How to Identify Three Phase Windings

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A procedure has been adopted by the IEC for identifying three phase winding connections. Letters and numbers are used as follows. The high voltage (HV) terminals have upper-case letters e.g. A-B-C, R-Y-B, U-V-W, L1-L2-L3 and the low voltage (LV) terminals have lower-case letters e.g. a-b-c, r-y-b, u-v-w, l1-l2-l3. Each winding has a start numbered 1 and a finish numbered 2.
The choice of letters and numbers tends to be a national preference, see the table below for a rule of thumb guide:

Geographical Area Letters and Numbers Used
 USA L1   L2   L3      or       1   2    3
Europe U     V    W       or       R   S   T
United Kingdom   R     Y     B        or       A   B    C