ECN Forum Forums Technical Discussion Electrical Theory and Applications Opinions? This is a long one.

All good points. I knew I would get some interesting comments on this.
Radar, Yes I plan to install a #10 ground along with the #10 circuits. All receptacles, boxes, and the like will be grounded.
resqcapt19, I agree with you about the documented reports. In all my years, I've never actually encountered a problem caused by harmonics. But...I've never installed a service dedicated to 100+ computers. I just want to play it safe. As for the single phase panels, I like that idea, and will look into that.
n1ist, Something that wasn't said before, is that I work at a prison, and this is for the prison VT program. The less receptacles I install, the better. Yeah, I know, our tax dollars at work.
Dnkldorf, I've never used them, but I'm somewhat familiar with the harmonic mitigating transformers that Cutler-Hammer sells. I originally wasn't going to use these because C-H states that the loads should be very closely balanced in order to get the best results. I can't guarantee that the system will always be balanced as the number of computers being used will change probably daily. Even though, I think I'll get a quote on these, and may use them anyway. I'll get some advice from a rep before I decide.
Ron & resqcapt19, I was thinking of trying to cut down on the harmonic loads on the neutral, and wasn't thinking of the imbalanced load that will be on the neutral because of using only two of the three phases. I think now that I will use dedicated circuits with one neutral per circuit.
That's where I am with this now. Please keep the comments and suggestions coming. The only additional info that I can add is that the loads may not always be balanced. There will never be a certain number of computers being used on a daily basis. The number will probably change daily. I have already received price quotes for the distribution equipment, and I'm now working on the list of materials for the raceway, wiring, etc. I'm going to go ahead and get a quote on the zig zags, too.

How many transformers are you putting in?

You can get the C-H zig-zags in positive shifting, neg shifting and non shifting.

They state they should be used in pairs, ie one pos and one neg, or 2 nons. That gives the best results..

The nons are the cheaper ones.....

Load balancing is always going to give you headaches, because not too much these days stays permanent.

Dnk...

I'm installing 2 transformers fed from the same distro panel. They in turn feed one sub panel each. Therefore, if I use zig-zags, I'll use one pos and one neg for the best results (according to C-H).

Nice design, but I think it is a bit too much.

First thing I suggest is to taka a True-RMS clamp meter and measure the actual current consumption in as many computers as you can. I’ very sure that the measure will be at least half what the plate says. 100 computers for a class room will have slight variations, but the totals will add pretty well if you sample a generic 10% and multiply.

Doing some calculations, I doubt that if we are talking about normal PCs (like Dell Dimension XXX) the individual power consumption could be more than 400 VA. So, we have 40 KVA. One 75 KVA K-type transformer will be more than enough and leave enough room to accommodate variations in the computers, accessories and future grow.

One transformer means only one input breaker.

About the lines, if you are going to share every circuit between two computers it will be more than enough to run number 12 wire. Two times 400 VA add to a current of 7 amps. 85 feet in the worst case is not a problem. I could say that you could even power four computers per line, but let’s give some rest to the wire.

But if you run number 10 wire then you could power four per circuit.

Remember that neutral considerations due to harmonics don’t apply to single circuits, neutral and hot, both number 10, will be more than enough for 1600 VA.

Zig-Zag transformers are a good option too. And, as some other members have noticed, maybe you could put two duplex receptacles per computer, you will need them.

Regards,

Joe.-

Sorry, but I won't size a circuit that way. If a motor nameplate says it operates at 50 amps, and you put a clamp-on on it and it read 35 amps, would you size the circuit by the amp reading instead? Not I.

My gut feelings:

Using #10 for these runs is overkill, unless you bundle lots of these circuits in a single conduit, and thus need to deal with derating for that reason.

Consider: the resistance of #12 is 1.6 ohms/1000 feet. In an 85 foot run you have 170 feet of wire. At 7.2A you would have a voltage drop of 2V or just under 2%. If these circuits are run as _balanced_ multiwire circuits (3 circuits sharing a neutral for 3 phase, 2 circuits sharing a neutral for single phase, then the voltage drop will be only 1%.

On the question of one versus two versus three ungrounded conductors sharing a single grounded conductor:
With one hot and one neutral, the neutral carries the same current as the hot, both fundamental and 'triplen' harmonic
With two hots and one neutral, the neutral carries the same _fundamental_ current as the hot conductor, but _twice_ the triplen harmonic load.
With _three_ hots and one neutral, the fundamental currents balance out, so the neutral doesn't carry any fundamental, but it carries three units of triplen harmonic.
My opinion is that unless you have special circumstances, you are better with three circuits sharing the neutral than two.

If you do use the approach of having two three phase transformers each carrying half the load, look at the phasing of the secondaries relative to the primaries. I don't know the details, but I think that by appropriately phasing these two transformers, you could reduce the harmonic load reflected to the rest of the facility with no additional expense. Even though this is probably _not_ a problem that needs to be 'solved', if a bit of thought can reduce the problem with zero cost, then it is probably worth it

Don's point about using single phase supplies is a good one. Harmonics on the neutral are not the problem. The problem is _unbalanced_ harmonics on the neutral. In a three phase system, the _third_ harmonic (and multiples of the third harmonic) do not balance on the neutral. These are the 'triplen' harmonics mentioned above. The 5th and 7th harmonics (and all 6n+-1 harmonics) balance just fine. In a single phase system, the harmonics that don't balance are the even order harmonics. Well a computer power supply essentially injects _odd_ harmonic currents into the system; and they essentially won't be a problem on a single phase system.

Something that I am curious about: do there exist _standard_ transformers that have a three phase primary and _three_ single phase secondaries? What I am thinking about is a transformer that is a delta on the primary side, but with three separate center tapped secondaries, sharing the same core like a standard three phase transformer. If one had such a transformer, would odd harmonics on the single phase sides show up as circulating currents on the primary side?

-Jon

Dear XtheEdgeX:

If you don’t like suggestions don`t ask for them.
I you do it then be polite.
Computers and motors are very different creatures. When you have a new project between hands you should try to learn what is it about first.
As I told you, you don`t need two 75 KVA transformers to supply power to a 100 PC-type computers classroom.
Reading again your original posting and “winnie” last post I should agree that sharing the neutral between two circuits is not a good idea. You should use superneutral (3 number 12 phase conductors, a number 12 ground conductor and a number 10 neutral conductor) and power 6 computers per run (with two duplex receptacles per comp.).
You can do what you want, but if I were the customer and read about my contractror trying to make me spend a lot of extra money just because he doesn’t understand properly the project needs I won’t be very happy. Remember this is not about an airport’s control tower or some highly-critical facility. Making good use of customer’s money is a good quality too.

Joe.

Joe, I didn't think I was being unpolite. You just didn't offer a very practical method for sizing a circuit. Also, you're commenting on things that I've covered in my other posts. If you want to contribute, please read my posts first before you submit something. I'll recap:
Two transformers and panels will be installed for future expansion. Other equipment in other classrooms will be added. I'm installing #10 on all wiring because I wanted the neutral to be oversized, and so I won't have to set up and install different sizes of wire. I won't be adding extra receptacles because I don't want inmates messing with them. You wouldn't believe the number of receptacles that we replace because of this. I wanted to stay away from three circuits sharing a neutral because the loads will not be balanced. The number of pc's being used at one time will vary. The wiring will be installed in a 12" X 4" wireway mounted horizontally along the walls of the room.

If you are placing all of the conductors in a single raceway, than you may be forced to use 10ga as well as 15A breakers because of derating requirements.

See 310.15(B)(2)(a)

I know that there are special rules for ducts... but you are talking about 100 conductors per panel.

-Jon

I was reading this thread and some of you stated that there is no real edvidence of problems with harmonics. And it's just a economic theory to sell products and service. I can tell you right now that it is a very real problem, that has infected an entire high school district I work for. After upgrading all the lighting (from T12 to T8 electronic ballast) and computer labs (switch mode power supplies).The electrical system has caused major problems on equiptment. And I personally discovered the building steel has been acting as the neutral.If I disconnected the ground (green) wire in a box while a load was in use,I would get shocked.As if you were to disconnect a neutral under load, the return end will shock you. To cure problem: All sensitive electronic equipt. on it's on system (K20 rated XFRMR, 200% neutral panels,dedicated circuits-one neutral for one phase conductor). Some have even gone further and run a extra green wire (w/yellow stripe) for isolated ground.