ECN Forum Forums General Topics of the Trade General Discussion Area When to use medium voltage instead of 480?

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