Transformer Sizing

[tarun]

I have three machines: 230/1/60 and rated 48.5A each. I have to feed these three machines using a 3-phase panel (thru 2 pole breakers)

Machine#1: fed from Phase A&B
Machine#2: fed from Phase B&C
Machine#3: fed from Phase A&C

I need to put a 3-phase, 480/240V transformer before this panel. I am confused if I need a 30 KVA transformer or a 45 KVA transformer.:angel::angel:

Calculation 1: Each machine needs 48.5 x 240 = 11640 VA
So, total 3 x 11640 = 34920 VA
Hence I need atleast 35KVA transformer, next available size will be 45KVA

Calculation 2: Full load secondary current of 30KVA transformer is 72.2A. I have a 100% balanced load on three phases, so basically I have 48.5A required at each phase. Then 30 KVA trnasformer should be enough.

 

[david luchini]

Calculation 2: Full load secondary current of 30KVA transformer is 72.2A. I have a 100% balanced load on three phases, so basically I have 48.5A required at each phase. Then 30 KVA trnasformer should be enough.

You have 84A required at each phase, not 48.5A. There are two machines on each phase (ie, one on A-B and one on A-C). The current on each phase will be 48.5*1.732.

 

[tarun]

Transformer sizing

Transformer sizing

You have 84A required at each phase, not 48.5A. There are two machines on each phase (ie, one on A-B and one on A-C). The current on each phase will be 48.5*1.732.

Thanks for the answer. So, the calculation 1 is correct and I need 45KVA minimum?

 

[charlie b]

So, the calculation 1 is correct and I need 45KVA minimum?

I agree.