Here's a good one for you all. I've designed an installation, and wanted to get some opinions on it. First off I want to say, that I know I'm not going to eliminate any harmonics that may occur. I'm simply trying to design this system so it will tolerate it. Personally, I have witnessed the existence of harmonics before, but have never encountered actual problems caused by harmonics, and I don't want to start now. With that said... I have to install a service in our VT area, that will be capable of feeding 100 computers for the classes. They will all be in one large classroom that is about 69' X 73'. Everything in that area is Cutler-Hammer, so I'm buying the same. All of our panels are 3 phase, and I have an existing 277/480Y distribution panel close by in an electrical room. From line to load, I plan to install two 3 pole breakers in this panel that will feed two 75kva transformers. These will be Cutler-Hammer's type K transformers, designed to tolerate harmonics. The secondary will be 120/208Y 3 phase, and each will feed a 200 amp sub-panel, with 42 circuits each (room for future expansion). These will have a 200% neutral bus, and I will oversize the neutral wire between the transformers and the panels. The computers' and monitors' labels rate their combined load at 7.2 amps. Each computer will have it's own duplex receptacle. Each panel will feed half of the computers installed. There will be two computers per circuit (14.4 amps), and two circuits per neutral. I'm not going to run a dedicated circuit for each one, due to the amount of wire to be installed. From the panels to the farthest computers is about 85', and I'm going to run #10 all the way to each duplex. I'm thinking that I've covered all my bases here, but for any of you that have dealt with harmonics problems before, you may be able to fill me in on anything that I've not anticipated. Thanks.
Edited for spelling
[This message has been edited by XtheEdgeX (edited 02-20-2006).]
I thought of something obvious which you already know, but I'll say it anyway (I have a gift for the obvious). Just make sure you run good grounds throughout, as I'm sure you will. I'd say you've done your thinking on this one.
There are 10 types of people. Those who know binary, and those who don't.
#130160 - 02/20/0608:02 AMRe: Opinions? This is a long one.
Take a look at 3 panels and transformers. If you make the transformers single phase 480-120/240, there is no harmonic problems on the neutral. That being said, I don't think you have a problem with your proposed installation. I have seen very few documented reports of problems caused by harmonics on three phase systems. Almost everything that I have ever read about this "problem" has been published by those with an economic interest in solving the problem. Don
#130161 - 02/20/0608:20 AMRe: Opinions? This is a long one.
Why "two circuits per neutral"? The more circuits on the neutral, the more harmonics on the neutral.
You need to use one neutral per circuit or put all three phases on a single neutral. Using only two phases with a neutral increases the neutral load. With two hots and a neutral on a 3 phase system the neutral current is equal to the phase current, assuming a balanced load. While the harmonics always add in a three phase system, the normal part of the neutral load will still cancel if you use three hots and a neutral. I still am not convinced that there is any real problem caused by non-linear loads. There are very few if any documented case studies...just theory promoted by those that have an interest in selling something to make the problem go away. Also as I said before the easiest way to make the neutral harmonic problem go way, if it really exists, is to use 120/240 volt single phase distribution for those loads.
[This message has been edited by resqcapt19 (edited 02-20-2006).]
#130166 - 02/21/0607:58 AMRe: Opinions? This is a long one.
The size of the transformers, the balancing of the loads, the heat they put out, the size of the wiring neeeded, and the power consumed(on a demand basis), are all down sides to that idea....
Maybe, but you elimintate the costs of "super neutrals", K-rated transformers, double neutral bus and things like that. I don't think that you would have any more heat as all of the heat from the harmonic loads is gone. The balancing is not any harder than with the 3 phase system. That being said I wouldn't do anything special... there is just no documented evidence of a big problem with non-linear loads....just a lot of theory and a lot of products in search of a problem. Don