ECN Forum Forums Code Related Discussion NEC & other Code issues Super Neutral Type MC Cable

These sketches might help illustrate what Al said.




Ed

[This message has been edited by electric-ed (edited 01-17-2003).]

I am learning, thanks ElectricAL and electric-ed.

How do I recognize the loads that cause this?

For a while I used to hook up concert power feeds, one night during a show I was checking the 200 Amp 208/120volt 3p 5w disconnect for the sound system feed.

I felt the cables near the terminations (before I new about PPE) the 3 hots and ground where fine but the neutral was buzzing/vibrating.

Amprobe showed aprox. (This was 8 years ago I might be mistaken) 70 amps on the hots and over 210 amps on the neutral I remember thinking at the time it seemed the current had added up, I did not put much thought into it at the time, other then hoping the neutral feeding the disconnect would not fail.

Was this caused by a Nonlinear Load?

Another question, a lot of the data centers I do work in, run the servers at 208 instead of 120. Does this take care of nonlinear load problem or just move it from the neutral to a Hot?

As resqcapt19 said why not avoid the problem and go 120/240 single phase?

I will read the articles Ron linked to.

I hope these questions are not too basic, I feel like I should know this as I install this stuff (super neutral) all the time.

Thanks Bob

Equipment that uses switch-mode power supplies produce harmonic currents. Most electronic equipment, such as computers, printers, photo-copiers, some electronic ballasts, etc.

Ed

Don,

When using the SDS 1Ø 3W system(s) from a 3Ø 4W source, would the 3Ø transformer(s) need any special consideration?
i.e. If a premise system is 3Ø 4160V then at a point a 3Ø 480Y is used for that building and then 1Ø 3W 120/240V system is used for these non-linear loads.
Is there any non-linear load reflected 'up-stream' though the transformers?

Gwz,
My understanding is that there are very few harmonics on the primary side of the single phase transformer used to supply nonlinear loads.
Don

brave soul Don.

considering all the markets opened up in recent years due to harmonics ,and the simplicity of a single ph fix putting the kebosh to them.....

rescapt — The description of using 3-wire 1ø-secondary transformers is not unlike a description of {something like} a media-production studio {previously described at this board?} that contained some fairly exotic grounding and a specialized delta/dual-wye transformer; possibly a 480/dual-208Y drytype intended to limit high-side line harmonic currents.

If one were to employ three 1ø 2-wire-primary to 3-wire-secondary transformers with primaries served in delta, with the center taps of secondaries bonded and grounded in a conventional manner, this would effectively mimic the earlier-described “6ø”-secondary application furnished for the studio electrical system.

A description of the transformer connections could be, using three similar 1ø transformers—rated 240x480 - 120/240 Volts, with each 1ø transformer fed phase-to-phase from a 480V 3ø 3-wire source—1H1 and 1H4 connected to 480V Aø and Bø, 2H1 and 2H4 connected to 480V Bø and Cø and 3H1 and 3H4 connected to 480V Cø and Aø, comprising a delta arrangement. For the secondaries—1X2/1X3, 2X2/2X3 and 3X2/3X3 are tied together and grounded; treated as a separately-derived system. This comprises a staggered or “asterisk” configuration of ungrounded terminals sequenced 1X1 - 3X4 - 2X1 - 1X4 - 3X1 - 2X4, for serving various ø-n 120V loads.

This is somewhat similar to moderator Scott35’s drawings at https://www.electrical-contractor.net/ubb/Forum15/HTML/000052.html

sparky,
I think that is exactly why we don't hear much about the use of single phase 120/240 volt systems to supply these loads. There is more profit to be made by all involved using K rated transformers, harmonic filters, oversized neutrals and other treatments for the symptoms of the problem.
Don

I understand that if there are too many conductors in a box, this can become a problem because of fill harmonics.

Sorry, I just couldn't resist

<comedy club drummer>
ba-dump bump *crash*
</comedy club drummer>

Very "punny" TG!