Non linear loads

[DavidFahey]

When calculating a non linear load what is the maximum unbalanced seen on the grounded ( neutral) conductor

Example:

A lighting load of 30 amps on all 3 phases would the unbalance load be 30 amps or additive to 90 amps?

 

[roger]

When calculating a non linear load what is the maximum unbalanced seen on the grounded ( neutral) conductor

Example:

A lighting load of 30 amps on all 3 phases would the unbalance load be 30 amps or additive to 90 amps?

If it were incandescent lighting it would be zero, discharge lighting would have some additive harmonics but probably not enough to worry about.

Roger

 

[wirenut1980]

It depends on the load and would probably need to be calculated. Need to look at how much of the load is non-linear, is the load a bunch of switch mode power supplies, flourescent lighting, variable speed drives?

Flourescent lighting and switch mode power supplies contain a lot of third harmonic and will add in the neutral.

 

[LarryFine]

When calculating a non linear load what is the maximum unbalanced seen on the grounded ( neutral) conductor

Example:

A lighting load of 30 amps on all 3 phases would the unbalance load be 30 amps or additive to 90 amps?

Well, it certainly wouldn't be 90a, and it most likely wouldn't be 30a. You'd really need to know the harmonic content of the loads, or measure the harmonic current.

You could always try the energize-it-and-feel-the-wire-after-one-hour method. Worst case is you need to use the #10 to pull in something bigger.

 

[steve66]

The unbalance could be anywhere from 0 amps (for loads with no harmonics at all) to 90 amps (for loads that are 100% 3rd harmonics) to anywhere in between.

Steve

 

[ron]

The argument is if the phase to neutral load was theoretically 100% 3rd harmonic (180hz), then you would get three times the load per phase on the neutral. Some say 1.73 times the line current, but when I've simulated this in excel, the third harmonic seems to fall perfectly on top of each other on phase A, B and C, which is why I believe that it can be as much as 3x

 

[brian john]

When calculating a non linear load what is the maximum unbalanced seen on the grounded ( neutral) conductor

Example:

A lighting load of 30 amps on all 3 phases would the unbalance load be 30 amps or additive to 90 amps?

In my experience with lighting loads negligible.

 

[Besoeker]

When calculating a non linear load what is the maximum unbalanced seen on the grounded ( neutral) conductor

Example:

A lighting load of 30 amps on all 3 phases would the unbalance load be 30 amps or additive to 90 amps?

Non-linear loads have a spectrum of frequencies in their current. For single phase non-linear loads the most prevalent and most troublesome is the third and multiples of it, often termed triple-n. As Ron has said, the triple-n contribution from all three phases add in the neutral.
But the load will not be 100% 3rd (that wouldn't be non-linear) so the neutral current won't be the total arithmetic sum of the line currents. Exactly what it will be depends on the nature of the loads. For example if it lighting is it non-linear it might be because it's CFLs with an electronic ballast or dimmers on incandescents.
In UK and elsewhere it is becoming more common to provide a distribution system with a double sized neutral to cope with increasing harmonic generating loads.