multiwire circuit

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JdoubleU

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Just trying to fully understand what I think I allready know. In a muiltiwire circuit, if phase A is pulling 5 amps and phase B is pulling 8 amps then the neutral current will have 3 amps on it. If in a multiwire circuit you are shareing a neutral with two circuits on the same phase, the current adds. For example, if phase A is pulling 10 amps and phase A is pulling 10 amps your neutral current will have 20 amps on it. If a single branch circuit with its own neutral is pulling 10 amps then the neutral current will have 10 amps. Is this all correct.
 

charlie b

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If we restrict the discussion to single phase 120/240 volt systems, you are entirely correct. If we talk of three phase systems, with phase C carrying no current in any of your examples, then only your second two statements are correct. As to the first statement, if the A, B, and C currents are 5, 8, and 0, then the neutral current would be 7.
 

JdoubleU

Senior Member
Thank you for the reply. How did you come with that on three phase? And how does it add up if for example, you have 3 circuits shareing a neutral in a three phase system and two of the phases are the same.
 

charlie b

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The neutral in a three phase system will carry current that is equal to the square root of (A^2 + B^2 + C^2 - AB - AC - BC). In your example, A = 5, B = 8, and C = 0. So,

N = SQRT(5x5 + 8x8 + 0x0 - 5x8 - 5x0 - 8x0).
N = SQRT(25 + 64 - 40).
N = SQRT(49).
N = 7.
 

Smart $

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Jakewhis said:
...And how does it add up if for example, you have 3 circuits shareing a neutral in a three phase system and two of the phases are the same.
The amperage of the two same-phase circuits are added together then you'd plug the result into the formula that c.b. demonstrated.

I should add that such a 4-wire circuit is atypical.
 

charlie b

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Jakewhis said:
And how does it add up if for example, you have 3 circuits sharing a neutral in a three phase system and two of the phases are the same.
Until I read Smart$’s reply, I understood this question differently. Now I see two ways that you might have intended the question.

I thought you meant that your circuit used all three phases, and that the current on two of the phases was the same. The formula I demonstrated would give that answer.

Now I suspect that you are asking about using Phase A, Phase A, and Phase B as the three circuits sharing a neutral. If that was your question, then Smart$’s answer was mathematically correct. But I must add that that would be a code violation! 210.4 allows us to use “multi-wire branch circuits.” The definition of that term, in article 100, explicitly requires that there be a voltage between the ungrounded conductors, and the same voltage from the grounded to each of the ungrounded. Therefore, you can’t have a MWBC with 3-phases, unless you use all three.
 

Chamuit

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Clarification . . .

Clarification . . .

Originally Posted by Tiger Electrical
A minor additional comment is that in single phase there isn't an A Phase and a B Phase.

Dave

What about 408.3(E) where it talks about phase "A, "B", and "C"? Is it or would it be more proper to say L1, L2 and L3?
 

George Stolz

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Chamuit said:
What about 408.3(E) where it talks about phase "A, "B", and "C"? Is it or would it be more proper to say L1, L2 and L3?
No, it's correct with 3?.

408.3(E) Phase Arrangement. The phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard or panelboard. The B phase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. Other busbar arrangements shall be permitted for additions to existing installations and shall be marked.

Three phase panels have three phases, whereas single phase panels are a single phase split in half for half the voltage from line to neutral.
 
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