2 computer circuits sharing a neutral

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I agree with the others, run a 12/3. If it's single phase why were you concerned about a MWBC?
 
steelersman said:
Ok this is something new to me. I now can understand this concept, however, what dereckbc said makes it sound like you will only reduce the voltage drop if you use the shared neutral on single phase. I don't see how this would make any difference. Another words this concept should also work on 3 phase just the same.
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If you run two hots and two neutrals from the panel to the loads you have 4xLength in conductors. If you run a MWBC, you have only three, so your voltage drop is limted to 3xLength in conductors.

Step up to three phase, and if you run 3 hots and 3 noodles, you have 6xLength, but with a MWBC, you only have 4xLength.

The diagram Bob posted makes no reference to single- or three-phase. The voltage drop would be the same.
 
480sparky said:
If you run two hots and two neutrals from the panel to the loads you have 4xLength in conductors. If you run a MWBC, you have only three, so your voltage drop is limited to 3xLength in conductors.

Step up to three phase, and if you run 3 hots and 3 noodles, you have 6xLength, but with a MWBC, you only have 4xLength.
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I don't think length is the story, load is. Look at the load on the neutral in Ed's diagrams.
 
iwire said:
I don't think length is the story, load is. Look at the load on the neutral in Ed's diagrams.
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It's not the whole story. Just half of it. Instead of I of both loads being run over two seperate wires, the difference between them is run over only one.
 
infinity said:
I agree with the others, run a 12/3. If it's single phase why were you concerned about a MWBC?
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why would he be concerned if it were 3 phase instead of single phase? Am I missing something here? Is it not acceptable to run 12-3 and share the neutral if it were a 3 phase panel? Wouldn't the 2 circuits still cancel each other out on the neutral assuming they are identical loads.

Don't know why I'm asking. I know the answer to be yes. It shouldn't matter if he's on 1 or 3 phase. As long as the 2 circuits are on seperate legs then they are subtractive, not additive.
 
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480sparky said:
It's not the whole story. Just half of it. Instead of I of both loads being run over two seperate wires, the difference between them is run over only one.
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We will remain in disagreement on this one, when you calculate the voltage drop you will use the 2x the circuit length, not 3x, 4x or 6x.
 
steelersman said:
why would he be concerned if it were 3 phase instead of single phase? Am I missing something here? Is it not acceptable to run 12-3 and share the neutral if it were a 3 phase panel? Wouldn't the 2 circuits still cancel each other out on the neutral assuming they are identical loads.

Don't know why I'm asking. I know the answer to be yes. It shouldn't matter if he's on 1 or 3 phase. As long as the 2 circuits are on seperate legs then they are subtractive, not additive.
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Harmonic currents will not be a concern in a single phase system but would be in a 3 phase Wye system. He said he's not a big fan of computers sharing a neutral I was just asking why?
 
steelersman said:
why would he be concerned if it were 3 phase instead of single phase?
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Because with 3 phase systems combined with non-linear loads harmonics may become an issue for the shared neutral.

With single phase that is not a concern. Personally I would not worry about the harmonics as most circuits feeding non-linear loads are designed very lightly loaded.
 
iwire said:
We will remain in disagreement on this one, when you calculate the voltage drop you will use the 2x the circuit length, not 3x, 4x or 6x.
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I don't think we're disagreeing, Bob. We're just using a different angle of attack on the same problem.

I'm using field artillery, and you've mounted an air strike. Taget eliminated just the same.
 
Only thing wrong with the diagram is that in the shared neutral version the arrows for the direction of flow are the wrong direction for the neutral and the 60 amp circuit.
 
steelersman said:
Only thing wrong with the diagram is that in the shared neutral version the arrows for the direction of flow are the wrong direction for the neutral and the 60 amp circuit.
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Actually if the circuit was DC it is drawn correctly.

If it's AC then the arrows should point both ways or not even be there.
 
iwire said:
I think your field artillery is firing blanks. :D
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I actually agree with 480 on this one. It does have to do with the one fewer conductor or neutral, but I can see your outlook on it as well. They are inverse opinions of each other. Both being cause and effect on one another. You can't have one without the other. :)
 
iwire said:
I think your field artillery is firing blanks. :D
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Not at all.

Take Ed's diagram, with 40amps on one circuit, and 60 on the other. Let's say the length of the circuit, from panel to load, is 100 feet. Vd with two seperate circuits would be a result of 40 amps over 100 feet plus 60 amps over 100 feet. Last time I went to school 100 feet plus 100 feet is 200 feet.

With a MWBC, you are only putting 20 amps (the difference between 60 and 40 [last I went to school ;)]) down 100 feet.

If you still say we disagree, then I'd say your planes are grounded instead. :grin:
 
480sparky said:
If you still say we disagree, then I'd say your planes are grounded instead. :grin:
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We still disagree and this is my plane. :D

SR-71A_04.jpg
 
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