When to use Table 430.7(B) vs 430.251(B)

[Dennis Alwon]

This was a question posted on a forum and I was curious about. As I read it I thought the disconnect is sized by 430.251(B) and the conductor by 430.7(B)???? But I don't think so now.... 430.6 states to use 430.247, 248, 249, 250 for all motors other than low speed, high torque or multispeed. I don't see anything that states what to do with Code Letter. Would you really just use 430.250 for conductor size.

This would mean a disconnect at 725 amps and a conductor at 101 amps? Yes, No

Using Table 430.7(B) I have a question about whether 430.251(B) should be used to determine Locked Rotor Current (LRC) for fire pumps. At the end of the table it gives some specific places the table can be used: "For use only with 430.110, 440.41. 455.8(C)." Do we use this table for the LRC on motors and in the case fire pump or do we use Table 430.7(B)? I was told that we don't use 430.7(B) to calculate motor LRC and must use 430.251(B). This does not seem right. The 430.251(B) LRC for the fire pump motor (480V 100HP Code letter C) is 725A and the Table 430.7(B) calculated maximum current is 480A. Which one is right? This makes a big difference in my mind.

 

[taylorp]

"Using Table 430.7(B) I have a question about whether 430.251(B) should be used to determine Locked Rotor Current (LRC) for fire pumps. At the end of the table it gives some specific places the table can be used: "For use only with 430.110, 440.41. 455.8(C)." Do we use this table for the LRC on motors and in the case fire pump or do we use Table 430.7(B)? I was told that we don't use 430.7(B) to calculate motor LRC and must use 430.251(B). This does not seem right. The 430.251(B) LRC for the fire pump motor (480V 100HP Code letter C) is 725A and the Table 430.7(B) calculated maximum current is 480A. Which one is right? This makes a big difference in my mind"

For Fire Pumps, the LRC applies only to the selection of Overcurrent Devices. (See 695.4, 695.5, and 695.6.) The LRC is not used for conductor or starter/disconnect calculations.

Motor conductors are sized according to 430.22 and 430.24
Motor disconnects are sized according to 430.109 and 430.110.

 

[dkidd]

LRC is used to make sure that the conductors are sized to comply with 695.7.

 

[taylorp]

LRC is used to make sure that the conductors are sized to comply with 695.7.

I agree. But the normal application of 430.22 and 430.24 should cover voltage drop in most cases, but it will be necessary to do calculations to make sure the conductor you select meets the requirements of 695.7 also.

 

[taylorp]

I'm sorry, I am combining Dennis' questions and the questions from the OP.

Yes 430.7 B is used to determine the Locked Rotor current for starting purposes. Table 430.251 is used for sizing the disconnect. But the point on disconnects is moot because Fire Pump disconnects are already Horsepower rated and designed. No need to jump through hoops with a listed Fire Pump controller/disconnect.

 

[Dennis Alwon]

Thank you :thumbsup: