120v/240v subpanel 100a
#3for the 2 hots,#8 for grounding conductor
can i downsize the neutral?? if so, to what size,and why????

Yes you can under the right conditions. You must calculate what the maximum line to neutral load will be and multiply by 140%. There are a few other variables so please see article 220-22.
Nick

Yes, but why? Most modern homes now have many solid state switching devices in all the microelctronics in every appliance, computers, etc. These cause the neutral to have harmonics that can overheat the neutral. You will save very little by downsizing the neutral and may cost more to you and the homeowner to replace it later.

To calculate the neutral load, it says to add all the120 volts circuits, and take 70% of the range and dryer circuits. Also it shall be possible to apply a demand factor of 75 % for four or more fastened in place appliances,other than range, clothes dryer, space heateing equipment or A/c.
Generally Aluminum SEU cable and the copper come wiht the neutral already downsized.
Somebody referenced switching power supplies inherant to computers, I have only seen this problem on 208/120 3-phase circuits where triplen harmonics are generated and this is usually in offices where many computers are used and transformer shielding and supersized neutrals have to be installed to help eliminate this problem.
The only problem with downsizing a neutral in a residence is future expansion. but circuit balance is also important such that there is as small amount of current on the neutral as possible.

Yes you can if you follow the above advice but as Warren1 said WHY. The cost saved will not pay for the trouble calls later.
I have a customer where this was done in an apartment complex. Most of the residents who use computers there are having problems from monitors that will not go full screen to lost data and very slow response times. The owner will not pay for ugrading the neutrals, wants me to bill the outstate electrician who built them. That company will not pay for that. The tenants are not going to pay for it either. Can't fix the problem without repulling the feeders. It a no win situation. It may go to a lawsuit. With more and more electronic items going into houses,apartments, and other places this problem will just get worse. Downsizing a neutral is not worth the hassle now when you justify to the inspector, or later when problems occur.

The schools run into this problem in the older schools not equiped for computer labs.
this is not a easy fix and can cause repeated problems.

Sam;
We ran into this problem with an air-tool manufactureer, in Cleveland. We wound up installing a special shielded transformer, re-pulled every circuit on the computer network, 208/120, 3-phase oversized the neutral in all cases had a special panelboard made just for the problem and even installed insulated/isolated gronding receptacles. And for all our trouble we eliminated about 95% of the problem. Which was a huge improvement.
It is true to correct a problem like this it is costly and with DC/AC drives there is a similar problem but different order harmonics.
However I have never seen this problem in a residence.

While I agree that down sizing the neutral is not worth it in most cases I would have to say that you have bigger problems than an undersized neutral in your apartment complex. Third order harmonics, the type generated by computer power supplies, do not multiply in 240/120V single-phase systems as they do in 208/120V systems. Also, oversized neutrals mitigate heating problems. They do not cancel harmonics in the system. They merely create the capacity to handle them.

thanks for all the input guys.. i agree..keep it the same...

Warren,
The harmonics caused by non-linear loads and or switching power supplies do not cause problems with neutrals on single phase systems. These harmonic currents only add on three phase systems.
Don(resqcapt19)

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.

Bjarney,
I haven't actually found any cases where the neutral current was high enough to cause any problems. I haven't even talked with anyone who has personally observed such conditions. I spoke at length with a design engineer for a firm that does mostly commercial office buildings, and he told me that he never uses any of the oversized neutrals or K rated transformers and has never found any problems in the system after the client has moved in and went into full operation. Some of these installations that he told me about had over 300 computers plus electronic ballast all on 120/208 wye systems. The was an article 3 or 4 years ago that suggested that where there are very large numbers of switching power supplies on the system that the neutral current actually goes down. The reason that was cited was that the harmonics are created randomly and where there are large numbers of generators, the random timing actually results in harmonics that do cancel.
But back to your installation, isn't the neutral current in a 3 wire system fed from a 4 wire system always at least equal to the highest ungrounded conductor current? The formula is the square root of (A^2 + B^2 + C^2)-((A x B)+ (A x C) + (B x C)). A^2 = A squared. If you use a 10 amp load for any 2 hots and solve you will have 10 amps on the grounded conductor.
Don