Sharing neutrals

[millvalemurph]

Is it true that in three phase services that I can share a neutral between 1,3, and 5. or share a neutral between two curcuits from the same phase. For example 1 and 7.

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[electric-ed]

Yes, it is true that three branch circuits can share a common neutral on a 3 phase system if -
1. they all feed from different phases, one red, one black, and one blue. Two circuits from the same phase CANNOT share the same neutral.
2. and, if the loads supplied by the circuits do not consist of electronic switching power supplies containing HARMONICS.
This type of non-linear load - computers, printers, photocopiers, soft-starters, variable-frequency motor drives and some electronic ballasts, do not cancel like linear loads do, and can cause overloaded (overheated) neutrals and other problems.
If this type of loads exist it is recommended that a separate neutral be pulled for each branch circuit (don't get them mixed up) or a double (200%) sized neutral installed.

Ed

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[millvalemurph]

thanks ed. Several Journeyman at my work feel that sharing a neutral between two branch curcuits feed from the same phase is premitted. One would think that all the load from this type of set up would be unbalanced. Clearly there is some confusion about shared neutrals. Rule 4-024 seems to address the topic in terms of feeds. where are the rules for the use of shared neutrals in branch services spelled out. Or do you have to compile them from the other rules such as ampacity. I know I'll have to be able to show them in the code that they can not share a neutral between two branches from the same phase.

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[bs3true]

I don't know where in the code that the rule governing current on a neutral is, but electrical theory is what really governs this. Since the sine-wave of the AC on each phase is 120 degrees out of phase with the current on each of the other phases, the current flowing on the neutral conductor is only one-third of the total flowing on the red or black or blue leads (if the current thru each phase was equal). There is a kind of current cancelling effect, so long as the current fed thru the neutral is from all 3 phases together. Feeding two branches from one of the phases unbalances (and increases) the current flowing in the neutral. This is more easily explained if you look at the 2-wire (red & back) 2-phase feed to a split-outlet in a residential kitchen. In this case, each phase is 180 degrees out of phase, with the result that the current on the neutral would be zero amps, if the loads (& current flow) on each phase are equal. If there is a 15 Amp draw thru one phase, but nothing on the other, a full 15 would be measured flowing in the neutral. As a load was added to the second phase, the current in the neutral would decrease, as the current from one phase would cancel out the current from the other phase. The same is true for a 3-phase setup, except the cancellation effect is not as great becuase the difference between each phase is not 180 degrees, it is only 120 degrees.

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Dave T

[millvalemurph]

Thanks Dave!
I explained the theory to some off the older journeyman on our job site and they grudenly agreed. I know now much I can learn from these guys so one has to step softly. You did a better job of explaining it than I did so I may borrow your explaintion. Thanks again. MURPH

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[electric-ed]

This sketch might help with the shared neutral. This is an example of a single phase, three-wire circuit, but a three phase, four wire circuit works the same way, except that the THREE phase currents would have to be subtracted vectorially to calculate the neutral current.



Ed

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