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Old 12-26-2008, 08:59 PM   #1
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Dumb Neutral wire question, just curious.


I know one not suppose to share neutral wires, now for the dumb question why when one uses say 12/3 wire. One hot red, one hot black and one nuetral white. If you are using this for two circuits, one has to share the neutral. Is it okay then?

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Old 12-26-2008, 09:29 PM   #2
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Dumb Neutral wire question, just curious.


One of the phase conductors (black/red) is on the breaker on the A phase bussbar, the other is on the breaker on the B phase bussbar. The A/B phase "fluctuations" ensure that the neutral is never overloaded. A common mistake is to put both phase conductors on the same bussbar, which guarantees a neutral overload in the right circumstances. That's called an imbalanced multiwire branch circuit.

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Old 12-26-2008, 10:31 PM   #3
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Dumb Neutral wire question, just curious.


KC is right. In a single phase panel, if the two hots are on different phases (legs), the neutral current will be the difference between the two hot currents. For example, phase A has 12 amps, while phase B has 9 amps, the neutral current will be 3 amps.

If both hots are on the same phase (leg), the neutral current will be the sum of both the hots. Using the same example, phase A has 12 amps, and Phase A (not B this time) has 9 amps, the neutral current will be 21 amps. This is obviously a serious hazard, as the neutral has no overcurrent protection.

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Old 12-26-2008, 10:44 PM   #4
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Dumb Neutral wire question, just curious.


we are discussing this in another thread as thats what i am going to do for a shed circuit, am i correct in thinking that the two breakers feeding this circuit have to be next to one another in the panel, and not across from one another, hence a 240v water heater breaker with the connecting rod removed in essence? if they were across from each other then they would be unbalanced?

can i just use a 20a 240 breaker and remove the mechanical lock between them?

Ratt

Last edited by rsriverrat; 12-26-2008 at 10:56 PM.
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Old 12-26-2008, 11:02 PM   #5
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Dumb Neutral wire question, just curious.


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Originally Posted by rsriverrat View Post
we are discussing this in another thread as thats what i am going to do for a shed circuit, am i correct in thinking that the two breakers feeding this circuit have to be next to one another in the panel, and not across from one another, hence a 240v water heater breaker with the connecting rod removed in essence? if they were across from each other then they would be unbalanced?

can i just use a 20a 240 breaker and remove the mechanical lock between them?

Ratt
Do not remove the rod. It needs to be left in place. If one breaker trips, they both trip. Using a double pole breaker is the best practice and is required on the 2008 code cycle. A double pole breaker ensures 1 leg lands on each leg of the panel. In most boxes it is on phase then the next phase one on top of the other. In some older boxes it is left and right.

Either way, a double pole breaker that is made for your panel will ensure the conductors are on the proper poles.
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Old 12-26-2008, 11:08 PM   #6
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Dumb Neutral wire question, just curious.


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Originally Posted by jamiedolan View Post
Do not remove the rod. It needs to be left in place. If one breaker trips, they both trip. Using a double pole breaker is the best practice and is required on the 2008 code cycle. A double pole breaker ensures 1 leg lands on each leg of the panel. In most boxes it is on phase then the next phase one on top of the other. In some older boxes it is left and right.

Either way, a double pole breaker that is made for your panel will ensure the conductors are on the proper poles.
Jamie

simple and clear enuf, i wont hijack this thread anymore and will continue any other questions about my situation in my thread... i was a forum admin for a few yrs and i hated hijackers.... lol

Ratt
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Old 12-27-2008, 01:04 AM   #7
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Dumb Neutral wire question, just curious.


And HelplessHandyman, there's nothing dumb about that question at all. Actually, it shows you're thinking safety. I can't tell you how often I come across improperly wired multiwire branch circuits in new construction...Happens all the time, and it is a BIG deal.

Builders often ask me why I spend so much time tugging on the black and red wires of the multiwire circuits during inspections (determining what two circuits share the neutral), and they always regret doing so because asking that question earns them a 3 minute lesson so they understand what I'm checking and why I'm checking it. Some listen, some don't care...But I've given that speech a hundred times.
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Old 12-27-2008, 10:01 AM   #8
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Dumb Neutral wire question, just curious.


Thanks to allthekctermite , thanks, so your saying I should use a double pole breaker? I have a Square D Panel# QOC30UF 30 circuit panel. I am just finishing the laundry room, and I ran 12/3 to a junction box 20 amp circuits. One circuit will do the washer, and the other the dryer (gas). Just want to be on the safe side.
Thanks Again for all your time you put into this forum. Happy Holidays to you and your Family.
Or should I put one circuit on one side, and the other on the opposite?
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Old 12-27-2008, 10:21 AM   #9
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Dumb Neutral wire question, just curious.


Be sure you are using a double pole breaker that takes up two slots and not a tandem or mini breaker that only takes up one slot. Also measure between the black and red wires. You should get 240 volts if installed properly. In some panels it is possible to install the double pole breakers with both poles on the same leg.
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Old 12-27-2008, 11:35 AM   #10
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Dumb Neutral wire question, just curious.


Definately true. There's no one size fits all answer to the double pole breaker question because panels vary. A meter is the way to do it! Most modern panels bussbars go

A B
B A
A B
B A
A B
B A

Some panels will go

A B
A B
A B
A B
A B

So you see, in the last example a double pole breaker would connect both phase conductors to the A or B phase, and that ain't good.

The only way to know for sure is to pull the breakers out and look at the orientation of the bussbars to determine how they lay out.

Thanks for your kind words by the way.
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Old 12-27-2008, 11:46 AM   #11
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Dumb Neutral wire question, just curious.


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Originally Posted by thekctermite View Post
Definately true. There's no one size fits all answer to the double pole breaker question because panels vary. A meter is the way to do it! Most modern panels bussbars go

A B
B A
A B
B A
A B
B A
Don't you mean

A A
B B
A A
B B
A B
B B

?
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Old 12-27-2008, 12:31 PM   #12
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Dumb Neutral wire question, just curious.


DOH! Yeah, that's right.
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Old 12-27-2008, 06:34 PM   #13
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Dumb Neutral wire question, just curious.


Okay, got it. I took out a few breakers and the panel goes :
A B
B A
A B
B A
A B
B A
Can I just use two seperate breakers? One under the other?
Thanks

Last edited by helpless handyman; 12-27-2008 at 06:37 PM.
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Old 12-27-2008, 07:42 PM   #14
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Dumb Neutral wire question, just curious.


Although I'm stuck in an older version of the NEC, it is my understanding that the 2008 NEC requires that the two breakers of the multiwire branch must be interconnected so that if one trips the other trips too. So no, two separate breakers is no longer code-accepted if your city is enforcing the newest NEC code.
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Old 12-28-2008, 12:48 PM   #15
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Dumb Neutral wire question, just curious.


Got it, I will go to the Electrical Supply House tomorrow and get that double pole breaker. I want to did it once and do it right.

Thanks thekctermite for all the help

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