Transformer banking Design

One of the difficult task of electrical engineer is how to design a banking transformer.
Banking means three single phase transformer combine to supply three phase line can be 240 or 480 volt by combining we loop each other end of the transformer bushing and secondary side.
The reason why banking of transformer born because not all single three phase transformer available in the market so in order to provide we bank a three - single phase transformer. And the question how to do that,1.) how many no. of wires to be looped, 2.) what kind of wire, 3.) what are the load limit of the transformer 4. The looping set – up of secondary side if 240 or 480 volt.
For further explanation and details, below are the example of 3-333 kva 240 v – 480 v transformer rated to supply 3 phase 230 volt, wye primary side and delta secondary line connection, ground mounted. Here it is..

1. Number of wires for looping secondary side :

where IL = Line current
IP = Phase current
KVA = Transformer size
V = voltage

IL = 3 x KVA / square root of 3 x V
= 3 x 333000 / 1.732050 x 230
= 2,2507.71 amperes @ full load

then
IP = IL/square root of 3
= 2,2507.71 / 1.732050
= 4343.47 amperes @ full load

since 336.4 AAC wire the current ampacity is 405 ampere

Number of looping wire = IP / 405
= 4343.47 / 405
= 10 or 11 conductors of 336.4 aac
see picture...




2.Kind of wire can be copper wire or all aluminum
3.The load limit of the transformer is 80% of the total KVA rated the 20% is the safety factor.
= 333, 000 x .80
= 266,400 VA

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