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?

Quote: "We are in the process of value engineering for a new office building that is running over budget."

Are you calling a design build, value engineering?

Is what you mean to say, we are trying to design a power distribution system, without having paying a PE.

Sure hope you din't commit a price to this job, without Professional Plans.

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.

John,

Well said !

The existing plan, sounds good to me.

Sounds pricy and dangerous to me...

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.

Roger

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.

If the design was changed to use the MV on each floor then the electric rooms will be much larger than with the 480 volt system.

I doubt that it would save cost. The MV cable and equipment cost more than the 480 and will require more room that will cut into the available sq/ft for lease.

You will still need a MV switch for the POCO somewhere, most likely outside.

I don't think you will save much from the POCO.

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.

In California contractors are limited to 800A Services. Larger loads must be designed by EEs.

Typical design out here would be:

External, pad mounted oil cooled POCO XFRMR

480V 3 phase service to NEMA1 Service in dedicated room close by the XFMR with two doors.

@ 3,000 Amps either 7 parallel 1000kcmil copper ( underground ) service feeders or ( above ground ) bus duct.

From the Service's integral Distirbution Board these loads at 480Y277:
Elevators 480
Firepump 480 ( with backup power ATS )
HVAC 480

1 Landlord/Site Lighting Panel 480Y277

4 to 8 Daughter Panels 225A @ 480Y277 with utility rooms each floor.

1 or more dry type XFRMRS at each 480 panelboard to provide 208Y120 (typically K-13 type with copper windings for office computers)

Common point grounding for all neutrals back at the service -- oversized grounding conductors from dry type XFMRS.

3/0 Ufer ground in footing, bonded to all necessary conductive paths via the Service 'rail'.

Connections between loads and the Distribution Board ( at the Service ) to be strictly 4/0 THHN copper -- no bus duct -- running below the slab ( PVC ) or above ( EMT -diecast )

Panels surface mounted in dedicated rooms.

Master LCC in main electrical room.

HVAC provided as a porta-pack atop the roof.

Elevators: hydraulic piston type up to four floors.

Structure to be tilt-up concrete curtain wall -- floors to be cast in place on pan decking with heavy steel post and beam on 30 foot centers. All interior trim to be 20 gauge tin or better.

TI Branch loads to be wired in 10-4 MC with occasional EMT.

No one makes a jump to vertical bus duct or medium voltages at your threshhold.



[This message has been edited by Tesla (edited 06-24-2006).]

Assuming you have a PE working on this, I think that the real estate required to go to having 15kV class gear in each floor will be unpalatable to the building owner. They make money on the usable floor space in the building, but pay taxes on the total floor space. Doubling the non-revenue generating floor space devoted to a higher voltage class of equipment (and I believe it will double) is going to look really costly to that owner.

On top of that you have the new Arc-Flash safety issues to contend with now too. 15kV class is a whole new ball of wax where that is concerned.

Tesla,
Your "sample" system is almost exactly what we are working with here in the original design. The only difference is that we are planning a separate 480:208 transformer in each electrical room (two per floor) since the facility is rather large and we don't want to distribute both 408and 208 separately all around the building at very high current. I believe we now concur with your statement that MV is not a consideration for a service this size. The cost of switchgear and the additional space requirements make it a poor choice. I'm curious, though, why you don't think vertical bus duct would be reasonable? It seems like a logical choice here, no?

I have seen the vertical buss duct on smaller installations that you describe.
I for one think it is a very good choice for this application.

John I have been stewing on your posts here for a few days.

You and I could not be further apart in our views of 277 volt circuits.

Maybe it is a regional thing or perhaps we just work on different size buildings.

At least 80% or more of the buildings we work in use 277 volt almost exclusively for lighting.

I can not imagine using 120 for lighting a commercial building other than small buildings.

I have no idea why you would say


We have no problem meeting energy codes and regardless of the voltage you have to buy the controls.


What issue, I call and ask for those items for 277 volt, no issue.


I have not seen any evidence that is a problem.

I don't see the office workers trying to change switches and if they do try to do it hot 120 can kill them just as well as 277.


We are not supposed to be working hot with either voltage.

Anyway I had to respond to your post as I find things much different in this area.

Also I agree with TKB we see a lot of 2000 to 3000 amp 480 volt vertical bus in office buildings, many of these buildings are only 3 or 4 floors tall.

Bus duct is more restrictive of the TI build out...

The Service is just inside the perimeter of the curtain wall. A direct rise vertically takes the bus duct right through the most valuable rental footage: the window zone.

Owners hate that.

Further, bus duct requires higher skill levels from the troops. Out my way the designs are dumbed down to our 'talent pool'.

Which also explains the pervasive use of MC. This selection is so automatic with some contractors that they ignore lower cost EMT runs.

EMT beats MC in retrofits of places open for business since you can assemble it stick by stick. It also beats MC in dead straight runs with multiple round robins. Yet I've seen this ignored time and again.

We all have our habits and ways of thinking.

Also bus duct is a little bit more of a lead time item. I think it's coming cross country. Not installing it makes all feeders pure bread and butter and no one loses their job.

Tesla;


Could you elaborate on this a bit please?

I am not trying to be a jerk, but I am curious as to why you say this.

I have performed Design / Build type Engineering on Projects with new or upgraded Services > 1,000 Amperes many times.

In fact, just completed an Industrial Design / Build Project, which included an Upgraded Service from the existing 1,200 Amp 480Y/277V 3Ø 4 Wire - driven by a 112.5 KVA Pad Mounted PoCo Transformer, to a 4,000 Amp 480Y/277V 3Ø 4 Wire Service - with new Service Feeders, Transformer Vault, Primary Feeder Duct, and 750 KVA Pad Mounted Transformer (provided by the PoCo).
This was completed in late April of this Year (2006).

The reason why I am asking is that maybe there are some restrictions in your area, which differ from my area.
(I am in Southern California, and deal with SCE, PG&E, SDG&E, LADWP, Pasadena DWP, Anaheim Public Utilities, Riverside DWP, etc.)

Scott35

Forgot to ask the Original Poster ("OP") a few questions:

<OL TYPE=1>

[*] Are the Tenant Suites to be "Separately Metered"?,


[*] How many "Leasable" Square Feet are on each floor?,


[*] Does your location have any Energy Conservation Codes?,


[*] Did the PoCo (Power Company) Service Engineer quote that they would supply this project with a 2500 KVA Transformer, or is this what your client would like to have?
</OL>

The idea of 12 KV distribution on a 4 floor Office Structure, would be something done "Long Ago" in my areas.

I have seen older projects with Medium Voltage Distribution, but these were in old Downtown L.A. Buildings.
The Electrical Rooms were designed for >2000 VAC Systems, which contained 12,470 VAC to 480 VAC Transformers in one vaulted area, and the Low Voltage Gear, Panelboards + 480V x 240V & 480V x 208Y/120V Transformers in another area of the same Electrical Room(s).

Currently, I do not think the KVA requirements of your Project would be ample to warrant the use of anything above a 600 VAC Service Voltage; but that's judging from what is common in my area (Southern California).

If the Tenants' Suites will be separately metered, then you might be looking at a Service Voltage of 208Y/120V 3Ø 4 Wire.

If everything will be from a single meter, then the 480Y277V 3Ø 4 Wire Service is the best choice.

As to the possible Energy Conservation Codes, you will need to allow each tenant's space to have lighting controls dedicated to them only. Verify this with your local Building Department, to find out what is involved for compliance.
If this project is in California, let me know, and I will post the necessary links to obtain compliance manuals and sheets.

I will check back for your reply.

Scott35

Never seen that done, our electric rooms are located in the core or in some cases the buss duct will travel horizontally through a lower floor before going vertical.

No way do we use up window space with electric rooms.


Sorry to here that...time for some training.


MC is always cheaper than MC in an overall picture.


I have been doing this work 20+ years and have no idea what you mean here.


Agreed, it is not on the shelf locally.



This is an odd statement to me.

It is the customers responsibility to foot the bill for more labor hours just to 'keep the guys working'?

Bob

Since we are speaking of talent pool, I am guessing that anything North of 600V is a special crew. When a regular guy goes to adjust the trip unit on the breaker and realizes that medium voltage breakers have their trip unit mounted separately (protective relays) and are very specialized, or grabs a fuse puller to replace a medium voltage fuse, there will be an increase in labor (and medical) costs.
Just like special EC crew's for medium voltage, there are many PE's that are not familiar with medium voltage equipment and protective devices.

*** BUMP ***

Hoping to get a reply from Tesla to the question I asked, per the quoted text regarding Service Capacity & EC design/build limitations.

Also hoping the OP can reply to the other questions I posted.

Scott35

Scott, here in Nevada the state limits us to "work within our classification."

Local towns have their own understanding on this issue. While Reno will accept an EC'c calculations to 400 amps, neighboring Sparks lets us go to 800.
It has been speculated that these levels are based upon:
-400 amps is the largest service we can have without requiring CT's for metering; and,
-800 amps is the PoCo limit for single phase services.

Which brings us to the ultimate AHJ- the PoCo. All new services, and service upgrades, ned to be reviewed, and inspected, by them. Chances are, anything the PoCo has signed off on will be approved by the locality.


Oh, and Iwire... you're right, my attitude is influenced by local conditions. 480 is pretty rare here, and the PoCo has a strong "you don't need 480" bias. Indeed, getting real three-phase can be a challenge!
To give an illustration.... the vast majority of sparkies I've met have no idea what a "delta" system is, or a "corner grounded," - though there are quite a few in the area. These guys have never noticed a service where the voltage to ground was different for different legs- all their measurements have been leg-to-leg!

In an even worse manner, maintenance folks are seriously under-trained and inexperienced. This is what you get when your town lacks any serious industry.

[This message has been edited by renosteinke (edited 07-08-2006).]

Reno, what do they supply to the big casinos/hotels? Surely it's not 208/120 at a bazillion amps?

Actually, they do something like that, at most of them....

The PoCo will deliver high voltage to transformers within the casino, where it will be stepped down to 208.

One twist on this is that, to discourage unqualified servicing of the equipment, several casinos have "Danger 480" stickers on the 208 equipment!

The problem has another, uglier, face. Supply houses don't stock all that much in the way of 480 gear (and they charge dearly for it).... so I HAVE found 480 installed using 250-rated disconnects.

[This message has been edited by renosteinke (edited 07-09-2006).]

The big casino's in Vegas have large central plants where they receive the standard distribution voltage (12-13KV)from the utility. The huge casino's receive transmission level voltage (69KV) at their central plants where it is stepped down to 12KV. At any rate the power company meters at the 12KV or 69KV in the CP substation then the casino's distribute their own, load side of the meter, 12KV class system. Usually 12,470V, to many unit substations around the casino. Some are 12.47KV-480/277 some are 12.47KV-208/120V.