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.
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
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: simple question from civil engineer re:kVA calculation
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
120V x 3000A x 3 = 1080,000VA
120V x √3 = 208V
208V x √3 x 3000A = 1080,000VA
1080,000 VA / 1000 = 1080KVA
Bill
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"Why not the best?"
Jimmy Carter
RE: simple question from civil engineer re:kVA calculation
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!
RE: simple question from civil engineer re:kVA calculation
RE: simple question from civil engineer re:kVA calculation
Radius?
An excellent description.
RE: simple question from civil engineer re:kVA calculation
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: simple question from civil engineer re:kVA calculation
RE: simple question from civil engineer re:kVA calculation
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!
RE: simple question from civil engineer re:kVA calculation
Bill
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"Why not the best?"
Jimmy Carter