120/240 vs 120/208

[Strombea]

Quick question:

Why do we have two different systems when one system 120/208 or 120/240 (3 phase or single phase) would be easier as all double pole equipment would be the same voltage?

Just curious as to the big picture difference, and if 120/ 208 Y is more efficient then why don't we put that in residence?

Reason for question is that I was called to a high school needing a 240 v hook up for a new espresso machine in the snack shack, but the panel was 120/208 3ph of which was not sufficient per manufacture specs, frustrating because if everything was on one system (except maybe industrial and heavy commercial), it seems that these situations would be much easier, efficient, and quicker. Otherwise you have to shop around for the correct equipment of which 240v is more common.

Is 3 phase 120/240 common?

Food for thought, no issues, just curious.

 

[ritelec]

Quick question:

Is 3 phase 120/240 common?

Yes it's around.

espresso machine in high school?

wow, the times they are a changing.

(not sure why your machine would not work with the 208 but they also make a small buck boost transformer that you could boost that 208 to 240 )

 

[GoldDigger]

Quick question:

Why do we have two different systems when one system 120/208 or 120/240 (3 phase or single phase) would be easier as all double pole equipment would be the same voltage?

Just curious as to the big picture difference, and if 120/ 208 Y is more efficient then why don't we put that in residence?

Reason for question is that I was called to a high school needing a 240 v hook up for a new espresso machine in the snack shack, but the panel was 120/208 3ph of which was not sufficient per manufacture specs, frustrating because if everything was on one system (except maybe industrial and heavy commercial), it seems that these situations would be much easier, efficient, and quicker. Otherwise you have to shop around for the correct equipment of which 240v is more common.

Is 3 phase 120/240 common?

Food for thought, no issues, just curious.

Think about the vectors and their lengths:

Can you take three 120V vectors from a common point and arrange them so that the vector distance between each of the three ends is 240V?
No, you cannot.

The closest you can come is to aim two of the 120V vectors back to back, with their tails joined at the neutral point, and then add a totally separate 240 volt vector to each end of that line, meeting in a phase line point above the line, in fact 208V above the neutral point.

The above is called a three phase four wire 120/240 or a high leg delta. It has the disadvantage that the third wire cannot be used for any 120V loads, so if the majority of the loads are 120V lighting, with only the occasional three phase 240V load, the total system produces an unbalanced three phase load on the POCO service. And it confuses young sparkies who have not encountered it before.

It is not as common as it once was, and in some areas the POCO is no longer offering that type of service.

BTW, what you abbreviate as 120/208 is properly (NEC) written as 208Y/120.

 

[GoldDigger]

(not sure why your machine would not work with the 208 but they also make a small buck boost transformer that you could boost that 208 to 240 )

Other than the 24% reduction in heating element capacity, the wiring may not be designed for a system in which the two hot wires are not symmetric about the neutral.
For example, the net leakage current resulting from winding capacitance can be zero in a 240 volt winding fed symmetrically from 120/240 but will not be zero from that same winding which joins two 120V wye phase conductors.

 

[ritelec]

The above is called a three phase four wire 120/240 or a high leg delta. It has the disadvantage that the third wire cannot be used for any 120V loads, so if the majority of the loads are 120V lighting, with only the occasional three phase 240V load,

My understanding was it was for operations that needed 3 phase, but also needed some lighting and receptacles.

Thank you for the explanations GoldDigger.