, the one I am using takes up only one position. I agree it is probably a bit of overkill for the switched receptacles.

With regard to JOED's suggestion abou measuring the breaker voltage, I expect there will be 220 Volts not 240 volts across the two poles. This is a residential distribution panel. I have two black feeds, one neutral and one ground coming in. I think they are all on the same phase - does this make a difference - e.g. of both circuits are drawing 20 amps will the neutral be carrying 40 amps on 12 guage wire? (although it would be drawing the same power as 20 amps at 220 volts)

Thanks again for the help,

David
 
  #5  
Old 11-27-04, 07:15 PM
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220, 240...same thing. This varies from power company to power company. 240 is the nominal value, but when you read 240 here, it means anything between 220 and 250 (but 208 is different).

If you wire it correctly, the current on the neutral is the difference of the current on the two hot wires, not the sum. If you don't understand why this is true, I suggest that you do not wire this circuit until you do. Shared-neutral circuits should only be wired by one who is fully familiar with the theory. We can help with that if you want.
 
  #6  
Old 11-27-04, 07:17 PM
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"As for the double pole breaker, the one I am using takes up only one position."

Make sure this is indeed a double-pole breaker and not a tandem or piggyback. A double pole will take up two positions.

"will the neutral be carrying 40 amps on 12 guage wire? (although it would be drawing the same power as 20 amps at 220 volts)"

In the shared neutral scenario, the neutral carries the difference in load between the two legs, not the sum.
 
  #7  
Old 11-27-04, 07:18 PM
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Sorry John...you posted basically the same message while I was editing mine...
 
  #8  
Old 11-27-04, 07:22 PM
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If your "double pole breaker" takes up the same amount of room in the panel as a regular breaker, then it does not sound like a double pole breaker, but rather it sounds like a tandem breaker. If this is the case (it is a tandem breaker) then you cannot use as you are describing.

Don;t know why you want two duplex receptacles at each location, but that is your choice. I would probably use a single duplex, and switch half of each receptacle, on a single 20 amp circuit. But again, it is your choice.

Do make sure that you understand what you are doing. Yes, your incoming power is single phase, but it is 240 volts, with 240 volts being the difference between the two hot wires. Each hot wire by itself measures 120 volts to ground/return.
 
  #9  
Old 11-27-04, 08:17 PM
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Thanks for the clarification

Thanks again for all the help and replies. This site and its members are a great resource

The single position breakers are, in fact, tandem breakers. My double poles do take up two positions.

I understood about the currenton the shared neutral as being the difference between two hots but I was unsure if this applied on single phase installations.

John, the moderator suggested this site could help if I needed to strenghten my understanding of the theory "Shared-neutral circuits should only be wired by one who is fully familiar with the theory. We can help with that if you want." Where would I look?

All the best,
David
 
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Old 11-27-04, 09:51 PM
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  #11  
Old 11-28-04, 12:13 AM
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Here's something to keep in mind:

Shared-neutral circuit on a double-pole double-wide breaker = safe and cool.
Shared-neutral circuit on a tandem breaker = fire and death.

If you have 240 volts between the red and black wires, then you are safe and cool. If you don't have 240 volts between the black and red wire, then you have fire and death.

There are other special considerations that apply to shared-neutral circuits (officially called "multiwire circuits"). There are special codes about how to handle the neutral connections in some situations, and if you need GFCI, very special care is needed.

I generally highly discourage shared neutral circuits. When 4800 watts of power are needed, the DIY is usually much, much better off simply running two 12/2 cables rather than one 12/3 cable.

And we still haven't determined whether or not a simple non-shared-neutral 20-amp circuit (2400 watts) would fill your need without all this complication. Simple is usually best.
 
  #12  
Old 11-29-04, 04:31 PM
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I installed a shared neutral or multi-wire circuit earlier this year and my research included advice at this site. But I don't recall covering a point made by the responses to this thread:

"In some panels it is possible to install a double pole breaker to get them both on the same leg and thus zero volts between the red and black."

I understand that there should be 240 volts between the red and black.
I installed a double pole breaker (takes up two slots in the panel) for my multi-wire circuit but have not checked the voltage between red and black wires. My current question is: what equipment or instrument do I need to conduct this check and how is it done?

Thanks
 
  #13  
Old 11-29-04, 06:11 PM
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"double-pole breaker" is not synonymous with "240-volt breaker". Some tandem breakers are sold as "double-pole". And in some panels (e.g., certain GE panels), the same tandem breaker can be installed on the same power leg, or moved one-half spot down and hit both legs. So I'm always reluctant to simply say "double-pole breaker" without providing more information. Testing for 240 volts between the two breaker screws is the definitive test. This is one of the few good uses for a multimeter in home wiring. Actually, if you have one of those $2 neon testers, you can test for any voltage at all between the two breaker screws. If the light comes on at all, it's certainly 240 volts (unless you are testing a GFCI or AFCI breaker and use the neutral screw for one of your test points).
 
  #14  
Old 11-29-04, 07:59 PM
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question:

I am a plumber with a pretty good understanding of electricity. ( Is that a dangerous combination?)

The above reference article on the multi-wire circuits and overloading of neutral was very informative .

My question is this: Is there ever any restrictions on having both legs of the 240 volt circuit ( i.e. a red and a black) in the same (120v outlets) receptacle box? I would think there is increased danger of 240 volt shock to a user in the event of a malfunction of a device or loose wires. Or am I jsut making too much out of this?

The comments on this thread have been excellent. A typical homeowner or "Sunday electrician" like me could easily do this job wrong.
 
  #15  
Old 11-29-04, 08:32 PM
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In some panels it is actually possible to install a double pole breaker and not hit both busses. You can tell when you look at the slot before inserting the breaker. The bus bar the breaker is attaching to will be split. If it is one continuous piece then it is not the correct slots.
 
  #16  
Old 11-29-04, 11:48 PM
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594, it is allowed by code to have wires from both power legs in the same box, even connected to the same (split-wired) receptacle. This is pretty common. Code requires a double-pole breaker (connected handles) and pigtailing the neutral if you do this. Yes, I suppose the risk is higher if you stick a paperclip in the hot slots on both the top and bottom receptacles at the same time. Don't do that.
 
  #17  
Old 11-30-04, 01:34 AM
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A related question

This has been a great thread - For everyone's information after reading all the posts I have talked myself out of using the shared neutrals. Just because I could do it doesn't mean it is a good idea.

The last couple of posts bring to mind a related question - can you have a 240 volt and 120 volt receptacle sharing the same box and wiring. More specifically can you have a double receptacle box. On one side have a 240 volt outlet feed by a double pole breaker. Then next to it, a 120 volt receptacle fed from one of the hots of the 240V and a neutral? I suspect you would need to pigtail the neutral on this also.

Thanks again to everyone for their help.
 
  #18  
Old 11-30-04, 04:25 AM
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594tough asks:

"My question is this: Is there ever any restrictions on having both legs of the 240 volt circuit ( i.e. a red and a black) in the same (120v outlets) receptacle box? I would think there is increased danger of 240 volt shock to a user in the event of a malfunction of a device or loose wires. Or am I jsut making too much out of this?"

Very common. You may find it's the easy and convenient way to get separate circuits to feed a dishwasher and a garbage disposal under a kitchen counter.
 
  #19  
Old 11-30-04, 06:27 AM
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Questions for John

"Shared-neutral circuit on a double-pole double-wide breaker = safe and cool.
Shared-neutral circuit on a tandem breaker = fire and death."

What is a Tandem breaker? How do you identify one?

"If you have 240 volts between the red and black wires, then you are safe and cool. If you don't have 240 volts between the black and red wire, then you have fire and death."

If installing a 12/3 circuit using a double pole breaker (for 120v applications), how/when do you measure for 240v? When you say between the red & black, do you imply measuring across the breaker terminals inside the panel with a DVOM?
 
  #20  
Old 11-30-04, 10:12 AM
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ghumphri, a "tandem" breaker (also called a "skinny" breaker or "twin" breaker) is a one-inch wide breaker (in most brands) that has two independent and skinny handles. They are typically sold as 15/15 (meaning both breakers are 15 amps), 15/20, 20/20, etc. They are not allowed in all panels, and even when allowed, not in all spaces in the panel. Because the NEC restricts the number of breakers to 42, they wouldn't be allowed at all in a 42-space panel. In a 20-space panel, they might be allowed in every location. In a 30-space panel, they might only be allowed in 10 of the spaces.

In a properly wired shared-neutral circuit (officially a "multiwire circuit"), you can measure 240 volts between the two screws on the breaker, or between the two hots (usually black and red) at any outlet.

Montanaro, yes you can have a 240-volt receptacle and a 120-volt receptacle in the same box on the same double-pole breaker. This would be very unusual, however, since ordinary 120-volt receptacles cannot be protected by a larger than a 20-amp breaker, and most 240-volt devices need something larger than 20 amps. That's why you would almost never see this in practice.
 
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