neutral size in 3 phase multiwire circuit

[paul]

I have always been told to increase my grounded conductor in non-linear multiwire circuits to 1 wire size bigger than the 3 ungrounded. I understand the reason, but my question is where in the code does this "1 wire size rule of thumb" practice stem from? (why is it not a requirement)?

What determines the maximum possible harmonic current that could amass on the ungrounded?

How could this be planned for in the rough-in stage to ensure that 1 wire size bigger is sufficient?

If the load served is nonlinear and only a 2 wire circuit, could this circuit's grounded conductor be affected?

What about a 3 phase multiwire circuit involving only 2 phases?

Do harmonics enter into 3 phase delta configurations or are they unique to wye?

OK, I have too many questions. I think I have been around the young-ens too long!!! Sorry, but I have always wondered these things and don't mess with 3 phase enough to experiment.

[Homer]

Give these documents a read. They provide a good overview of the theory of harmonics in 3-phase systems.

They are written in Europe, so be careful when you read statements about "upsizing the breaker for the neutral pole". :eek:

Harmonics In Practice (http://www.cda.org.uk/megab2/elecapps/pub145lo.pdf)

Power Quality Application Guide (http://www.cda.org.uk/megab2/elecapps/pub503/3_5_1_lo.pdf)

The maximum sum of the harmonics is limited by the mathematics of the summation.

If you have a multiwire circuit with only 2 out of the 3 possible branches, you can still have harmonic addition, it's just not as bad.

If you have non-linear loads on 2-wire branch circuits, the neutral that feeds those two or three 2-wire branch circuits will have harmonic addition. The actual 2-wire branch circuits will just have normal expected current flow.

This phenomenon is characteristic of WYE connections as they provide a pathway for the harmonic components to flow.

Homer

[Wgoodrich]

While increasing the neutral size when major nonlinear loads are on a circuit is not required by the NEC rules, it is a very good practice for increased safety.

There is a rule in 220.22 that forbids a reduced neutral size on a neutral service conductor when that service has a major part of its load to be nonlinear loads.

There are many FPN notes in the code that addresses the concerns of added harmonic heating concern when dealing with nonlinear loads. This is a major cocern and is being approached more and more each new Code version coming out.

The NEC does demand that any neutral conductor carrying nonlinear loads be considered as current carrying conductors same as hot conductors.

You will be hearing over the years more and more about harmonic heating concerns in my prodictions of the future of the electrical field.

Hope this helps

Wg

[paul]

I have been browsing the internet on this and not able to find much in the line of practice, just theory. Thanks for the info!