Look up 430.22 C. In the 2005 edition, it states that the conductors between the starter and motor are to be sized at 58% of the full load current of the motor.
In the wye connection (starting), the motor is configured so that each coil gets about 1/2 voltage. To do so, the starter must apply line voltage to 3 of the 6 wires that run to the motor, and short out the other 3. (Center of the wye).
In the delta connection (run), all 6 wires are at line voltage.
Obviously, the wye connection must be opened before the delta connection is closed, or bad things will happen. Both mechanical and electrical interlocks are employed. Closed or open transition has no effect on this. The timer in such a starter is not an ordinary one at all. In fact, it's called a wye-delta timer. The start contacts open, then there's a brief (1/2 second or so) delay, then the run contacts close. This is to allow the shorting contactor (start) to drop out before the line (run) contactor closes.
During starting, all 6 wires have starting current on them. This is not much more than full-load current, due to the nature of the connection. During running, each set of 3 wires has about 1/2 the full-load current on them. Because a perfect motor is impossible to manufacture, the current will not be exactly half, hence the 58% requirement.
An open transition starter interrupts all current to the motor during the start-run transition. A closed transition supplies limited current, through resistors, during the transition.
The reason for a closed transition is because a low-inertia application (like a close-coupled fire pump) will slow considerably, and might stop completely during the transition.
Rob.
