Circuit Breaker Rating for 4 Pumps, Each 7.5kw/ 15.4A

First of all, you didn't mention about the rating and setting of the individual motor protection. I am guessing (1) the 63 A CB is installed at the motor feeder circuit, (2) there is no other load in the feeder and (3) motors will not start at same time.

You must have individual protection for each of the motor branch circuit. The individual protection should have both overload and short circuit setting or rating. See NEC 430.32 (A)(1) for overload setting and Table 430.52 for short circuit settings. Per NEC 430.62, the feeder circuit breaker rating or setting should be equal to (but not greater than) the rating (or setting) of the short circuit protection plus sum of the full load current of other three motors.

Let's say the short circuit setting of breaker (Inverse-time) each of the motors is 35 A. The feeder circuit breaker rating should be 35 + 3 x 15.4 = 81.2. So, you can select 80 A Inverse-time circuit breaker. The 63 A circuit breaker is not enough in this case and it is causing the nuisance trip during the starting period.

Hope it helps.

- MS