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Thread: LV relation between t/f rating and load demand

  1. #11
    Join Date
    Jul 2006
    San Francisco Bay Area, CA, USA
    ...and/or the desire to be capable of some future expansion without needing to change out the transformer, cable, main bus etc., etc. when the time comes. There is a price to pay for that besides just the initial cost of that larger transformer in that there will be slightly higher losses in the larger transformer, but that difference would likely be insignificant compared to a change-out at a later date.
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  2. #12
    Join Date
    Nov 2007
    Quote Originally Posted by saad778 View Post
    Please can you guide me on how 500kVA IS GOOD FOR 450Kw load. I want to know how to arrive at this t/f value
    You have to factor in the diversity factor and the power factor.

    Transformer size = connected load(in kW)/PF X DF X 1.30
    where: 30% fudge number to take care of future loads(your choice)

    IMO, your 750kVA trafo and CB selection is a result of a power factor of 0.8, a diversity factor of 0.7!
    See the math:
    connected load = 476.47 kW
    transformer size = (476.47/0.8) X 0.7 X 1.3 = 541.98kVA. The nearest size would be 750kVA!

    Sizing of the secondary protection cb also depends whether the system is supervised or not (NEC 450.3). 1200A if not supervised and 2000A, if supervised(supervised = only qualified personnel monitor and service the transformer installation is ensured).

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