How to calculate transformer KVA

If you know the rated LV current (I) then for a three-phase transformer it would be I x Vln x 3 where Vln is the phase-neutral current, or I x Ipn x sqrt(3) where Vpn is the phase-phase current so 400A would imply 276kVA....


... But the fuse may or may not be directly related to the rating of the transformer, depending on what it's been put there by the designer for. They might well suggest a higher rating than the transformer actually is if they're only there for fault protection, for example, with overload dealt with by other means.


It should be pretty obvious from the nameplate though, which if you can see the fuses you should be able to see!

Thanks jam that's a nice easy to work out way to do it. The reason for my question is simple curiousity

Hello Kelly, you might find this video interesting.

https://www.youtube.com/watch?v=rp1OdR7fFGI


Z.

Yes that video was good took a few minutes to get used to that guys accent but was very informative

You can make a rough estimation by doing the following calculation:


First of all the following information should be available.


a) Type of LV fuse (HRC/ Wired/Glass fuse)

b) Voltage rating of LV. If the same is not available you can take the working voltage of LV winding as voltage rating. If this is not possible then see the fuse, generally they also have rated voltage printed on it. 

c) Percentage impedance of transformer


Calculation:


In general, the fuses have a rating between 125% to 250% of the rated current of the LV winding. If you want a conservative estimation of transformer VA, you may assume that the fuse you have in hand is 250% of rating. Hence rated current can be estimated as 400/2.5  A. With voltage data available you can calculate the VA rating using standard formula.


Please note this is a rough estimation as a number of factors such as whether the transformer has tap changer or has multiple windings etc shall affect the calculation.

Hope this is of some help.

That's interesting I will use both suggested methods hopefully both will come up with similar results  il post when I've done it