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Sizing 3 Phase Generator #39774 07/01/04 01:18 PM
Joined: Jul 2004
Posts: 6
E
Estimator Offline OP
Junior Member
I have a customer installing a new computer / server room.
They will have (280) pieces of equipment @ 1.5 Amps /120 volt and (10) pieces 2A / 208 volt - All single phase loads....
Plus they would like to have a 60A /3 Phase Rooftop unit to run of the generator also.

280 x 1.5A x 120V = 50,400 Watts
10 x 2A x 208V = 4,160 Watts
1 x 60A x 208V x 80PF x 1.732 / 1000 = 17,300 Watts


Calculated load = 71,860 Watts.

Is this correct?

Work Gear for Electricians and the Trades
Re: Sizing 3 Phase Generator #39775 07/01/04 01:40 PM
Joined: Oct 2003
Posts: 156
D
dereckbc Offline
Member
Not exactly, you are assumming a PF 0f 1 on your single phase loads. Which is an error on the good side. You used VA on your single-phase loads. If you assume a .9 or .8 PF you will get something less. Long story short, you need a 100-KW generator based on the numbers you gave with not much room for growth.

Is there a UPS between the generator and data room. If not your client is really waisting a lot of money for nothing gained. If there is a UPS you size the generator at 150% of the UPS input power requirement with walk in capability, plus 125% of the HVAC load.

Edited to ask UPS?



[This message has been edited by dereckbc (edited 07-01-2004).]

Re: Sizing 3 Phase Generator #39776 07/01/04 01:43 PM
Joined: Jul 2004
Posts: 6
E
Estimator Offline OP
Junior Member
So you would figure a PF of .9 for electronic equipment? Thanks for the response.

Re: Sizing 3 Phase Generator #39777 07/01/04 01:50 PM
Joined: Oct 2003
Posts: 156
D
dereckbc Offline
Member
Go back and read my edited statement first. I would not go lower than .9, but I would not even talk to the client unless they are willing to put in dual mode UPS.

Re: Sizing 3 Phase Generator #39778 07/01/04 02:15 PM
Joined: Jul 2004
Posts: 6
E
Estimator Offline OP
Junior Member
YES! A UPS will also be part of this installation. I figured I would start with the generator first to keep the question as simple as possible. UPS has not been sized yet.

Any suggestions are greatly appreciated.

Re: Sizing 3 Phase Generator #39779 07/01/04 02:17 PM
Joined: Jul 2004
Posts: 6
E
Estimator Offline OP
Junior Member
Please discribe "Walk-in capability"
Thanks.

Re: Sizing 3 Phase Generator #39780 07/01/04 02:33 PM
Joined: Oct 2003
Posts: 156
D
dereckbc Offline
Member
Higher end UPS designed to work with generators like Powerware and MGE will have control circuitry to "Walk In" the rectifiers, rather than all at once to prevent the generator from tripping. Also look for UPS systems with power factor correcting circuitry. Get with one of the UPS manufacture reps and let them get invloved in equipment specification.

Re: Sizing 3 Phase Generator #39781 07/01/04 03:35 PM
Joined: Apr 2002
Posts: 2,527
B
Bjarney Offline
Moderator
Consider getting quotes from at least a couple of genset vendors based on your list of loads. Motor starting, switchmode power supplies, [UPSs] and generator automatic voltage regulators can be a problematic combination.

Producers like Kohler and Cat have sizing software with a ‘built-in’ checklist on stuff to consider for genset loads—often free for asking.

Otherwise, it can be a no-win dogfight—first between connected equipment, and soon to follow—between you and your client.




[This message has been edited by Bjarney (edited 07-01-2004).]

Re: Sizing 3 Phase Generator #39782 07/02/04 12:58 AM
Joined: Jun 2001
Posts: 642
N
nesparky Offline
Member
All comments above are good. But experience has tought me to make sure the UPS controls and the generator's regulator are compatable. Make sure you get this in writing from both the generator and UPS suppliers. If they are not compatable in the load enviroment you will have the dog fightmentioned above.
Load sizing alone is not going far enough.


ed
Re: Sizing 3 Phase Generator #39783 07/02/04 03:01 PM
Joined: Apr 2002
Posts: 2,527
B
Bjarney Offline
Moderator
Boy Howdy, NE. Excellent advice. Sounds like you may have learned this 'the hard way.'

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