Ship Electric motor control, cabinet design requires specifying the following:
1. The continuous and sustained short-circuit amperes that the bus must
withstand without thermal and mechanical damage. Typical continuous
current ratings of the motor control centers are 600 A, 800 A, and 1000
A, with typical short-circuit ampere rating of 40 kA, 65 kA, and 100 kA,
respectively.
2. The motor starter size (usually Y-Δ starter) as specified by NEMA frame
size below:
As an example, we read from the foregoing table that a 30 hp motor would
need NEMA frame size 3 starter (rounded up to the nearest standard size).
3. Enclosure type, that is, ventilated dip proof or sealed watertight.
4. Protection type, that is, fuse or molded-case circuit breaker. The moldedcase breaker trips all three phases together to eliminate single phasing and is usually of single element type without the thermal element. For this reason, the fuse protection, having a quick fusible link that melts quickly at higher current, has generally higher short-circuit current ratings and limits the damage.
