When an installation is to be supplied directly from a MV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking into account the following considerations:

The possibility of improving the power factor of the installation (see chapter L)
Anticipated extensions to the installation
Installation constraints (e.g. temperature)
Standard transformer ratings

Apparent power [kVA]	In (A)
	- 237V -	- 410V -
100	244	141
160	390	225
250	609	352
315	767	444
400	974	563
500	1218	704
630	1535	887
800	1949	1127
1000	2436	1408
1250	3045	1760
1600	3898	2253
2000	4872	2816
2500	6090	3520
3150	7673	4436

Figure. 1 : Standard apparent powers for MV/LV transformers and related nominal output currents

3-phase transformer

The nominal full-load current In on the LV side of a 3-phase transformer is given by:

where:

• P_a = kVA rating of the transformer
• U = phase-to-phase voltage at no-load in volts (237 V or 410 V)
• I_n is in amperes

Single-phase transformer

For a single-phase transformer:

where

• V = voltage between LV terminals at no-load (in volts)

Simplified equation for 400 V (3-phase load)

• I_n = kVA x 1.4

The IEC standard for power transformers is IEC 60076.

SOURCE: Schneider Electric
http://electrical-engineering-portal.com/how-to-choose-transformer-rating