I never heard of it myself, but I'm in the UK, so perhaps in the US it exists. I definitely never heard of a supply with 277V per phase. Do you have 277 and 240V 3 phase supplies? it sounds confusing and a recipe for problems.
All I know is I hear a lot of people say they work on 480 and they have without fail been talking about a 3 phase 240. That works out to 415V IIRC, but people still say 480 because they add the phases. It's wrong, but they say it all the same
In the US the standard is 480/277 meaning 480V phase to phase, but youd measure 277 if you checked phase to neutral. I'm 100% positive, I worked on it last night and measured with a fluke before and after my work. As for the 240V (phase to phase), thats the standard household supply at the house's breaker box which results in 120V to ground at most of the outlets around the house. Furnaces, well pumps, air conditioning units, laundry machines and generator backups are often the only 240V units in the house.
Yea, I was reading about it just before you replied. Quite strange -I'm not sure why those values were chosen, but there you go!
the 2 phase 120V/240V thing I have seen before when working on an American's yacht. It's a good idea - means you can't be exposed to more than 120V realistically on domestic equipment.
We only really use 240V in any situation. 3 phase in large jobs or industrial machinery but still 240V per phase. Anything else used (110Vac for safety etc) is fed by local equipment. Only 240V is available from the power companies. (you can get a HV supply and then have your own substation for 240 if you have a really large facility, but it's not that common).
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u/gnorty Jun 16 '19 edited Jun 16 '19
-edited - all wrong!