On Dec 2, 2010, at 8:46 AM, Leo Bicknell wrote: > In a message written on Thu, Dec 02, 2010 at 11:32:16AM -0500, Jay Ashworth > wrote: >> No, I'm pretty sure he means "across the 2 high legs of a 120/208 3ph >> Wye service", and I'd never heard that idea suggested before. I can see >> why it reduces the amount of copper you need to run, but it seems as if >> it would have compensating disadvantages, though I can't think precisely >> what they might be at the moment. > > In most residential / small business construction in the US you > will find "240V single phase with neutral". There are two hot wires > and a neutral from the provider. Hot to hot is 240, hot to neutral > is 120. > > Most colos run their back end plant (e.g. UPS's, Gensets, etc) on > 480v 3-phase power. The typical way they get 120v power is to > transform that to a 3-phase Y wired output, also known as 3-phase > 4 wire. Each hot leg is 120v to the neutral (the fourth wire). > > You can run hot to hot here as well, where the voltage is 208v. > The trick with 208v loads in this situation is you want to keep the > load across each pair of phases roughly balanced. > > What can be particularly confusiong here is the panels look exactly > the same. The same physical panel layout your house gets with 2 > phases in plus a neutral is now two of the three phases from the > three phase power go in, plus a neutral. Same breakers are used, > with hot to hot being 208 volt. The difference is, in the colo > there are three of them: > > A N B B N C C N A > | | | | | | | | | > Panel 1 Panel 2 Panel 3 > > With A, B, and C being the 3 phases, and N being the neutral. > It is not uncommon for three-phase panels to be different and have all three phases in the panel each phase feeding every third breaker slot.
Owen
