Crane calculations

[wbc]

In Sec. 610-33 for sizing the disconnect it says not less than 50% of the combined short-time ampere rating of the motors or less than 75% of the sum of the short-time ampere rating of the motors required for any single motion.
My questions is if you're sizing your conductors out of 610.14E and your crane has three motions and you only use two(100% of the largest and 50% of the next largest and omit the 3rd motion)is it 50% of the two that you used or all three? And if it's less than 75% of the sum of the motors for any single motion do I compare the 3 motions and take 75% of the largest motion? And what should one use for the short-time rating? Any illumination would be greatly appreciated.

 

[charlie b]

Re: Crane calculations

There are two separate issues here. 610.14 deals with the conductors of the power feed to the crane, and uses full-load currents. 610.33 deals with the disconnecting means (i.e., switch or breaker), and uses "short-time" currents. Table 430.22(E) may shed some light on this later term, but I think you need to get the actual ratings from the motor manufacturer.

For the conductor sizing, you look at all three motions. You consider every motor associated with the motion as a group. You take the motion with the highest rated group at 100%, and the motion with the next highest-rated group at 50%, and you ignore the third group.

For the breaker sizing, you look at two numbers, and use the higher of the two. The first number is 50% of the rating of all motors on board the crane. The second number is 75% of the motion with the highest rated group (ignoring the other two groups).