Take a number and divide it by six, the remainder (or fraction) is your phase. Example: circuit number 27.
27/6= 4 with 3 remaining. Three is phase "B"
If you have a remainder of 1 or 2, it is "A"
If you have a remainder of 3 or 4, it is "B"
If you have no remainder, or a remainder of 5, it is phase "C".