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Old 01-29-2011, 02:20 PM   #1
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Shared neutral on more than one circuit


Hello,

In my shop I will be putting in a 20 amp duplex receptacle where the 2 outlets are on separate 20 amp circuits. Can I share the neutral wire between these 2 circuits? The 12 gauge hot wire is rated for 20 amps but when you share the neutral for circuits totaling 40 amps, is a single 12 gauge wire OK?

One the same note, if I am wiring a 240V receptacle downstream of a 120V receptacle, can I just use the same neutral/ground or do I need to home run a separate one to the panel?

Thanks for your help

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Old 01-29-2011, 02:24 PM   #2
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Shared neutral on more than one circuit


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Originally Posted by Richo View Post
Hello,

In my shop I will be putting in a 20 amp duplex receptacle where the 2 outlets are on separate 20 amp circuits. Can I share the neutral wire between these 2 circuits? The 12 gauge hot wire is rated for 20 amps but when you share the neutral for circuits totaling 40 amps, is a single 12 gauge wire OK?

One the same note, if I am wiring a 240V receptacle downstream of a 120V receptacle, can I just use the same neutral/ground or do I need to home run a separate one to the panel?

Thanks for your help
you need to hire an electrician so you don't kill somebody. The fact you asked:

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can I just use the same neutral/ground
means you have no idea what you are doing here. stop now and save a life. It might be yours you save.

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Old 01-29-2011, 02:27 PM   #3
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Shared neutral on more than one circuit


I have to agree with nap here. At least I must ask how you plan on "wiring a 240V receptacle downstream of a 120V receptacle".
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Old 01-29-2011, 03:27 PM   #4
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Shared neutral on more than one circuit


Nap, I appreciate your concern but please don't insult my intelligence. I have been doing my own electrical wiring for over 10 years. The obvious answer is when in doubt, use a separate neutral wire for each circuit which is what I've always done in the past.

Quote:
At least I must ask how you plan on "wiring a 240V receptacle downstream of a 120V receptacle".
I'm talking about on the same run of EMT conduit, each receptacle having their own wires run from the panel. I always thought that the neutral carried the same amperage as the hot but recently I opened up an outlet box that contained 2 circuits and they were sharing the same neutral wire. Apparently it was not wired by an electrician.

I got my answer.
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Old 01-29-2011, 03:43 PM   #5
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Shared neutral on more than one circuit


Now that you cleared up some questions, yes you can share one neutral between two hots as longs at the hots are on two different phases in the panel. Contrary to what most people think when hooked up to two different phases the neutral canels each other out, so if you have 18 on one hot and 15 on the other you will only have 3A on the neutral. If you hook the two hots up to the same phase you would have 33A on the neutral.

You can share the EMT with both plugs. Your 240V plug does not need a neutral and your emt can be used for your bond. So in your pipe you would have 3 hots(2 for the 240V, 1 for the 120V) and you will also have a neutral. This would give you 4 current carrying conductors and depending on where you are you may to have to derate your wires.
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Old 01-29-2011, 03:46 PM   #6
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Shared neutral on more than one circuit


You should not be using 2 120 volt circuits to build a 240 volt circuit "down stream". Run separate conductors from a dedicated 2 pole breaker. And I agree with the others, if you would even consider doing when you planned, you need professional help.
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Old 01-29-2011, 03:47 PM   #7
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Shared neutral on more than one circuit


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I'm talking about on the same run of EMT conduit, each receptacle having their own wires run from the panel.
Then it is NOT downstream of a 120v receptacle. It is a different 240v circuit you are asking about, just using the same conduit.
Also, a 240v circuit does not have/need/use a neutral. Are you talking about a 120/240v circuit?



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I always thought that the neutral carried the same amperage as the hot but recently I opened up an outlet box that contained 2 circuits and they were sharing the same neutral wire. Apparently it was not wired by an electrician.
NO, apparently it was wired by someone who knew what they were doing. The person currently looking at it apparently does not.
A shared neutral (multi-wire branch circuit) circuit is a perfectly safe, common and efficient way of wiring.
The neutral of a properly installed MWBC carries only the current imbalance, NOT the combined current.



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I got my answer.
Now you do.
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Old 01-29-2011, 03:51 PM   #8
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Shared neutral on more than one circuit


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Richo;580480]Nap, I appreciate your concern but please don't insult my intelligence. I have been doing my own electrical wiring for over 10 years. The obvious answer is when in doubt, use a separate neutral wire for each circuit which is what I've always done in the past.
so, explain to me what a neutral wire has to do with a 240 volt circuit then? and then combining them as a "neutral/ground".

Quote:
I'm talking about on the same run of EMT conduit, each receptacle having their own wires run from the panel. I always thought that the neutral carried the same amperage as the hot but recently I opened up an outlet box that contained 2 circuits and they were sharing the same neutral wire. Apparently it was not wired by an electrician.
again you show your ignorance. What you found was a multi-wire branch circuit and as long as the hot legs are on opposing legs or phases, it is quite legitimate. Real electricians put them in with regularity all across the country every day.
I have not insulted your intelligence as much as you have done that to me. I do this for a living and I know what is correct and what isn't here. Especially since you claim to have been doing this for over 10 years, you show an extreme lack of knowledge concerning the question at hand. I knew more than that as a 1st year apprentice.
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Old 01-29-2011, 04:08 PM   #9
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Shared neutral on more than one circuit


The statement "I have been doing my own electrical wiring for over 10 years" is pretty damn scary considering the obvious lack of knowledge and expertise that exists.
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Old 01-29-2011, 04:40 PM   #10
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Shared neutral on more than one circuit


Quote:
Originally Posted by Richo View Post
Hello,

In my shop I will be putting in a 20 amp duplex receptacle where the 2 outlets are on separate 20 amp circuits. Can I share the neutral wire between these 2 circuits? The 12 gauge hot wire is rated for 20 amps but when you share the neutral for circuits totaling 40 amps, is a single 12 gauge wire OK?

One the same note, if I am wiring a 240V receptacle downstream of a 120V receptacle, can I just use the same neutral/ground or do I need to home run a separate one to the panel?

Thanks for your help

You can share a neutral between two 120V circuits as multiwire branch circuit (MWBC), with some caveats:
  • The neutral connection should pigtailed to all receptacles/devices, not passed through. If the neutral connection fails at any point, the two hot legs will essentially try to become a single 240V circuit.
  • The hot legs of the MWBC must be installed on opposite hot legs of your panel, and must be protected in such a manner that both legs will trip simultaneously in event of overload. You need to use either a 20A/15A (depending on the gauge of your circuit wiring) double-pole breaker, or two paired single-pole breakers connected with a common trip handle.

Both of these requirements must be met! Make sure your neutral connections are securely pigtailed at every junction and that your breaker is a either a double-pole or paired single poles with a common trip handle.

I would run a separate circuit for the 240V receptacle. This would allow you to use whatever size breaker (with appropriate wiring) necessary for your 240V application, since the MWBC will be limited to 20A.

I do have a couple of concerns about your proposed set up, though:

You mentioned that you were splitting a receptacle between two circuits. If you are using 20A breakers, you MUST use a 20A receptacle, not a 15A. You cannot place a single 15A receptacle on a 20A circuit (if it is a duplex receptacle, that counts as two receptacles on a single circuit and a 15A duplex receptacle is permitted on a 20A circuit).

The other question is how are you providing GFCI protection to this circuit? You will not be able to split a GFCI duplex receptacle into two circuits, although you could easily gang two GFCI receptacles into a single box and put them on separate legs of a MWBC with a shared, pigtailed neutral.
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Old 01-29-2011, 04:44 PM   #11
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Shared neutral on more than one circuit


Please disregard the word "downstream" as I didn't realize it had a special meaning with wiring, and all I meant by that was further down the conduit, but on its own breaker.
Quote:
You should not be using 2 120 volt circuits to build a 240 volt circuit "down stream". Run separate conductors from a dedicated 2 pole breaker. And I agree with the others, if you would even consider doing when you planned, you need professional help.
Apparently you misunderstood my question if you thought I was considering something foolish like that. Apparently my wording was not clear as to what my question was. As I reread my first post (2nd question) about the 120/240, I was meaning the ground wire, not the neutral. There are currently 2 floor boxes of PVC conduit which have 120V receptacles in each. I need to put a NEMA 6-15 into one of them. I know I have to run 2 hots from a 2 pole breaker but I simply wanted to know if they can share the same ground wire that is already there or if I have to run a separate one for the 240V circuit. I used the term neutral/ground because they are tied together in the panel.
Quote:
yes you can share one neutral between two hots as longs at the hots are on two different phases in the panel.
That was all I wanted to know from my first question. Thanks.
Quote:
The statement "I have been doing my own electrical wiring for over 10 years" is pretty damn scary considering the obvious lack of knowledge and expertise that exists.
Electricians are formally trained. For 20 years I've read books and watched DIY shows. Just because someone didn't go through the training to become licensed doesn't mean that they don't know the proper measures to do something as simple as run conduit or romex and pop a breaker in a panel, install lights and switches, etc. Anything more than that is undeniably beyond the scope of my knowledge and I would not attempt to do it without proper research.

Thanks for the info.

Last edited by Richo; 01-29-2011 at 04:59 PM.
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Old 01-29-2011, 04:58 PM   #12
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Shared neutral on more than one circuit


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You can share a neutral between two 120V circuits as multiwire branch circuit (MWBC), with some caveats:
  • The neutral connection should pigtailed to all receptacles/devices, not passed through. If the neutral connection fails at any point, the two hot legs will essentially try to become a single 240V circuit.
  • The hot legs of the MWBC must be installed on opposite hot legs of your panel, and must be protected in such a manner that both legs will trip simultaneously in event of overload. You need to use either a 20A/15A (depending on the gauge of your circuit wiring) double-pole breaker, or two paired single-pole breakers connected with a common trip handle.

Both of these requirements must be met! Make sure your neutral connections are securely pigtailed at every junction and that your breaker is a either a double-pole or paired single poles with a common trip handle.
Very well explained...and yes, I do have 20A receptacles. That helps alot
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Old 01-29-2011, 05:30 PM   #13
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Shared neutral on more than one circuit


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You mentioned that you were splitting a receptacle between two circuits. If you are using 20A breakers, you MUST use a 20A receptacle, not a 15A. You cannot place a single 15A receptacle on a 20A circuit (if it is a duplex receptacle, that counts as two receptacles on a single circuit and a 15A duplex receptacle is permitted on a 20A circuit).
the circuits must be on a common trip breaker as well (if it hasn't already been addressed)
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Old 01-29-2011, 05:40 PM   #14
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Shared neutral on more than one circuit


Nitpicking. You can use 15 amp receptacles including singles on a 20 amp circuit so long as there are at least two receptacles altogether.

For multiple circuits in a conduit, run one ground wire sized for the largest amperage circuit present. For example for one 30 amp circuit and two 20 amp circuits, run one 10 gauge ground wire.

While a 120/240 volt circuit i.e. a common multiwire branch circuit can serve both 120 volt and 240 volt receptacles, most 240 volt appliances use enough amperes that you would not want to share a 20 amp circuit.

While 30 amp 120 volt receptacles exist for use on circuits of that capacity, you may not connect a fixture not rated for a 30 amp circuit and you may not modify a light or appliance to have a 30 amp plug for use on that circuit.
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Old 01-30-2011, 05:44 AM   #15
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Shared neutral on more than one circuit


If you insist on using a shared neutral.

There are several points to be aware of when dealing with multiwire circuits.
The first is that the two HOT wires must be connected to opposite legs of your service in the panel. This ensures that the current in that "shared neutral" wire is the difference and not the sum of the currents in the two HOT wires.
To confirm this, you should be able to measure 240V between the red and black HOT wires while seeing the typical 120V between either HOT and the white Neutral wire.


Installing multiwire branch circuits needs to be done in compliance with 210.4 of the 2002 NEC.
The important issue here is ensuring the ungrounded conductors end up on different phases. If not, the neutral currents will not cancel out and will overload the neutral conductor.
However, placing the ungrounded conductors on the different phases also has its problems. If the neutral were to open during operation, you essentialy have placed the two circuits in series with each other. This in effect cause low and high voltages outside of nominal ranges that can damage equipment.
Great care must be taken when installing multiwire branch circuits.

Is all of this worth saving $10 on wire?

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