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simple question from civil engineer re:kVA calculation
2

simple question from civil engineer re:kVA calculation

simple question from civil engineer re:kVA calculation

(OP)
I am a civil engineer reviewing an assessment report concerning a multi-building facility.
They have a photo of the main switchboard with the label reading Main 3,000 amps, 3 pole, 120/208 volt.
The report then makes the following calculation, which I believe is wrong:
The calculation they have is 3000*3*120 to get 1,080 kVA.
It is my understanding that three phase power should be the square root of 3, so I would get:
3000*1.782*120 to get about 641 kVA.
These are commercial/residential buildings, so I am not concerned with power factor for this study phase of the project.

I would appreciate any corrections of my assumptions.

RE: simple question from civil engineer re:kVA calculation

The calculation is correct. It uses phase-neutral voltage. It is only when you use line-line voltage that you use sqrt(3).  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: simple question from civil engineer re:kVA calculation

(OP)
Thanks Skogs.  I was just reading your posts on kVA from a post in 2005.
Is there a simple explanation of the difference between phase-neutral and line-line voltage?
I am ignorant of electrical engineering, but I try to have some understanding because I have to deal with some electrical issues from a project management standpoint.

RE: simple question from civil engineer re:kVA calculation

Put another way.
120V x 3000A x 3 = 1080,000VA

120V x √3 = 208V

208V x √3 x 3000A = 1080,000VA

1080,000 VA / 1000 = 1080KVA

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: simple question from civil engineer re:kVA calculation

jgailla,

For a simple explanation of how 120V and 208V can co-exist, try this. Draw a circle of diameter 120 units. Draw three radial lines out from the centre at 120° angular displacement. The distance from the centre to each intersection between radial line and circle represents the individual phase-neutral voltages and are 120 units in length, which we'll equate to 120 volts. If you measure the distance between two intersections of a radial line and the circle you'll find that the distance is 208 units, which we'll equate to 208 volts.

At an elementary level a three-phase generator has three coils physically arranged at 120° angular displacement, as has a three-phase motor. The reality is slightly more complex, but the principle holds.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: simple question from civil engineer re:kVA calculation

(OP)
Thanks all for your comments.

RE: simple question from civil engineer re:kVA calculation

Quote:

...diameter...

Radius? smile


An excellent description.
 

RE: simple question from civil engineer re:kVA calculation

(OP)
I did Scotty's exercise on paper with the radius (not diameter) 120 units without even thinking about it.
Funny how the mind glosses over errors when the intent is known.

Great example, by the way, Scotty.

RE: simple question from civil engineer re:kVA calculation

Nice catch VE1BLL! As jqailla mentions the mind, I read it and just substituted radius in my mind's picture.  LOL!

Keith Cress
kcress - http://www.flaminsystems.com

RE: simple question from civil engineer re:kVA calculation

Funny, I did the same...and me not even a sparky.

RE: simple question from civil engineer re:kVA calculation

VE1BLL,

Oops! blush

jgailla,

Glad it helped, even with the silly error.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: simple question from civil engineer re:kVA calculation

I know how that happens, Scotty. Grin.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

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