Neutral Current

bmadhu srao; the main panel is balanced almost perfect. your lighting circuits are unbalance, try switching off & on different loads & watch the neutral current change. perry

<...why is there a difference in neutral currents in main and its distribution panel...>

Because other single phase loads elsewhere on the system downstream of the distribution board are contributing currents that reduce the neutral current on the main feeder.

<...what is the maximum value of neutral current allowed in a 3phase circuit...>

If there were three phases, say, red, yellow and blue, and there were a single phase load connected to the red and no loads connected to the yellow or blue, then the neutral current would be equal and opposite to the red phase current. It can never be greater than this value.

I think that it is simply single phase loads that all draw their current from a phase and neutral.....so having current on neutral on single phase loads is normal. the current will be the sum total of all the single phase currents.....

If you have 3 single phase loads, one on each phas, each taking 10 amps per phase, you will have 30 amps running through the neutral.....

If the loads are only 3 phase, with no neutral connection, then something is drawing unbalanced currents I believe or a short on a phase possibly.........but I really feel that it is the single phase loads that are causing the current, then it is fully correct.....

Actually, if you have 3 equal single phase loads, one on each phase, it will be balanced on neutral, and no current flow on neutral.

Sorry, but how can that be?

Are you maybe confusing the neutral line with the neutral point where they all come together?

Here is a simple diagram that might help you:-

The red blobs are the loads, connected between each phase and neutral, located in 3 different rooms in a building. The Earth is connected to the frames of the machines simulated by the red blobs.....

Each circuit takes say 10 amps, what current will you measure at points "a", "b" and "c" in the neutral lines running to each room?

Next question:-

If one load takes 10, the next 20 and the last 30 amps, what will you amps will you measure at points "a", "b" and "c"?

In this case, the neutral point will have a voltage on it due to the unbalanced loads.......how much will depend upon the distance the cables are run, the resistance of the cable runs, the difference in the loads etc etc etc....

Therefore it makes a massive difference WHERE the currents are measured....

Does this help? Or do you need further assistance?

Andy, it should be explained to the questioner that when a multiple phase system shares a neutral conductor, the currents flow from phase to phase through the single phase loads as much as possible. The neutral conductor carries only the difference between the two or three phase currents, and is therefore a great indicator of the system balance.

True, when the branch circuit neutral serves only one conductor, the line and neutral currents are the same, as you have drawn. The question was about the neutral measurement at the panel, the neutral bus measurement there will reflect the difference between the phase currents only, being, as you have pointed out, "the neutral point where they all come together."

For the questioner, it is very normal for the neutral current values to differ between the main panel and the individual sub-panels, since the balance of the phase currents in each sub-panel will reflect in the neutral line feeding the point where all of the neutrals meet in each panel. Lighting panels are especially difficult to balance within the lighting variations possible in many facilities, however the over-all load balance may be quite good, as reflected at your main panel neutral.

This is where paying attention to the service load patterns of the various lighting loads will be helpful, but again, this is usually the hardest panel to balance. Having lighting that uses line-to-line voltages can be helpful, but may not always be possible.

regards, CJM

Guest is correct about the zero amps on the neutral; the currents flow between the phases across the neutral connection and not down the common neutral. Regardless of the mis-communication about the point of measurement, 3 different phases of ten amps each can never result in a measurement of thirty amps at any point on a neutral. In fact the highest measurement in this scenario is ten amps on one of the individual load neutrals.