Unbalanced Current

A

Alwayslearningelec

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What is the unbalanced current?


A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(C)(1).
 
In an MWBC, the two hot lines are required to be from opposite poles of a two pole breaker, sharing a common neutral wire. That way, the neutral current on either line occurs at the opposite sequence from the other and they don’t combine. The neutral currents may not be equal, but they are not happening at the same time on the wire.
 
Alwayslearningelec said:
Well that doesn't:) help me understand.
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In a two wire 120V circuit the line and neutral currents are the same. In a three wire 240V circuit the neutral current is the difference between the L1 and L2 currents because the line currents are inversions of each other and send current to the neutral that are "180 degrees out of phase" from each other, and they cancel so that if L1 and L2 currents are equal, the neutral current is zero.
 
Going from the opposite perspective: the balanced current is the current that is matched in the two hot legs, so that the same current flows from hot 1 through load 1 then through load 2 then to hot 2. The unbalanced current is any current in either load 1 or 2 that isn't matched by corresponding current in the other load.

The neutral is there because load 1 and load 2 are rarely perfectly matched, so you have both balanced and unbalanced current in an MWBC, and you need a path for the unbalanced current.
 
roger said:
View attachment 2578885
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IIRC, the sum of all current vectors = 0. Normalize the graph with right is pos, left is neg vector.

I think this graph confuses many with the "notice equal current flowing both directions". That's not really the case. At any given time the current vector in the common leg will be neg, zero, or pos. Vector addition agrees with Xneg + Xpos = 0, but at no time is there actual amps in the wire, hence why evaluating each leg ckt separately tells us a lie from that view. When the common is zero, the 1st C in CCC is nill, no heat..
 
Alwayslearningelec said:
What is the unbalanced current?


A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(C)(1).
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Essentially, the idea of this rule, is that if it is possible for there to be zero current on the neutral, when all ungrounded conductors run at maximum capacity, then you do not need to count the neutral. Consider all possibilities of how current is distributed among all conductors that could carry it. The sum of heat generated by all conductors including the neutral, will never exceed what it would be, if the ungrounded conductors run at full load. This assumed no harmonic distortion, which is another issue to consider later.

As a split-phase example, consider a 100A feeder, with 100A on the black line, and 60A on the red line. 60% of the amps on the black line are carried back on the red line that is 180 degrees out of phase with it. The remaining 40A return on the neutral. Consider each milliohm of wire resistance and length for each phase and each neutral. This means that the heating is (100^2 + 60^2 + 40^2)*0.001 = 15.2 Watts of ohmic losses. Had both red and black lines carried 100A, with nothing on the neutral, we'd get (100^2 + 100^2)*0.001 = 20 Watts. No matter what the imbalance is, we'd never get more than 20 W of heat generated.

Similar example, but 3-phase, with 100A on phase A, 75A on phase B, and 60A on phase C. Corresponding neutral current = sqrt(SOS - SOP) = sqrt(100^2 + 75^2 + 60^2 - (100*75 + 75*60 + 100*60)) = 35A. Total heating per milliohm of 1-way length, is 20.45 Watts. For a balanced system with 100A on all 3 phases and no neutral current, the heating is 30 W. No matter what imbalance you try, you'll get less than 30 Watts in this situation.

Where neutral is required to count as a CCC:
1. If neutral is a mandatory part of the return path, such that it isn't possible to have zero current on it at full load, even with balancing. Examples being individual phase-to-neutral loads, and two phases + neutral derived from a 3-phase wye system. The neutral is mandatory to allow the current to add up (as vectors) to zero, as is required per Kirchhoff's laws.
2. Harmonic intensive loads, particularly triplen harmonics (multiples of 180 Hz in the US), where currents accumulate instead of cancel on the neutral. Less of an issue, but still a requirement to consider for non-linear loads, where current is a different wave shape than the voltage.
 
I am surprised how such a simple concept is made to complicated by people trying to make it even simpler.

Back in school we learn Kirckoffs Current Law which effectively says current entering a point (node) must be equal to the current leaving a point.

I am not convinced this confusion is not in due in part to the focus on using the neutral as the reference for voltages. Look at the current path through the transformers windings in post #12, they are in phase not 180° apart.
 
jim dungar said:
Look at the current path through the transformers windings in post #12, they are in phase not 180° apart.
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They are 180 degrees apart, if you look from the point of view of the wires outside the transformer.

I'm aware that they aren't really produced by a 180 degree phase delay, but the two waveforms are mathematically equivalent to having a 180 degree phase delay between them.
 
Carultch said:
They are 180 degrees apart, if you look from the point of view of the wires outside the transformer.

I'm aware that they aren't really produced by a 180 degree phase delay, but the two waveforms are mathematically equivalent to having a 180 degree phase delay between them.
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There is no reason to have to switch points of view.
Not all electrical circuits involve a neutral conductor.

I have no intention of starting at discussion about 'in phase' or ' out of phase'.
My point was the analysis of the diagram shown in post 12 and Kirchhoff's Current Law (KCL).

Edited spelling of Kirchhoff.
 
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jim dungar said:
There is no reason to gave to switch points of view.
Not all electrical circuits involve a neutral conductor.

I have no intention of starting at discussion about 'in phase' or ' out of phase'.
My point was the analysis of the diagram shown in post 12 and Kirchioff's Current Law (KCL).
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Post #7's diagram on the left, shows two waveforms, 180 degrees out of phase from each other. That's what you get when you assign outbound current as positive, and inbound current as negative, and put the L1 & L2 currents together on the same plot. Sorry if I see it from a different point of view from how you see it.
 
throw this in to complicate calculations: If any of the loads are non-linear (e.g rectifiers, etc) you need to consider the harmonics, especially the tripletts.
 
ggunn said:
In a two wire 120V circuit the line and neutral currents are the same. In a three wire 240V circuit the neutral current is the difference between the L1 and L2 currents because the line currents are inversions of each other and send current to the neutral that are "180 degrees out of phase" from each other, and they cancel so that if L1 and L2 currents are equal, the neutral current is zero.
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I'm glad you put the 180 degrees part in quotes. A better way of expressing it would be 'of opposite polarities at any point in time'.

And yes, the two voltages being of opposite polarities are indistinguishable from a 180-degree phase shift FOR A PURESINE WAVE.

I see others have taken up the case further down the page, so I'll quit here.
 
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