Motors Locked Rotor Current - NEC Question regarding Tables 430.251 and Table 430.7(B)

[akyip]

Hey guys,

I have a question regarding motor locked-rotor currents from the NEC. This comes from several practice exam questions regarding calculating max locked-rotor current of motors.

Table 430.7(B) is for locked-rotor indicating code letters, and applies to all motor design code letters.

Table 430.251(B) is for locked-rotor currents specifically for 3-phase or poly-phase Design B, C, and D motors.

I understand that for most motors (not Design B, C, and D), you can find the max locked-rotor power from Table 430.7(B) and then use basic power equations to solve for max locked-rotor current.

But my question specifically for Designs B, C, and D 3-phase motors is: for finding max locked-rotor current do we just simply use Table 430.251(B) to find the max locked rotor current? Or do we calculate which locked-rotor current is higher between Table 430.7(B) and Table 430.251(B)?

Thanks for any input on this!

 

[BebeshKing PE]

Hey guys,

I have a question regarding motor locked-rotor currents from the NEC. This comes from several practice exam questions regarding calculating max locked-rotor current of motors.

Table 430.7(B) is for locked-rotor indicating code letters, and applies to all motor design code letters.

Table 430.251(B) is for locked-rotor currents specifically for 3-phase or poly-phase Design B, C, and D motors.

I understand that for most motors (not Design B, C, and D), you can find the max locked-rotor power from Table 430.7(B) and then use basic power equations to solve for max locked-rotor current.

But my question specifically for Designs B, C, and D 3-phase motors is: for finding max locked-rotor current do we just simply use Table 430.251(B) to find the max locked rotor current? Or do we calculate which locked-rotor current is higher between Table 430.7(B) and Table 430.251(B)?

Thanks for any input on this!

Table 430.251(B) is the maximum LRC for the selection of Disconnecting means and controllers.

Hope that will enlighten you. 

 

[Zach Stone P.E.]

Table 430.251(B) is the maximum LRC for the selection of Disconnecting means and controllers.

Hope that will enlighten you. 

Bingo.

 

[akyip]

Alright, thanks!

 

[Angel]

Bingo.

Table 430.251(B) is the maximum LRC for the selection of Disconnecting means and controllers.

Hope that will enlighten you.

So when we are selecting short circuit protection, and obviously we need to account for the inrush current, for a design B motor we use table 430.7 or 430.251 (B) to estimate the starting current?