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why 277 volts for lighting?

why 277 volts for lighting?

Why do so many commercial buildings use 277 volts for their fluorescent and metal halide lighting power? Much of the surplus light fixures one comes across are 277 volts.

I don't understand this. Is the concept that you get 277 volts across 2 legs of 3 phase power? I don't think that is the case.

Do they run these through transformers? If that is the case, that seems really silly because you loose effieciency in a transformer, not to mention being another point of failure and fire hazard.

So- how are 277 volt fixtures wired in a commercial setting? And is it reasonable to consider powering them somehow from 240 single phase either directly or via a stepup transformer?

I am trying to put together a case for the local power company to provide a 3 phase drop to my barn. They run their @#$%^ power lines right across my farm and maybe 30' from the barn. I'd like to be able to justify to them why they should give me the drop (" i have the following 3 phase motors on machines, and my shop lighting is 277 volt")...but I don't know how you power 277 volt lighting, or if its reason for being is related to 3 phase.

RE: why 277 volts for lighting?

Try multiplying 277V by sqrt(3).

RE: why 277 volts for lighting?

3 phase 4 wire Y systems, what is now supplied by most utilities in the US for large comercial and industrial users, is 480V phase-to-phase, and 277V phase-to-neutral. They use it for lighting BECAUSE they do not need a transformer that way.

"Venditori de oleum-vipera non vigere excordis populi"

RE: why 277 volts for lighting?

Yah, lkasdorf,

If you've done your multiplying,

277 is the line-to-neutral voltage on a 480 Y.  Don't expect too much cooperation from the power company though.  They can just as easily give you 208 Y / 120.  

Best to ya,

Old Dave

RE: why 277 volts for lighting?

If you are running a long lighting circuit, such as one for a parking lot, you are better off using 277 V because of the voltage drop.  277 V also requires less current for a given lighting load, so it is advantageous if your load is large.  208 V is also possible and is better than 120, although not as common.  If I only have 120/208, then I usually use 120.  I may also use 277 if I risk running out of space in my 120V panel.  The purpose of a lighting contactor is to allow a 277V lighting circuit to be switched with a 120V circuit.

See NEC 210-6(c) and 225-7(c).


RE: why 277 volts for lighting?

The answers given are all accurate, but let's put the whole soultion together. In a large building it is more economical to use a higher voltage for distribution since voltage drops are less (if you double the voltage, the current is cut in half AND for a 3% voltage drop the permissable drop is doubled so for a given wire size the allowable run distance is multiplied by four - the elctrical contractor saves money for both wire and number of branch circuits).

Standard three phase low voltages in the US are 277/480, 120/208, 120/240 three phase(special and dying) and 120/240 single phase. The first number the phase to neutral voltage, and the second the phase to phase voltage. In a large building all possible loads are either 480 three phase or 277 (to neutral) single phase, including lighting. Transformers are set to provide 120/208 for receptacles only, normaly. It increases the cost of the installation to provide other loads through the transformers since the transformers and associated switch gear would be larger.

The reason your power company doesn't want to provide you with 277/480 is that the transformer they must set to serve you is expensive - there are no small 277/480 transformers available to them since the transformers of this voltage rating are normaly large enough to provide a large building or industrial shop and the power company doesn't want to waste a perhaps $10,000 transformer to provide a customer who will only be paying perhaps $100 per month.

I might add that if you did get 277/480 you would not be happy with the cost of the switch gear - it goes up by a factor of five or so. Home Depot offers a complete panel with breakers for 120/240 single phase for about $125, while for a 200A 277/480 panel with breakers your cost would be aboput $800-$1200, and you would still need to set a transformer and 120/208v switch gear for receptacles, probably another $1000 minimum.

For you, the economical solution is to use either 120 volt fixtures, or try to find used fixtures with a multi-tap ballast (the fixture will operate on 120/208/240/277 volts, just connect the proper wire). Incidentaly the most economical shop fixture (light output per $ cost) is a 400w metal halide high bay fixture - cost about $90, output equivilent to three 8 foot two tube flourescents.

RE: why 277 volts for lighting?

the reason there is so much 277 volt fixtures is because large plants change all the fixtures out at once during planned outage so it will not stop productions. these fixtures are old and at the end of thier life, most people cannot use these fixtures because of the 277 volt industrial voltage so they build up. so don't buy them.

large buildings use higher voltages like 480 and 277 because it provides more power or punch to operate large machines and a lot of fixtures using lower amps therefore smaller wire, also you can run the lines futher without losing much percentage voltage.

for example on 120 volts you can run about 14 light fixtures on a 20 amp breaker with number 12 gage wire but on a 277 volt circuit you can run 30 of the same wattage light fixture from a 20 amp breaker and 12 gage wire.

277v and 480v voltage are a real danger to work with. a lot of guys get killed with it. don't use in your home or  barn especially since you are not use to it.

You are thinking of the savings but you may just be dead right.

good luck

RE: why 277 volts for lighting?

Lots of people come here with questions about 480/277 volt equipment etc.  About one ever two weeks someone asks about how to run their 3 phase equipment they bought at a bargain price on 120/240 volt single phase system. Very simply lighting is run on 277 Volts because it cheap (less cost to install).
My suggestion is to look at the ballast and see if they are dual voltage. If so you can wire the fixtures to run on 120 or 240 volts.  
If not take one of the ballast down to the local Farm and Fleet store or your local electrical supplier and see what a replacement ballast would cost.  You'll have to replace the ballast in all of them.  My guess the ballast is going to cost $15-20 each. Always be leary of that cheap stuff you never know how much it will cost.

RE: why 277 volts for lighting?

We design Healthcare facilities, schools, transportation,penal,civic, commercial and industrial facilities.  We do many millions of dollars worth of construction every year.  It is very rare that we design a primary service at less than 277y/480v, 3ph, 4w.  Unless it is a very small facility.  It is more efficient for larger loads, less costly to install, and the industry norm for medium to large sized facilities.  In fact, we look at going to medium voltages, 5kV, 12kV, etc..., for large sites such as a college or some larger high schools.  It is only to be installed and serviced by qualified personnel, and is not for residential or light commercial use.

Just another thought, carefull in purchasing older ballasts, there are still some old PCB filled units out there, you don't want anything to do with those.  We run into them in old buildings in Los Angeles and it's surrounding areas all the time.  They should have been abated years ago, but....

Best of luck.

RE: why 277 volts for lighting?

I have seen a 277Y480 volt service that was supplied using three 15 KVA single phase transformers. This was for the groundskeeping building of a condominium neighborhood that needed to send the power to irrigation pumps and common area lighting.

One of the economies of 277 volt ballasts is that they "live" longer because they do not work at home. That is, 277 volts controls employee theft. However, that may change if new houses are required to have electric thermal storage airconditioning that freezes and thaws a block of ice on a daily/weekly basis. For a 4 bedroom house that would otherwise need a 5 ton air conditioner you would need 30 horsepower of motors as follows:

7.5 HP freezer compressor #1 (off peak)
7.5 HP freezer compressor #2 (off peak)
5 ton add on heat pump with 8 HP motor (off peak and shoulder)
5 HP for heat pump water heater
1.5 or 2 HP for pumps and blowers

Installing a 400 amp service using a pair of 200 amp single phase service panels is actually quite expensive. You can only have 100 amps of motors on each panel so that motor contribution to short circuit current will not create problems. Since most electricians only have 1/2 to 3/4 of copper wire strands conducting when a service is new, a 400 amp service would only live about 10 maybe 15 years with this amount of load. A 600 amp or 800 amp single phase service is even more expensive and if you have a whole neighborhood would exert negative oral pressure on the Great Cosmic Banana.

Now if you were to upgrade house current in this situation to 120/240+277Y480 volts 6-wire upstream of the meters, you would have a more reasonable situation. A 200 amp single phase service plus a 100 amp or 130 amp 277Y480 volt service would be so much more reasonable. If all of the 480 volt branch circuit are under utility control as to when they work, then the utility does not care.

You still cannot use 277 volts for lighting inside of a house partly because of a number of National Electrical Code rules. Another rule is that general contractors who wire new houses will not hire anybody who has industrial or commercial experience or is otherwise "too smart" read too expensive". Employers are required by the equal pay for equal work law to pay somebody what the employer pays to someone else for the same work, not what other employers pay an emplyee for the work. If you previous wage rate was too little or too much they do not want you.

RE: why 277 volts for lighting?

I also forgot to add:

1. Most electrocutions occur on 120 volts, partly because there is more of it, people take a cavalier attitude, and because 120 volts normally does not knock people away. On 277Y480 volts you are more likely to be killed by a lethal haed bonk if it knocks you off of a ladder. Does not matter how it kills you. Some people are unnaturally afraid of 480 volts. 120 volts will burn your fingers just as easily.

2. If you let enough perspiration salts build up on your skin, picking up a 1.5 volt dry cell by the ends will give you a very nasty shock. Any voltage that is hot enough to light a light bulb is a hazardous voltage under the right circumstances.

3. The jaw spacings in kilowatthour meter sockets are inadequate for voltage over 120/240. With high capacity 120Y208 volts such as in large apartment buildings the spacing is also not adequate. Some states requires that 480 volt and 600 volt meter sockets be deenergized when pulling or installing a meter. If there is not a safety switch ahead of the meter then they have to pull the supply transformer primary.

4. There have been some instances where electronic meters have had a power supply explosion when put into an energized 480 volt socket. One solution is class 200 form 9s meters and sockets that allow self contained meters to be use with voltage transformers that step down to 110Y170 volts.

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