ECN Forum Forums General Topics of the Trade General Discussion Area downsizing neutral

Resqcapt:
That is absolutely correct, single phase will not generate harmonics of a say 208/120 3-phase system.

To keep computers and electronic equipment, especially high end audio systems running smoothly, would there be a possible benefit to an oversized neutral on a 120/240 1 phase residential system?

No, as they said above, the harmonics are a result of 3Ø 4 wire circuits. (120/208, 277/480. etc)

Many apartments are using only 2 pahses of a three phase system.
When looking at the panelboard in a apartment it may apprear to be a 120/240V system.

The apartment complex that is having these problems is served by a 208/120V 3phase transformers. The meter center is used to split up the 3 phase to 3 wire feeders to each individual unit. Most of the buildings are 24 units. 212 units total.
We did a larger complex across the street 246 units same owner but did NOT down size the neutrals. Same type of service, same meter centers, same tranformers, fed from same substation. Have not had a call about problems at second (246 unit) as have had with the first (212 unit). Since a lot of college students rent these places, computers and other electronic consumer equipment are all over the places.
Maybe the owner will allow us to fix it one day but right now it's too expensive and time consuming for him.

Don (rescapt19), Scott 35, others;

Explain the problems with harmonics on the 1 Ø panelboards on the 3Ø 4W 208/120V.

Is the problem only at the main panelboard equipment where the 1Ø systems are split-up or at the computer terminals in each apartment unit?

I would think that the neutral can not be reduced in such installations just due to 220-22 (last paragraph) .

Glenn

[This message has been edited by Gwz (edited 06-09-2002).]

Glen, one of the biggest problems we have with explaining additive harmonics is our generic terminology of single phase when we are refering to a double pole connection from a wye connected source. This would also be the case in our use of the term multi wire branch circuit which should be multi phase branch circuit. In the original question of a 120/240 single phase service (one winding center grounded) the harmonics would not be additive due to this truly being single phase. In the muti phase panelboards in your question, served from the 120/208 wye system, due to the phase angles the harmonics would be additive even if we were to use single phase to describe the panelboards. You are correct in refering to 220.22 in this case.

Roger



[This message has been edited by Roger (edited 06-09-2002).]

Glenn,
The harmonic problem with the neutral is made even worse when the system is feed from two hots and the grounded conductor of a 208/120 wye system. Not only do the harmonic currents add in the grounded conductor, the normal neutral currents do not cancel in this arrangement. The neutral is aways a current carrying conductor in this system. The neutral conductor current will always be at least equal to the highest hot conductor current. Then you have to add for the harmonic currents.
Don

How can one determine the critical effects of the harmonics on a 3W system from a 3Ø 4W 208/120V system.

If supplied by an utility transformer where the Grounded ( Neutral ) is installed with a lesser size than the ungrounded conductors, at what point of harmonics does the transformer ( even if premise owned ) or the Grounded Conductor size need to be up-graded?

Would be helpful if a formula was available that would determine what amount of harmonics would be permitted on a particular transformer and/or grounded conductor size during planning stages.

Do the manufacturers of " K " transformers have such formulas and would one determine the harmonic problems of non-linear loads in the planning stages?

[This message has been edited by Gwz (edited 06-10-2002).]

resqcapt19/Don—

I mean no offense, but have you found installations like that yourself? I certainly figured that would the case, and went looking for it and couldn’t find it. I looked at numerous campus-setting trailers that were shipped with their little 12-circuit pool-fence plugin-breaker panels that were prominently labeled as “100A, 120/240 volts, nominal, 3-wire ac, with grounded neutral” {article 550-4(c) style] that had a couple of roomfuls of 120v workstations and fed from staggered pairs phases on the secondary of ~75-300kVA 208Y/120V dry-type transformers.

I thought that this was to be a goose-gravy find, and would be a surefire harmonic cesspool, but I was embarrassed to tell anyone of my serious lack of discovery. There was the expected garbage on the drytype’s XO stub, and two (87 and 27 with tidy met stickers) meters and two bench-tested-by-the-leaflet-50-turn-meteod current probes showed equally pathetic neutral loading. Found out about a year later the several other hotshots independently tried it too, and of course we all read EC&M. No 3-wire trailerhouse neutral currents equaled or exceeded phase currents; there were no warm neutrals and of course, we were all p*ssed that none of our individual genius saved the day and half a million dollars worth of workstations. A leadman laughed at all of us, calling us idiots, but we knew he didn’t have a clue, and if he didn’t shut up we’d kick his butt.

Harmonics or not, there is one case where neutral conductor current will always be at least equal or exceed to the highest phase-conductor current, and that is for two-wire øø-connected are running {ie, 1ø wall-pack HVAC stuff common to trailers in this area.} They would have a tendency to swamp out—or dilute—the per-unit phase-to-neutral load current in fundamental or triplen-harmonic loads. Agreed—they would not reduce the actual current loading on the trailer “network service” neutral leads.