simple question from civil engineer re:kVA calculation 2

[jgailla]

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.

 

[Skogsgurra]

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

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[jgailla]

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.

 

[waross]

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

 

[ScottyUK]

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.


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

 

[jgailla]

Thanks all for your comments.

 

[VEBill]

...diameter...

Radius?


An excellent description.

 

[jgailla]

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.

 

[itsmoked]

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

 

[btrueblood]

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

 

[ScottyUK]

VE1BLL,

Oops!

jgailla,

Glad it helped, even with the silly error.


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

 

[waross]

I know how that happens, Scotty. Grin.


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