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480V 3-phase service load calc. #52112 05/19/05 09:28 AM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
Member
Just want to double check how I'm doing this. Can anyone tell me if I am missing something or is this right? I'm finding the existing load on an 800 amp 480V service to a building to consider different expansion options.

convert all 120/208 & 277/480V loads to VA.

480 3phase 100 HP motor with RLA 90 amps:
480 x 90 x 1.732 = 74,822.4 VA

277v 400watt MH high bays at 1.75 amps each:
277 x 1.75 = 484.75 VA each

208 3 phase well RLA 24 amps
208 x 24 x 1.732 = 8646 VA

208 1 phase heater 3.75 Kw = 3,750 VA

misc. 115V loads total 65 amps
115 x 65 = 7475 VA


I guess the main thing I'm looking for some confirmation on is the treatment of the 3-phase loads. when going from RLA to VA I do V x A x 1.732 = VA. then all my 1-phase loads I just do V x A = VA.

To get total load on the service, then I add all my VA together and do VA / (480 x 1.732) or the same as VA / 480 / 1.732 as entered in to calculator.

Does changing all the different loads into VA this way and then dividing them by the service voltage and use the 3-phase fixed value (1.732) account for everything? Do I need to consider anything else for the loads that convert from 480 to 208 within the space?

I think it is this simple, but I just want to know, am I overlooking anything. Thanks y'all.

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Re: 480V 3-phase service load calc. #52113 05/19/05 12:26 PM
Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
By converting all of the loads to VA allows you to caculate the amperage at 480 volts.
You need to remember that when making the caculation you are balancing the load equally across all 3 phases at 480 volts.


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