Y nutral wire size for unbalanced loads.

[jmrockwell]

I have been told that in a 3 phase Y lighting panel that the nutral must be larger than the phase wires because of the possible imbalance of the loads. The worst unbalance I can think of would be to have one phase to nutral loaded and the other phases with no load!? That would provide the same current in the nutral as the phase. It seemes any other combination would be less in the nutral!? What am I missing?

 

[davidbeach]

Add in some 3rd harmonic.

 

[Skogsgurra]

N area > phase lead area is not common. But it probably should be. An office installation with lots of flourescent lights and lots of computers has a lot of third harmonic (triplenes) in the phase leads. Triplenes add in the Neutral and the sum of them can easily be more than current in each phase. Hot neutral leads are getting more and more common.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

One caveat:

In theory, if you have 100% 3rd harmonic load, the N current could approach 1.73 times the phase current in a balanced three phase system.

On the other hand, if the lighting panel is oversized to begin with you may not need to oversize the neutral of the panel or its feeder neutral.

For example, you have a 200A panel which feeds 80A of lighting load it may be a non-issue. I am yet to see a fully loaded lighting panel. But again, you need to make the decision based on your system conditions.

 

[BraJacTech]

I've never thought to question the "why" of it before, but 99% of industrial specs I've worked with for purchasing require 200% neutral in all panels.

Now it makes more sense. Thanks for the insight!