Voltage between 277 & 120 V

Status
Not open for further replies.
V

vicdelta

Member
I am just curious what would be the voltage between Phase A of 120V & 277V
Branch circuit From the same source of Power.120V Branch circuit off the 480-120/208 3P4W Transformer.
Thanks
 
B

bcorbin

Senior Member
Don't take this answer as gospel, because there are really a lot of correct answers. I'm just going to give you a reasonable answer.

Given a typical delta-wye transformer, you will have a 30 degree phase shift between the primary and secondary. Don't ask me why...I could never quite get my head around it. Maybe I need to go back and study it a little more.

Given that 30 degree shift, if you take a delta feed from a 480Y/277V system, feed a delta-wye transformer down to 208Y/120V, you would have about 183 volts between 277V phase A and 120V phase A. Not that I would ever consider hooking anything up between them.
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I just did a quick model in AutoCAD. The voltage from a Phase A 277 volt line to a Phase A 120 volt line derived from that same 480/277 source measured out at 183 volts and change. If you go from a Phase B or C on the 277 volt side to a Phase A on the 120 volt side, you get either 303 volts or 386 volts.
 
V

vicdelta

Member
charlie b said:
I just did a quick model in AutoCAD. The voltage from a Phase A 277 volt line to a Phase A 120 volt line derived from that same 480/277 source measured out at 183 volts and change. If you go from a Phase B or C on the 277 volt side to a Phase A on the 120 volt side, you get either 303 volts or 386 volts.
Click to expand...

You Made my Day.Thanks very much
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
You can also solve this question using the law of cosines to add the voltage vectors. It matches up with what Charlie posted except I get 302 where he got 303. It is possible to get 397 volts between two conductors, but for that to happen you would have to have both primaries of both the 480/277 and 208/120 transformers fed from the same source, but have the phase rotation different on the primary side. This give on secondary a +30 degree shift and the other a -30 degree shift making it possible to have one phase of one system 180 degrees out of phase with the other system.
Don
 
Last edited:
Status
Not open for further replies.
Top