480/277v Vs. 208/120v

[new_ee]

Is there any rules of thumb that dictate whether you use a 480/277V vs. 208/120V three phase power system? Obviously the type of construction (industrial, commerical, office, residential) and the equipment installed would dictate which power system you choose. If theres no direct need for 480/277V, what other rules do you use? Sq. footage, length of runs, etc?

 

[peter d]

deleted - nonsense

 

[jtester]

Since the EE can often influence the service voltage, I look at 480 volts when the lighting circuits are longer than 100', when the service size is 600 amps or greater at 208 volts.
If there are lots of receptacles or other 120 volt loads, I shy away from 480 volts unless the service is larger than 600 amps at 208 volts.
The cost of copper has made 480 volt systems more attractive recently.

There are many reasons, here are a few of mine.

Jim T

 

[Tori]

Distance of runs

 

[infinity]

You should look at the connected load and the type of building. For a commercial building with a mix of lighting, receptacle and mechanical loads the cutoff for 208 volt system, if I remember correctly, should be about 500KVA. Anything above that would make it more economical to use 480 volt system with transformers for the 208/120 volt loads.

 

[charlie b]

It really is an economic decision. However, the key economic factors are changing rapidly these days. Most notably, the cost of copper, a key player, is rising. So any ?rules of thumb? that I may have used 5 years ago (or maybe even last week) might not serve in light of today?s economic realities.

Factors to consider include:
1. Total calculated KVA.
2. Size of building.
3. Availability of space for electric rooms (e.g., can you send 480 volts across the building, and transform to 120/208 in a small electric room?).
4. Cost of equipment and cables.
5. Cost of installation (i.e., 480V cables are smaller, and therefore easier to install, than 208V cables).
6. Calculated power losses in cables (naturally, these would be higher in a 208V system, but that might not be enough to make a difference in the overall costs).
7. Heat load from transformers (the mechanical people would have to remove this heat with additional air conditioning capacity, which in turn you would have to provide with power).
8. Availability of power at one voltage or the other from the utility, and the differences in the price the utility would charge for the service.

In general, unless you are talking about a single family residence, I am going to start with the presumption that the backbone will be 480 volts. That is my ?default? value. Someone would have to give me an overwhelming reason to change to 120/208. Such reasons might include a very small building, or the owner?s wishes, or a utility constraint.

 

[hmspe]

With due deference to Charlie, my approach is exactly the opposite -- I design at 120/208 whenever I have the option. Two main reasons.

First, 120 to ground is, in my opinion, safer than 277 to ground. It would be nice if maintenance was always done by trained electricians, but frequently it's the building owner or the janitor.

Second, 277/480 requires two sets of panels, transformers, etc. My observation is that any savings from smaller conductors disappears rather quickly when you add in the costs for the extra panels and the transformers. The equipment costs for 277/480 panels are significantly higher than for 120/208 panels. I also have to find places for the extra panels and the transformers, and provide cooling for the transformers.

I'll design either way, but for most of the jobs i do simple is better.

Martin

 

[charlie b]

Each to his own poison, Martin. Perhaps I just wind up doing bigger jobs (i.e., larger facilities) on average than you. For example, my present project involves six buildings, the largest of which is over 100,000 square feet.

By the way, I think your second reason is better than your first. I would not want a building owner or janitor to be thinking he or she is safer working on 120 volt equipment than on 277 volt equipment.

 

[hmspe]

Each to his own poison, Martin. Perhaps I just wind up doing bigger jobs (i.e., larger facilities) on average than you. For example, my present project involves six buildings, the largest of which is over 100,000 square feet.

By the way, I think your second reason is better than your first. I would not want a building owner or janitor to be thinking he or she is safer working on 120 volt equipment than on 277 volt equipment.

I seldom do jobs at 100,000 s.f., although I've done a number of hotels and schools that size. The schools have been 480, but most of the hotels have been 208. We could probably have a lively discussion about how to select the voltage for a project and why we made the selections we did.

I don't want for a building owner or a janitor to think he's safe working on anything electrical, regardless of voltage, and I didn't mean my statement as an endorsement. The fact remains that owners, janitors, and hotel housekeepers try to fix electrical problems in many cases. They can get badly hurt at 120/208 or at 277/480, but they're more likely to survive with 120V.

 

[dahualin]

The principal advantages of using higher secondary voltages in buildings are:
1. Smaller conductors: If you have same big load, you can use smaller conductors with higher voltage.
2. Lower voltage drop: If you have long run circuit, the voltage drop is much smaller with high voltage.
3. Fewer or smaller circuits: If you have same amount of lights for example, the total amount of lighting circuits will be much less with higher voltage.
4. Lower I2R losses (more energy efficient). If it is a big building, the total load is very big. The energy loss is much less with higher voltage.
5. Step-down transformers can be used for re-regulation of voltage. If the voltage is too low, you can change the tap for higher voltage.

The major disadvantages of using higher voltages have been mentioned on the item 2 by hmspe.

Which voltage is the best choise for a buidling is really depends on the size of the building, how many loads can go with higher voltage.

 

[iwire]

I am confused.

If we are using 'safety' and the likelihood of the unqualified working on electrical systems as the factors used in choosing a service voltage why not use 12 volts?

My opinion is that if the systems are installed correctly there really is little added danger in the use of 480.

I would say 90% of the buildings I work on are supplied with 480Y/277 service.

Also IMO if you think about the 480/277 circuits are not normally in reach of the public other than light switches.

 

[hmspe]

Also IMO if you think about the 480/277 circuits are not normally in reach of the public other than light switches.

I guess I've seen too many cases where janitors changed fuses in the service, or replaced switches or ballasts on hot circuits. Scary stuff.

I agree that 277/480V systems that are properly installed and maintained are pretty much as safe as 120/208V systems -- I've just lost the belief that people in general are willing to wait for an electrician.

As for 12V, there's an issue with voltage drop...


Martin

 

[hmspe]

The principal advantages of using higher secondary voltages in buildings are:
1. Smaller conductors: If you have same big load, you can use smaller conductors with higher voltage.
2. Lower voltage drop: If you have long run circuit, the voltage drop is much smaller with high voltage.
3. Fewer or smaller circuits: If you have same amount of lights for example, the total amount of lighting circuits will be much less with higher voltage.
4. Lower I2R losses (more energy efficient). If it is a big building, the total load is very big. The energy loss is much less with higher voltage.
5. Step-down transformers can be used for re-regulation of voltage. If the voltage is too low, you can change the tap for higher voltage.

These are generalizations. As with most generalizations they may or may not apply to a specific situation.

Which voltage is the best choise for a buidling is really depends on the size of the building, how many loads can go with higher voltage.

... and a host of other things. I wish is was that simple.

Martin

 

[infinity]

With due deference to Charlie, my approach is exactly the opposite -- I design at 120/208 whenever I have the option. Two main reasons.

First, 120 to ground is, in my opinion, safer than 277 to ground. It would be nice if maintenance was always done by trained electricians, but frequently it's the building owner or the janitor.

Second, 277/480 requires two sets of panels, transformers, etc. My observation is that any savings from smaller conductors disappears rather quickly when you add in the costs for the extra panels and the transformers. The equipment costs for 277/480 panels are significantly higher than for 120/208 panels. I also have to find places for the extra panels and the transformers, and provide cooling for the transformers.

I'll design either way, but for most of the jobs i do simple is better.

Martin

If you only design for 208/120 IMO you're doing the guy paying the bill a disservice. There is a point where transformers and high and low voltage panel boards are still more economical.

 

[hmspe]

If you only design for 208/120 IMO you're doing the guy paying the bill a disservice. There is a point where transformers and high and low voltage panel boards are still more economical.

As I said, I'll design either way, or at medium voltage if it makes sense. Unless there are unusual circumstances it makes no sense to design a warehouse that's 100,000 s.f. at 120/208, but it also makes no sense to design a 3000 s.f. office at 277/480. I've seen both.

Bottom line for me is that each job needs to be evaluated on it's own, taking all available data into consideration. There's no one answer that's best all the time.