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Old 02-14-2012, 12:16 PM   #1
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Convert 12/3 to two 12/2s


Took a wall down, so now rerouting the electrical. They had run a 12/3 cable, and in the panel black went to a 20a circuit and the red went to another 20a circuit. So, I think I get it, a quick easy way to run two circuits with just one cable.

But when I rerouted the cable, it is too short to make it back to the panel. Can I splice in two 12/2s wired like this:

Black on cable 1 to black on 12/3
Black on cable 2 to red on 12/3
All Whites connected together
All grounds connected together

Then bring them into the panel as two circuits, with the black from one going to the circuit that was red, and the other black going to the circuit that was black?

Logically, this makes sense to me, but would like to hear from the pros!

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Old 02-14-2012, 12:47 PM   #2
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Convert 12/3 to two 12/2s


No you can not. You are breaking two rule at least. That would be running two neutrals wires in parallel. Parallel wires are not permitted for size 6 and under. Also all conductors must be in the same cable.

You can do it the other way by splitting them apart after the 12/3. But you can not run two circuits and merge them together later on.

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Old 02-14-2012, 01:47 PM   #3
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Convert 12/3 to two 12/2s


Buy some 12/3 and do it right (and easier)
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Old 02-14-2012, 02:01 PM   #4
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by dougp23 View Post
They had run a 12/3 cable, and in the panel black went to a 20a circuit and the red went to another 20a circuit. So, I think I get it, a quick easy way to run two circuits with just one cable.
It is called a multi-wire branch circuit

As long as each of your hots from the 12/3 are attached to a breaker on separate buses of the panel...meaning the breakers are put in next to each other(like photo below)..... the 2 hots in the 12/3 can share the same neutral. You may have to make sure that the breakers are tied together with a handle-tie.

But I still cannot understand why you would even consider using two lengths of 12/2 when you can use only one length of 12/3.

And I'll add a part to what joed said: Also all conductorsTHAT SHARE THE SAME NEUTRAL must be in the same cable.(OR RACEWAY)
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Last edited by hammerlane; 02-14-2012 at 02:07 PM.
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Old 02-14-2012, 02:06 PM   #5
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Convert 12/3 to two 12/2s


Because he has some 12/2 wire he can use. He wants to finish the job with supplies that he has, rather than go to the store and buy some 12/3.
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Old 02-14-2012, 03:08 PM   #6
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by gilbo125 View Post
Because he has some 12/2 wire he can use. He wants to finish the job with supplies that he has, rather than go to the store and buy some 12/3.
So you think money has something to do with it??? No way.
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Old 02-14-2012, 07:00 PM   #7
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by gilbo125 View Post
Because he has some 12/2 wire he can use. He wants to finish the job with supplies that he has, rather than go to the store and buy some 12/3.
Guilty as charged.

Picked up some 12/3 tonight.

As an electrical engineer (which really has very little to do with residential wiring), my logic says that since the entire neutral bar is just one connection, it wouldn't matter that I would have two different locations where the same ground would attach.

I think that was probably my disconnect. But you're all correct: spend a little money and do it right, don't be so lazy, etc.

Thanks everyone for not nuke-ing me at least!
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Old 02-14-2012, 07:05 PM   #8
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Convert 12/3 to two 12/2s


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Originally Posted by hammerlane View Post
It is called a multi-wire branch circuit
So, I need some clarification, because I'm a true DIY'er, and electrical is not my area of expertise.

This circuit leaves the electrical panel, red is carrying 20a for Circuit 1, and black is carrying 20a for circuiit 2, the neutral supports both of them. If when they hit the first stop, Circuit 1 goes left towards bedroom1, and circuit 2 goes right towards bedroom2, how do you extend the neutral? and if a short or something occurs on circuit 2, doesn't it sort of run down all of circuit 1 as well (as well as into the panel)?

Not being a smarta**, just kind of confused on how that works!

There was no tie between breakers, this was two distinct and separate 20a breakers.
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Old 02-15-2012, 02:48 AM   #9
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by dougp23

So, I need some clarification, because I'm a true DIY'er, and electrical is not my area of expertise.

This circuit leaves the electrical panel, red is carrying 20a for Circuit 1, and black is carrying 20a for circuiit 2, the neutral supports both of them. If when they hit the first stop, Circuit 1 goes left towards bedroom1, and circuit 2 goes right towards bedroom2, how do you extend the neutral? and if a short or something occurs on circuit 2, doesn't it sort of run down all of circuit 1 as well (as well as into the panel)?

Not being a smarta**, just kind of confused on how that works!

There was no tie between breakers, this was two distinct and separate 20a breakers.
Now the nec requires a handle tie or double pole breaker for all mwbc
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Old 02-15-2012, 07:06 AM   #10
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by dougp23 View Post
So, I need some clarification, because I'm a true DIY'er, and electrical is not my area of expertise.

This circuit leaves the electrical panel, red is carrying 20a for Circuit 1, and black is carrying 20a for circuiit 2, the neutral supports both of them. If when they hit the first stop, Circuit 1 goes left towards bedroom1, and circuit 2 goes right towards bedroom2, how do you extend the neutral? and if a short or something occurs on circuit 2, doesn't it sort of run down all of circuit 1 as well (as well as into the panel)?
At the first stop as you call it there most likely is a junction box correct?
In this J-box the 12/3 would be cut.
So now you have 3 conductors red, black, white.
Now added to the J-box are two separate 12/2 cables.
Each of the 12/2's have black and white conductors.

First all 3 whites get wire nutted together. Neutrals are intact.

The red wire from the 12/3 for your circuit #1 gets wire nutted to one of the blacks from one of the new 12/2 cables.

The black wire from the 12/3 for your circuit #2 gets wire nutted to the black of the remaining 12/2 cable.


Each of the 12/2 cables then continue to their destinations.

Like the photo below. Imagine one of the cables is 12/3, forgot the crossed out one is there and the other two cables are 12/2 going to separate bedrooms or whattever

in all descriptions above of course you would have your bare grounds also
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Convert 12/3 to two 12/2s-dsc01711.jpg  

Last edited by hammerlane; 02-15-2012 at 07:13 AM.
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Old 02-15-2012, 08:16 AM   #11
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Convert 12/3 to two 12/2s


Another way of saying it is that there may be only one neutral path from any point "out in the field" back to the panel. (And the hot wire(s) associated with that neutral must accompany that neutral.)

Quick summary of your other question: If the red (of the multiwire branch circuit) goes one way and the black goes another way, then yes, the neutral may (must) branch so it can accompany both.

Downstream of that branch point the two subcircuits (one hot wire in each) are no longer treated as MWBC's. Should the two subcircuits (or any two different circuits) per chance come together again downstream into the same box (allowed), the neutrals may not be interconnected there.
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Last edited by AllanJ; 02-15-2012 at 08:20 AM.
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Old 02-15-2012, 09:00 AM   #12
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by AllanJ View Post
Should the two subcircuits (or any two different circuits) per chance come together again downstream into the same box (allowed), the neutrals may not be interconnected there.
Good point out.
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Old 02-15-2012, 10:49 AM   #13
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Convert 12/3 to two 12/2s


Quote:
Originally Posted by hammerlane View Post
At the first stop as you call it there most likely

First all 3 whites get wire nutted together. Neutrals are intact.

The red wire from the 12/3 for your circuit #1 gets wire nutted to one of the blacks from one of the new 12/2 cables.

The black wire from the 12/3 for your circuit #2 gets wire nutted to the black of the remaining 12/2 cable.


Each of the 12/2 cables then continue to their destinations.

Hammerlane, that's not what he's asking. His question involved two pieces of 12/2 coming from the panel and splicing with the 12/3 in a jb, which we already established is not allowed.
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Old 02-15-2012, 11:58 AM   #14
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Convert 12/3 to two 12/2s


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Originally Posted by hawkeye11 View Post
Hammerlane, that's not what he's asking. His question involved two pieces of 12/2 coming from the panel and splicing with the 12/3 in a jb, which we already established is not allowed.
hawkeye: yes two 12/2s joining with a 12/3 has been established is not allowed, but I was answering his question in post #9 where I think the poster is asking about a MWBC.

I think the poster is asking about a MWBC because:
1. The poster quotes it, namely: It is called a multi-wire branch circuit.
2. Further in post #9 he states: This circuit leaves the electrical panel, red is carrying 20a for Circuit 1, and black is carrying 20a for circuiit 2, the neutral supports both of them.

So I think he is talking about a MWBC.

I may be wrong in my interpretation of what the poster is asking in post #9 or you may be wrong. That is the problem when people use terms like This circuit, that wire, this wire, the box , the wire. One never knows if the wire I am thinkikng of is the same wire the poster is referring to.

Last edited by hammerlane; 02-15-2012 at 12:14 PM.
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Old 02-15-2012, 12:13 PM   #15
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Convert 12/3 to two 12/2s


Oh yeah, just an additional question asked. Gotcha! My bad..

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