How am i going to size my 3 phase X'mer with 1 phase load using current as reference

Peping · #1 May 11, 2007, 02:43 AM

Hi, I'm Peping:

I have 90 pieces of single phase loads with current of 2 Amperes. This will be loaded to a 3 phase transformer, Wye connected having a line to line voltage of 220 Volts. What will be the size of the transformer required for the mentioned loads?

Many thanx,

Peping

tkrussell · May 11, 2007, 10:45 AM

Assuming the load needs 220 volts, 90 units at 2 amps each at 220 volt 1 phase will require 22.890 KVA minimum.

180 amps x 220 volts = 39600 VA 1 phase / 1.73 = 22890 VA or 22.89 KVA

Add at least 25% KVA to arrive at the minimum size transformer. If the loads are motors and will be started at the same time, add the locked rotor KVA to size the transformer.

ctomasch · May 22, 2007, 02:28 PM

This seems to me like it is a question for school; however, I would add that in North America 220V line-to-line three phase is not a readily available voltage. The Three phase voltage which is readily available near 220V would be 208 line-to-line / 120V line-to-neutral and that transformers (around this size) are available in 30kva and 45kva (i.e. you can't get a 28kva transformer).

ctomasch · May 22, 2007, 02:29 PM

Also, the 1.73 comes from the square root of 3 in case you are trying to show your work.



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