Hello experts! I am trying to study from this Youtube lecture--https://youtu.be/xAYzYPV9xk4?list=PL9D285A166673D052&t=7483 and in the beginning of lecture, we are told that loads A and B are the receptacle and lighting loads respectively which both run on 120 V. My question is how can we size a panel feeding these loads using 208 V and not 120 V? In other words, in our calculation for current why are we using 208 V and not 120 V? Is it possible that for sizing a panel we use a higher voltage (208 V) than our load requires (120 V)? Please help me with figuring out this dilemma.

 

Is this regarding a typical U.S. installation? This uses a 240v feed from a center-tapped transformer. This gives two 120V lines - relative to the grounded center tap - that are 180 degrees out of phase with each other. Circuits get assigned to one side or the other to balance the load on the two sides as much as possible. Big load appliances can get full 240 by bringing both sides to them.

 

Yes, this is for a U.S. power system. Below I have attached my own notes which show the 120 V required power for the receptacle and lighting loads including the final Load Schedule for phase and neutral. Also, these loads are for an office building which is 50 000 sq. ft., 208/120V with no more transformers inside the building. So, the 240 V idea does not seem clear to me. The only thing I can think of is that in the formula for where for Vphase we multiply 120 by 1.732 and plug into the above formula. However, how do we always know that 120 V would be phase-neutral voltage?

 

50 sq ft?! My bathroom is bigger than that.

 

50 sq ft?! My bathroom is bigger than that.

Yes, my mistake, it is 50 000 sq. ft. Now, what about the rest of the problem? Can we assume that the receptacle and lighting loads are connected in Y configuration and the heating and cooling 3 phase loads are in delta configuration without the neutral?

 

Ah, got it. Commercial 3-phase, not residential.

Maybe this will clarify?

 

wayneh,
Thank you for the explanation. I do have some questions, however. "If the resistance of the star was 1.5 ohm we could calculate the output". From where exactly do we get this resistance, what exactly does it represent? I have attached an image which includes some of my questions. Please clarify these for me. I appreciate your help.
In regards to the problem I originally posted, we just need to multiply the 120 V by 1.73 to get 208.