Locked rotor starting current

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Dale001289

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I’m trying to determine if the Client specification needs to be revised. They want the MCP settings from 700% to 1300% (460V induction motors) and motor locked rotor must not exceed code G per NEC T430.7(B). Is this realistic? Would it be better to have the range up 1700% (per NEC exception) in case they have a Design B or E high efficiency motor? Is that even possible for Code G?
We’re focusing on 15HP motors (up to 100Hp).
Is it likely the MCP will trip with Code G when set at its maximum setting of only 1300%?


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LRA for type G 15 HP 460 V motor would be 106-119 A. The MCP range at 700-1300% for a 15 HP 460 motor would be 147-273 A. If anything, seems like the MCP is oversized.

Edit: I take back my last comment, the MCP is sized appropriately for 800% of FLA.
 
Last edited:
EC Dan said:
LRA for type G 15 HP 460 V motor would be 106-119 A. The MCP range at 700-1300% for a 15 HP 460 motor would be 147-273 A. If anything, seems like the MCP is oversized.

Edit: I take back my last comment, the MCP is sized appropriately for 800% of FLA.
Click to expand...

How did you determine the max 119A?


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Max KVA/HP for type G = 6.29 based on table 430.7(B)
LRA = 6.29 * 15 * 1000 / (460 * 1.73) = 119
 
EC Dan said:
Max KVA/HP for type G = 6.29 based on table 430.7(B)
LRA = 6.29 * 15 * 1000 / (460 * 1.73) = 119
Click to expand...

Thanks Dan.
Would happen to know if this range, i.e. LRA G, restricts motor selection to certain NEMA design motors, such as A, C and D? What about high efficiency Design B?
 
FWIW most of the cage motors I have experience with have been around 6PU (600%)
Mostly 30kW upwards.
 
Dale001289 said:
Thanks Dan.
Would happen to know if this range, i.e. LRA G, restricts motor selection to certain NEMA design motors, such as A, C and D? What about high efficiency Design B?
Click to expand...

Not sure exactly, but I found this source which indicates designs B-D are compatible with up to code J motors for a 15 HP 460 V motor, and anything higher would typically be a design E motor:

1638543723287.png
 
EC Dan said:
Not sure exactly, but I found this source which indicates designs B-D are compatible with up to code J motors for a 15 HP 460 V motor, and anything higher would typically be a design E motor:

View attachment 2558575
Click to expand...

Nice research. My understanding is Design E motors are very rare. Looks like LRA code G will cover the vast majority of motors. Thanks again.


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