Transformer claculations

[forabit]

480p 3 phase
220s 3 phase
112.5 Delta/Delta transformer (required because 75kva too small)
Secondary load 80 kva (stand alone machine tool)

Do I have to size the system to the transformer?
or
Can I size the system to the load?

If sized by the size of the trans: 175a p breaker, #2/0 p conductors
400a disconnect,
#750 kcmil s conductors

If sized to the load:
125a p breaker, #2 p conductors
400amp disconnect (with 300amp fuses)
#4/0 s conductors
Why is this (or not) ok?
Thanks

 

[petersonra]

Re: Transformer claculations

if by "sizing" the system to the load, you mean reducing the wire size and OCPD(s), below what would be needed if the full output of the transformer was used, the answer is yes. The OCPD and wire sizes listed in the code are maximum sizes for a given transformer. As long as the wiring is protected, you can always reduce the OCPD size.

I think you will need to consider that you generally have to supply a feeder that can provide 125% of the continuous load.

There are also some other issues. It is generally to your advantage to make the OCPD on the primary side of any transformer as large as possible. Some have a tendancy to have very high current on occasion (such as power up) that can trip the instantaneous side of the OCPD, even though under normal circumstances the load is quite low.

 

[jtester]

If sized by the size of the trans: 175a p breaker, #2/0 p conductors
400a disconnect,
#750 kcmil s conductors

Your high side breaker could be larger if you choose, as was posted earlier. I can't figure how you came up with 750 kcmil secondary conductors. I figure 112.5 kva at 240 volts 3 phase to be 270 amps full load on the secondary side. Did you leave out the square root of 3 in your secondary calculation?

Jim T