Calculating single phase loads to a 3 ph Y sys.

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tck

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Given: (5) Separate loads 208v, 1 ph, 12KVA each connected to a 208Y three phase system.
Question 1.) Show each of the (5) single phase loads on each of the three phases (A, B, C)in amperage. Show calculation of total current on the 208Y system.
Question 2.) Same as question #1 but show each load in KVA on each phase. Show calculation of total KVA on 208Y system.
Question 3.) Calculate the size of a 480/208Y transformer for the above (5) loads. Please show work.
tck :confused:
 
Re: Calculating single phase loads to a 3 ph Y sys.

This is obviously a test question.

Why don't you attempt to work out the solutions and post your answers so they can be discussed?
 
Re: Calculating single phase loads to a 3 ph Y sys.

I'm Looking for the correct way of calculating this type of three phase load. Which one of these processes is correct? The Amp and KVA solutions should come out with the same answer. Thanks for the interest.
Here is some of my calculations:
Question 1.)

AMP
Phase A B C
58 58
58 58
58 58
58 58
58 58
__ __ __
174A 232A 174A
232AMP

Question 2.)
Illustation #1
KVA
Phase A B C
6 6
6 6
6 6
6 6
6 6
__ __ __
18 24 18

18+24+18= 60KVA/1.732(208) = 166.5A
or
24KVA/120V = 200A

2nd Illustation:
KVA
Phase A B C
7 7
7 7 Recip of 1.732=.58
7 7 12KVA (.58)=7KVA
7 7
7 7
__ __ __
21 28 21

21+28+21=70KVA/1.732(208)=194.3A
or
28KVA/120V=233A

Illustation #3:
Total VA Method
Load #1 12KVA
Load #2 12KVA
Load #3 12KVA
Load #4 12KVA
Load #5 12KVA
_____
Total 60KVA/208V (1.732)=166.5A

Question #3 Sizing the transformer:
Illustation #1:
VA=A X 208V X 1.732
VA=232A (208)(1.732)
VA=83.6KVA
Illustation #2:
VA=A+B+C
VA=18+24=18
VA=60KVA

Illustation #3:
VA=A+B+C
VA=21+28+21
VA=70KVA

Here it is.
tck
 
Re: Calculating single phase loads to a 3 ph Y sys.

I'm confused. 5 single phase loads can't be evenly balanced over a 3-phase system. Do the questions specify to which phases the loads are connected?
 
Re: Calculating single phase loads to a 3 ph Y sys.

Are the loads delta connected? Wye? Or a combination?
 
Re: Calculating single phase loads to a 3 ph Y sys.

I'll take a stab at the line currents, and the total kva.

1. There is no such animal as "total current" in a three phase system.
Referring to the sketch below, the line currents would be approximately -
Phase A - 154 amps
Phase B - 200 amps
Phase C - 154 amps
(Edit - corrected error and typo)

2. Kva per phase - again, referring to the sketch below -
A-B = 24 kva
B-C = 24 kva
C-A = 12 kva

If you meant " minimum kva per phase required for the transformer", it would be 24 kva.

Total load kva is 12 kva x 5 = 60 kva

3. Minimum transformer rating = 72 kva

Ed

208%20Loads.gif


[ February 27, 2005, 11:18 AM: Message edited by: Ed MacLaren ]
 
Re: Calculating single phase loads to a 3 ph Y sys.

First question:

From Ed's diagram,

Vab and Vbc each provide 116A to their parallel loads.

Vac has a single load which draws 58A.

Ib = 116A x 2 x cos(30) = 201A

Ia = Ic = cos(30)x(116A + 58A) = 151A.

Adding these currents algebraically is meaningless because they are vector quantities.

Second question:

This is trivial, Ed has already done it.

[ February 27, 2005, 01:52 PM: Message edited by: rattus ]
 
Re: Calculating single phase loads to a 3 ph Y sys.

Thank you for your work on my problem. I'm taking the answers into class for discussion.
Terry
 
Re: Calculating single phase loads to a 3 ph Y sys.

Ed,

Thanks for your drawing. This is exactly what we are dealing with.
 
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