480 vs. 208 volt service

[hbendillo]

I had a construction manager for a client get all bent out of shape because I suggested a 208-volt service for a couple of buildings for a project on which I was working. We were only in the schematic design phase for the project but based on what I knew, we did not need a 480-volt service for the buildings. One building was a hospice (elderly care) facility of about 18,000 square feet and the other an admininstration building of about 22,000 square feet (two story). I still haven't been able to talk to the construction manager but apparently he thinks practically all commercial buildings require a 480-volt service. He also may have been involved in similar projects for which the engineers showed a 480-volt service.

In my opinion too many engineers fall in love with having 480-volt services designed for buildings on projects with which they are involved. A big reason is flexibility I think. They want to be ready for all possible loads and the voltages required. But they are having the owners pay a premium for the flexibility unnecessarily I think. What are your cut off points for 480-volt vs. 208-volt services?

 

[coulter]

hbendillo -

First my disclaimer. I don't do buildings. And for the industrial stuff I am generally involved in, 208 is pretty unwieldly for a main power source. This should make my opinion highly suspect and biased And I'm guessing that none of this will be news to you. So, it's just a philosophy opinion.

... What are your cut off points for 480-volt vs. 208-volt services?

Here are some of the things I look at:
1. What is available from the utility - or in my case, generation? That may well answer all of the questions.

2. What will the available SCC be? The load is - no changing that. Regardless of voltage, the MVA is the same. As the supply voltage gets lower, the available SCC goes up. The worst I have ever dealt with was a 500kVA, 208V, 2%Z. SCC at the xfmr was just under 70kA. Check the price on the lighting and appliance panelboards, 208V, 200A with the breakers rated for 70kA ($$$)

3. How far do you have to go? At 400', 208V is going to increase the copper 2 sizes. At 480V it will hold the same size.

4. What are the majority of the loads? If it's motors, I'd be looking for 480V.

...One building was a hospice (elderly care) facility of about 18,000 square feet and the other an admininstration building of about 22,000 square feet (two story). ...

For the size buildings you are discussing, it could well be 208V is fine. What is your longest run - 300'? What I don't know is the typical load for these types of use.

... In my opinion too many engineers fall in love with having 480-volt services designed for buildings on projects with which they are involved. ...

In my case, it isn't LOVE. I just don't want to fight big wire, overwhelming SSC and arc-flash, and large voltage drops. Higher voltage = lower VD, smaller wire, less arc-flash. I like those things.

... A big reason is flexibility I think. ...

Yes. In my limited experience (don't forget - industrial), the one thing I can guarentee is the installation will change - bigger, faster, higher duty cycle - possibly the day after it goes in. Again, this may not count for the stuff you work on.

Just curious, what are your typical xfmr sizes? That would be the biggest clue about the size of the service.

carl

 

[charlie b]

I don?t have any hard and fast rules or cutoff points. Lately, all of the buildings that I helped design were large enough to make 480 the only viable option.

I agree with the considerations that Carl provided. But he left off some of the advantages of a 120/208 volt system. One is that you don?t need transformers scattered about. That reduces the amount of space needed in electrical rooms or closets, a consideration that architects tend to like. It also reduces the heat load for which the mechanical engineers must provide ventilation. Another advantage is fewer and smaller enclosures, most of which could be discretely placed in walls (i.e., only one electrical room is needed).

One disadvantage of a 120/208 volt system, in addition to those that Carl mentioned, is that the more efficient lighting systems these days are rated for 277 volts. Another advantage a 480 volt system might have is that the transformers can help reduce the influence of harmonics (i.e., at least the triplens harmonics generated in one section of the building will not be reflected onto another section).

The bottom line is an economic one. Add the cost of the materials and labor for both the 120/208 and the 480/277, and compare.

 

[kingpb]

Most likely the roof top AC units will be large enough that 208V would make it more costly as the manufacturer of the units will not be willing to change from 480v to 208V. The other reasoning would be that lighting would probably be 277V. You can get a lot of fixtures on a circuit that way.

I'm sure there is a limit at which it would seem pratical to stay at 208V, but practical and politics don't always mix.

 

[hbendillo]

Well coulter most of the faclities I am talking about have no bigger than 300 KVA transformers. Neither building has significant lighting load with the hospice being very light. One of the determining factors with buildings this size is the HVAC system. We will probably be have gas heat so that helps. If I had fan coil units throughout with electric heat that would change my thinking. Here are some factors I presented to the owner and would be good for this discussion. By the way I almost exclusively do commercial engineering design.

480-volt vs. 208-volt Service Question

It is not necessary that a commercial building have a 480-volt service. The decision for using either a 208-volt or 480-volt service should be based on the size and the nature of the loads. The buildings in question are not of a size or type which would lead us to think that a 480-volt service is necessary. Installing 480-volt services on small commercial buildings increases construction costs unnecessarily. You must have two layers of distribution, the 277/480 volt incoming service and a 120/208 volt layer which is created through step down transformers. The costs for the transformers, wiring and other equipment to provide for the 120/208 volt service we need in a building is not offset by the 480-volt service unless the following factors are present:

1. Significant load amounts can be served at 480-volt.
2. A large quantity of 277-volt lighting and equipment loads will be present.
3. Multiple electrical rooms are planned in the facility which are relatively long distances from the main source and require relatively large feeders.

Types of projects that typically require strong consideration for 480-volt services are industrial projects, large office buildings (approaching 30 to 40,000 square feet and larger), hospitals, schools, and large shopping centers.

We do not seem to meet any of the criteria in any significant way. So unless the load increases quite a bit from what we have now, a 208-volt service fits the needs for the building. However, we are not going to make our final decision until all factors are known, especially for the office building, which is a more likely candidate for a 480-volt service than the hospice.

Because the hospice building requires a generator to provide emergency power, the cost for electrical distribution equipment increases significantly. If we show a 480-volt service for the hospice, we would also have to show a 480 generator. This may be necessary if we end up having a large percentage of emergency loads that requires 480-volt emergency power, but it does not seem so at this point. To provide a 480-volt service would require that we have two layers of distribution for both the normal and the emergency power. Additional equipment would be needed and additional equipment space to house the equipment.

If you have experience with other engineers designing 480-volt services for buildings of similar size, configuration and type for other projects with which you have been involved, it may not have been necessary. Engineers like the flexibility of having a 480-volt service because they want to be prepared for any eventuality, but the premium that you pay for the flexibility may not be justified. The sizes of equipment used for buildings of this type can always be obtained at 208-volt. If there is some rare instance whereby that is not true, one can always use a step-up transformer to serve a 480-volt load.

Among the many types of commercial buildings for which we have provided 208-volt services are small medical clinics or doctor’s offices, small to medium size hospices, churches, small warehouses, office buildings at and below 30,000 square feet, small strip shopping centers, and many types of restaurants and other food service establishments.

 

[sceepe]

I think your right on. If the building is small, and HVAC loads are small (gas heat). 208V, 3 phase is fine. An elderly care building and an admin building don't have any large loads and the architect probalbly only gave you a couple of closets to put panels in anyway. In this situation, I think it is fine avoid the transformers.

As for construction managers, don't get me started. Most are "big hat, no cattle" if you know what I mean. Grandstanding in front of the owner is common. Lastly, who are they to dictate the engineering design? Are they liscenced to make engineering decisions = nope. Are they liable when something goes wrong = nope. I have never figured out why they get paid as much as the architect but have no professional credability or liability. Guess its all in the marketing...