We are in the process of value engineering for a new office building that is running over budget. We are considering the option of running POCO primary voltage (12470V) to the electrical room on each of the four floors. The present plan calls for 480V service at 3000A, with step-downs to 120/208 on each floor for the receptacle loads (250KVA each). If we sent primary to each floor, we would need one 250KVA 120/208V transformer and one 350KVA 277/480V transformer on each floor, but we would eliminate the 2.5MVA service transformer (which we must pay the POCO for) and the 3000A bus duct. Is there a general guideline for when MV distribution becomes cost-effective, in terms of service capacity? Is 3000A too small to even consider upping the voltage, or might it be an option?
I agree with LK. There are several decisions to be made- and they can only be made by those with some experience.
277 may sound attractive for lighting... but it poses a significant additional safety hazard- especially if the maintenance folks are more of the "Handy Hank" sort (or, worse yet, the tenants themselves). In terms of power efficiency, there's not much value in 480 if all the motor loads are relatively small.
Your total load has some bearing; there are practical limits to the size of the wires you can handle.
Too cheap to pay the PoCo? Yea, right. Pay instead for $$$ 480 panels, $$$ for 277 switches, long waits for HVAC equipment- and don't think you'll be able to have all the lights come from a single panel, locked in the basement!
I guess what I'm saying is that the price of the equipment should be the least of your considerations. You need to design the system that will best serve your customers' needs. As cynical as I can be regarding engineers and architects, there really is a need for good design work.
#66997 - 06/23/0607:55 PMRe: When to use medium voltage instead of 480?
Can somebody explain what we are talking about here?
The OP is asking for opinions of running 12470 into the electrical closets on each floor verses (1) 480/277 v service to the whole building. I don't think there will be any real VE savings either way after all is considered and priced, but I'm not there either.
As far as 480/277 being an additional safety hazard over having 12470 in these rooms, I'm lost as to why this would be.
As far as any other concern, 9 out 10 jobs we do are 480/277 and the maintenance staffs of these buildings are not in the news papers with injuries or deaths any more than those maintaining 120/208 systems.
Maybe I'm missing something.
With out knowing anymore details though, I would agree that the original plan sounds good to me too.
However, I have done a number of buildings with MV run to rooms / vaults through out areas of the buildings.
One in particular is 25KV run to 7 substations throughout the building.
#67000 - 06/23/0610:32 PMRe: When to use medium voltage instead of 480?
Roger, unless this is a serious high-rise or very big sprawling complex, that voltage is a little too high for distribution within the building. Not that the voltage is the hazard... If properly done. Maint, and the space for vaults in all those locations, added liability for maint within the structure.... well... just dont sound right.
The hazard of pricing, and engineering it, different ball game.
Mark Heller "Well - I oughta....." -Jackie Gleason
#67001 - 06/24/0607:50 AMRe: When to use medium voltage instead of 480?
I consider 277 to be considerably more dangerous than 120.
From the numbers I've seen, the vast majority of electrocutions that occur while changing ballasts "hot" involve 277 volt ballasts. It just seems more likely that 277 won't let you 'let go'.... much more easily than 120. There is also the issue of 277 volt switches, motion sensors, timers, photocells, and the like.
As I see it, if a building will have one large tenant, with a dedicated maintanance staff, it is very possible that the staff will at least have a competent supervisor. A building with numerous small tenants, of sundry businesses, is far more likely to attempt to 'improve' things on their own. I don't want Joe Salesman running to the home center for just any old switch, then trying to work 277 hot.
This, in turn, bring up another design consideration. If you're going to have numerous small offices, I'd like to see everything go to a panel in that office. Again, I don't want the maintenance guy to have to kill all the lights on the whole floor just to change a bulb. If the entire floor will be an "open office", with one tenant, then going to a panel at the end of the hall isn't such a bsd thing.
"Building cheap" seems to directly interfere with "building good" sometimes. Sure, it's a lot simpler to put all the lights on a single 277 circuit- but that makes it much harder to comply with energy rules, or to have a scheme that the tenants will find a pleasure to use. I've seen way too many places where every light in the building was controlled by ONE switch by the back door. All on- or all off. Why pay extra to give every room it's own switch?
IMO, and lacking any details about this building, I would be biased in a single pad-mount transformer outside, with switchgear feeding 208 to banks of meters on each floor. I would then run to a panel in each rental unit, plus one for the common areas on each floor.
#67003 - 06/24/0607:16 PMRe: When to use medium voltage instead of 480?