Neutral Current
- Thread starter JdoubleU
- Start date
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- Massachusetts
Do you know that we can take two 120 volt lamps wired in series and supply those with 240?
Hold on, let me just direct you to a good demonstration
Hold on, let me just direct you to a good demonstration
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Jake, see if this helps: http://forums.mikeholt.com/showpost.php?p=708650&postcount=4
That was really helpful thank you. I still having problems understanding how the currents cancel out. I know this is a stupid question but where does that current go? If you have 3 amps on phase A going back and forth 60cycle a second and you share a neutral with phase b and it has 3 amp they cancel out. The canceling part I can visualize.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Let me give it a try.
Consider a 120/240 panel. Only two breakers are in use, and they set up as a multiwire branch circuit. Three wires (Phase A, Phase B, and a shared neutral - the EGC is not a player in this discussion) go out to a room that has two identical overhead lights, controlled by separate switches. Both breakers are on, both switches are off, so no current is flowing.
Turn on switch A. On the positive half of each cycle, current flows out from the panel along the Phase A conductor, through switch A and light A, and back to the panel on the shared neutral. On the negative half of each cycle, current flows current flows out from the panel along the shared neutral, flows "backwards" through light A and switch A, and back to the panel on the Phase A conductor. This pattern repeats 60 times per second. Now turn off switch A. No current is flowing.
Turn on switch B. On one half cycle, current flows out from the panel along the Phase B conductor, through switch B and light B, and back to the panel on the shared neutral, and the current reverses direction on the other half cycle.
Keep switch B on, and turn on switch A. Taking measurements at the panel, you will observe that the Phase B current has not changed, that the Phase A current is the same value as the Phase B current, and the shared neutral has no current. This is because the Phase A current will be on its positive half cycle at the same moment that the Phase B current is on its negative half cycle. Here is what is happening on one half cycle:
? For light A, current flows out from the panel along the Phase A conductor, and returns to the panel along the shared neutral.
? For light B, current flows out from the panel along the shared neutral, and returns to the panel via the Phase B conductor.
? Thus, the shared neutral sees two currents, one going into the panel and the other going away from the panel, and these two currents cancel each other out.
? In a practical sense, then, current is actually flowing out from the panel via the Phase A conductor, through switch A and light A, then “backwards” through light B and switch B, and back to the panel via the Phase B conductor.
? On the other half cycle, the pattern reverses, and still the shared neutral wire sees no current.
Consider a 120/240 panel. Only two breakers are in use, and they set up as a multiwire branch circuit. Three wires (Phase A, Phase B, and a shared neutral - the EGC is not a player in this discussion) go out to a room that has two identical overhead lights, controlled by separate switches. Both breakers are on, both switches are off, so no current is flowing.
Turn on switch A. On the positive half of each cycle, current flows out from the panel along the Phase A conductor, through switch A and light A, and back to the panel on the shared neutral. On the negative half of each cycle, current flows current flows out from the panel along the shared neutral, flows "backwards" through light A and switch A, and back to the panel on the Phase A conductor. This pattern repeats 60 times per second. Now turn off switch A. No current is flowing.
Turn on switch B. On one half cycle, current flows out from the panel along the Phase B conductor, through switch B and light B, and back to the panel on the shared neutral, and the current reverses direction on the other half cycle.
Keep switch B on, and turn on switch A. Taking measurements at the panel, you will observe that the Phase B current has not changed, that the Phase A current is the same value as the Phase B current, and the shared neutral has no current. This is because the Phase A current will be on its positive half cycle at the same moment that the Phase B current is on its negative half cycle. Here is what is happening on one half cycle:
? For light A, current flows out from the panel along the Phase A conductor, and returns to the panel along the shared neutral.
? For light B, current flows out from the panel along the shared neutral, and returns to the panel via the Phase B conductor.
? Thus, the shared neutral sees two currents, one going into the panel and the other going away from the panel, and these two currents cancel each other out.
? In a practical sense, then, current is actually flowing out from the panel via the Phase A conductor, through switch A and light A, then “backwards” through light B and switch B, and back to the panel via the Phase B conductor.
? On the other half cycle, the pattern reverses, and still the shared neutral wire sees no current.
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Instead of thinking that the currents cancel, think that there's no current even trying to flow on the neutral. As Charlie said at the end of his last post, the two loads are effectively in series across the 240v, each seeing half of that voltage.
If you were to open a shared neutral, and the voltage across the gap would be zero, then no current will flow when you close the gap. If there would be a voltage, the neutral brings that back to (or at least, toward) zero by carrying some current.
If phase A has 4a on it, and phase B has 6a, the neutral carries 2a. If you open the neutral, phase A's load would see about 60% of the line-to-line voltage, and phase B's load would see about 40% of it. (The larger load has lower resistance.)
The currents do not cancel out. In fact it is not even plural. It is the same current passing through both loads.
The current across load 'a' from Line A, travels to the neutral junction then on tthrough load 'b' to Line B. This is a balanced condition.
An unbalanced condition is when one load (or load bank) has either more or less current across it than the other. The same amount flows through both loads as the lesser of the two, while the 'difference' amount travels through the neutral conductor. An example is 6A and 4A loads. The circuit has 4A traveling through both loads, while the unbalanced 2A (6A - 4A) returns to the source on the neutral conductor.
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