OK, this is a new one on me. The building I am working on requires all major applicances (air conditioners, heaters, hot water heaters etc) to be 277 Volt.
No worries on installation, it is pretty, easy, just a single hot leg off the 480, and a neutral and a ground.
My question is this:
I have never heard of 277 volts. 120 fine. 240 OK, 480 okeydokey. But 277???
Apparently, per the building engineer, the 277 is up there, and is basically one hot leg off the 480, which makes no sense to me. I metered it, and yes, it is in fact 277 volts. Christ, I even saw 208 volt motors up there.
Can anyone explain this to me?
Regards,
Boris
Boris
“Sir, I may be drunk, but you’re crazy, and I’ll be sober tomorrow” — WC Fields, “Its a Gift” 1927
Replies
Where do these 277 volt appliances come from? Joe H
It is 3 phase. In 3 phase systems the phases are 120 degrees apart. Think of an xy graph with the neutral at 0,0. Phase A is 277 volts at zero degress. Then draw phase B at 120 and phase C at 240 degrees. A connection between the "tips" of 2 of the vectors is 480. This is a 480 volt Y system.
BTW, I don't think that the AC will be 277, but rather 3phase 480 volt motors.
Actualy, I stand corrected. Yes the small airconditioning units are 240. The evaporators on the building are 2160 volts.
What do you mean 120 degrees apart? Like a degree, like in a protractor?
So one leg is 277 volts, another leg is 120 volts and the third is 240 volts? Why doesn't this add up to 480 volts?
Help me out here. I really am interested. I am a glorified handy man, not an electrician.
By the way, there are suppliers for this stuff, e.g., 277 volt. All the lights in the building are 277 volt. Special switches, bulbs, etc. The engineer showed me a collection of screwdrivers which had the tips melted off them from working on the light switches.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
Here is a link that think will explain it all.
http://www.climaticcontrol.com/info/jjzgate/Infotec/Info-Tecs11-20/Infotec_17(rev1).htm
Are at least get you started. Fig 6 shows the system that you probably have.
Thank you Bill, that link was very helpful. I really appreciate it.
Regards,
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
480 Y is 277 volts to ground.
You could buy a transformer "N" type or get a buck boost to drop the voltage to 240.
It is a common voltage in the industrial world.
Come to Milwaukee and see our grounded B" phase 480. It's a spooky world out there.
Hey Boris,
I've always thought your posts were very informative,so,can I offer you a friendly opinion about 3 phase power?
You don't say what's in your scope of work,but working on 3 phase 480/277 panels is REALLY serious business.DON'T let the building engineer or anyone else tell you "it's simple,it's just a hot leg and a neutral....".Young electricians make the mistake of thinking 277 is just a little bit more than 240,or that 480 is just twice as much as 240.As the voltage doubles the power increases by a factor of four.Several times I've seen innocent nicks in the wire on 277 volt lighting circuits deliver enough current,fast enough,to go right through the circuit breaker and take out the whole panel (once in a hospital).You may have heard a 120 volt circuit go to ground with that spitting, sizzling arcing sound.With a 277 volt grounded circuit it sounds like a cannon or a .357 going off in the room.A 480 volt phase to phase mistake? Bad JuJu, nobody sees it up close without getting seriously burned.I've gotten bit once by 277 in 22 years (thanks to an ambitious apprentice turning the breaker on too soon) and I knew instantly I was just a step away.
If you have to work on it,watch your butt.
Barry
I got whacked briefly by 277v once (my own careless error) and it DEFINITELY felt different :)
Ed
Find a friendly electrician to work on this with you. 277v bites hard. More people die from 120v because of how common it is and complacency. Last time I got hit with 277v my lower arm was numb for hours and it just about jitterbugged me off the top of a ladder onto a display rack. Be safe and do it right. You will sleep better at night knowing the building isn't lying in wait and planning your demise.
Boris - 277V comes from 480V divided by the square root of 3, i.e. 480V/sqrt(3)=277V, which works the same as 208V/sqrt(3)=120V for 208Y/120 systems. That explains the odd-ball voltages, and no more. Any decent textbook on 3-phase will explain how and why it works.
Be seeing you...
Boris, OSHA say anything less than 600v is low voltage...BS! I work for a utility company, and we have had more burns and near miss accidents from from voltages below the OSHA treshold than deaths from high voltage. We modified our rules to beyond OSHA regs. Anywhere in our system or plants if we must work at voltages of 240 and above, we are reguired to wear PPEs. Those PPEs are: Flash jackets, face shields and safety glasses, and low voltage gloves with leather glove protectors. We are also required to wear 100% cotton clothing.
I am not suggesting you do all of that, but at the least wear 100% cotton clothing, safety glasses, and low voltage gloves if you can find them. Do not work it hot if you can avoid it. Trust knowone! Buy whatever lockout device that fits your applicationand use it. Put the key in your pocket, and they won't be searching your body for it! Another suggestion is to take off any jewelry you may normaly wear. Gold and silver are the best conductors, that is why it is used in electronics.
Most industrial lighting is single phase 277v, and as others have pointed out it can kill.
Be safe.
Dave
Yeah 277 is dangerous. Once at a factory I worked at, an electrician was found dead hanging over the lighting buss. He had been there for hours, but no one noticed until the day shift arrived. That was the end of anyone working solo in any department. From then on at least 2 people on a job site.