single phase vs 3 phase

Drain434

New User
Location
85282
Occupation
Civil Engineer
If I have a 277/480V system, and pull 2 hot legs to a stepdown transformer (converting to 120/208), is the 120/208 considered a single phase system or 3 phase system?
Also, can 277/480V 3 phase be stepped down to 120/240V?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Yes you have 1Ø-120/208. You could also use a 1Ø transformer to get the 120/240 output.

Welcome to the Forum. :)
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If I have a 277/480V system, and pull 2 hot legs to a stepdown transformer (converting to 120/208), is the 120/208 considered a single phase system or 3 phase system?
Also, can 277/480V 3 phase be stepped down to 120/240V?
Why would you step down from 480/277 to 208/120 on only two legs? Do you need 208 single phase? This would be a non-standard xfmr.

You can step down 2 legs of the 480/277 3 phase to be single phase 240/120.

Why don't you tell us what you need to do and we can suggest the best options.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Welcome to the forum.

If you run only two lines to one transformer, you can not derive 120/208v. You can get 120/240v.

If you run three lines, you can get either 208Y/120v 3ph OR 240D/120v with a high-leg.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yes you have 1Ø-120/208. You could also use a 1Ø transformer to get the 120/240 output.

Welcome to the Forum. :)
No such thing as single phase 208/120V. If it is three wire single phase and 208V phase to phase, phase to neutral is 104V.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
No such thing as single phase 208/120V. If it is three wire single phase and 208V phase to phase, phase to neutral is 104V.
My understanding is that 'single phase 120/208' (note the switch of voltage placement in notation) is sometimes used as shorthand for two hots and a neutral brought to a service or feeder from a 3-phase 208/120 wye system. However in that case the transformer still has a 3rd ungrounded conductor that's used elsewhere. It is not the same as what the OP seems to describe.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
My understanding is that 'single phase 120/208' (note the switch of voltage placement in notation) is sometimes used as shorthand for two hots and a neutral brought to a service or feeder from a 3-phase 208/120 wye system. However in that case the transformer still has a 3rd ungrounded conductor that's used elsewhere. It is not the same as what the OP seems to describe.
OK, but it's not single phase. The two hots are 120 degrees out of phase relative to the neutral.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
OK, but it's not single phase. The two hots are 120 degrees out of phase relative to the neutral.
🤣🤣
OK… it’s not three phase because all three aren’t there.
Its not two phase since they aren’t 90 degrees from each other.
Its not split phase because it’s not 180 degrees apart.

what is it…🤔
 

Carultch

Senior Member
Location
Massachusetts
🤣🤣
OK… it’s not three phase because all three aren’t there.
Its not two phase since they aren’t 90 degrees from each other.
Its not split phase because it’s not 180 degrees apart.

what is it…🤔
I prefer the term "Open Wye" for this kind of circuit, but this term hasn't become standard. It is two phases and a neutral derived from a 3-phase wye system. Another term one could use is "apartment-style single phase", since it is common that individual dwelling units in an apartment building are fed from a master 3-phase service to the building as a whole, while each dwelling unit gets a pair of phases and the neutral, and each of the neighbors get a staggered pair of phases from one another.

For historical reasons, it is still called single phase. I don't agree with the terminology, and I don't know of any other industry standard term for it, that would distinctly distinguish it from split-phase, but it is what we have.

Most loads on it are connected phase-to-neutral, and a select few Watt-intensive loads are connected to the 208V. Some loads are connected to both phases and the neutral (like dryers, and ovens), using the phase-to-neutral 120V circuits to power voltage-sensitive loads like motors, and the 208V circuit to power loads like heating elements that don't care about running on 83% of the standard voltage. It does mean you only run your heating elements at 75% power, if they were originally built for 240V.

It's unfortunate that "two-phase" is reserved for a system we no longer use anymore, because this would be a perfect use for that term.
 
Last edited:

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I prefer the term "Open Wye" for this kind of circuit, but this term hasn't become standard. It is two phases and a neutral derived from a 3-phase wye system. Another term one could use is "apartment-style single phase", since it is common that individual dwelling units in an apartment building are fed from a master 3-phase service to the building as a whole, while each dwelling unit gets a pair of phases and the neutral, and each of the neighbors get a staggered pair of phases from one another.

For historical reasons, it is still called single phase. I don't agree with the terminology, and I don't know of any other industry standard term for it, that would distinctly distinguish it from split-phase, but it is what we have.

Most loads on it are connected phase-to-neutral, and a select few Watt-intensive loads are connected to the 208V. Some loads are connected to both phases and the neutral (like dryers, and ovens), using the phase-to-neutral 120V circuits to power voltage-sensitive loads like motors, and the 208V circuit to power loads like heating elements that don't care about running on 83% of the standard voltage. It does mean you only run your heating elements at 75% power, if they were originally built for 240V.
That’s where I am. It is what we have, but it gets real confusing when trying to explain or teach it.
 

gar

Senior Member
220114-2017 EST

Hv&Lv:

It is a 2 phase source.

The vast majority of electricians do not understand the meaning of the word phase.

If you have a two wire electrical supply, then you definitely have a single phase power source. More than two wires, and you can have more than one phase.

Phase is the angular displacement between two or more sine waves of identical frequency where the same points in relative amplitude and slope are being considered. The easiest point to work with is a zero crossing of a specified slope because its location time wise is independent of amplitude. In a different approach phase angle may be considered as the angular displacement of two points on a single sine wave.

See
for a good discussion.

.
 

Carultch

Senior Member
Location
Massachusetts
220114-2017 EST

Hv&Lv:

It is a 2 phase source.

The vast majority of electricians do not understand the meaning of the word phase.

If you have a two wire electrical supply, then you definitely have a single phase power source. More than two wires, and you can have more than one phase.

Phase is the angular displacement between two or more sine waves of identical frequency where the same points in relative amplitude and slope are being considered. The easiest point to work with is a zero crossing of a specified slope because its location time wise is independent of amplitude. In a different approach phase angle may be considered as the angular displacement of two points on a single sine wave.

See
for a good discussion.

.
The industry terminology is not always consistent with the mathematical theory behind it. "Two phase" is an obsolete voltage system, that had a 90 degree phase shift. Indeed, the value of phi in the general equation V(t) = A*sin(omega*t + phi), is the concept to what the word "phase" ultimately refers.

Both 120/208V "single phase" and 120/240V split phase, consist of two voltage-to-neutral waveforms of the two ungrounded conductors that are shifted in time from each other, just like the original "two phase". Granted, split-phase is really produced by a different method than a time delay or phase delay, but in the ideal case, it is mathematically identical to a phase shift of 180 degrees.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Let's save the 120/208 phase count thing for another thread....

If you bring two hots from the 480V supply you only have single phase going to the transformer and cannot produce 120/208.

If you bring 2 hots and a neutral from the 480V supply then you could generate 120/208 with a suitable (non standard) transformer. But once you are pulling 3 conductors you might as well bring the full three phases and generate 208/120 3 phase.

Jon
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
220114-2017 EST

Hv&Lv:

It is a 2 phase source.

The vast majority of electricians do not understand the meaning of the word phase.

If you have a two wire electrical supply, then you definitely have a single phase power source. More than two wires, and you can have more than one phase.

Phase is the angular displacement between two or more sine waves of identical frequency where the same points in relative amplitude and slope are being considered. The easiest point to work with is a zero crossing of a specified slope because its location time wise is independent of amplitude. In a different approach phase angle may be considered as the angular displacement of two points on a single sine wave.

See
for a good discussion.

.
The industry terminology is not always consistent with the mathematical theory behind it. "Two phase" is an obsolete voltage system, that had a 90 degree phase shift. Indeed, the value of phi in the general equation V(t) = A*sin(omega*t + phi), is the concept to what the word "phase" ultimately refers.

Both 120/208V "single phase" and 120/240V split phase, consist of two voltage-to-neutral waveforms of the two ungrounded conductors that are shifted in time from each other, just like the original "two phase". Granted, split-phase is really produced by a different method than a time delay or phase delay, but in the ideal case, it is mathematically identical to a phase shift of 180 degrees.

I understand the word phase, the concept of angular displacement, and the associated math behind the workings of electricity.
I had this same discussion with a math professor in college regarding the mathematical concept of phase and the industry concept of phase.

My comments were more of a discussion nature on the industry terminology rather than a question asking others to explain phases…😂
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Let's save the 120/208 phase count thing for another thread....

If you bring two hots from the 480V supply you only have single phase going to the transformer and cannot produce 120/208.

If you bring 2 hots and a neutral from the 480V supply then you could generate 120/208 with a suitable (non standard) transformer. But once you are pulling 3 conductors you might as well bring the full three phases and generate 208/120 3 phase.

Jon
That’s what I was asking in post 8 regarding finding a transformer (off the shelf) that will do that..
 

gar

Senior Member
220115-0824 EST

With two hots and a neutral from a three phase Y, and two 1 to 1 transformers you can create a full 3 phase Y of the same voltage as the original Y source.

In my area we have many open deltas that use two transformers with one providing a 240 V center tapped secondary, and the other to provide the 3rd phase. This is less costly than three transformers. If 3 phase loading increases, then a third transformer is added.

.
 
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