Correct sizing of MCP

[rharris]

I have an application where I have a 150 hp motor, Design B, 460 VAC. NEC states that the FLA is 180 amps. The question is I am trying to correctly size the overcurrent protective device, an MCP or instantaneous trip (NEC), feeding the motor. I understand the function of this device, but still not clear on how to apply NEC Article 430.52 and Table 430.52. I see that the maximum rating or setting from NEC Table 430.52 is 1100% for this motor. Not exactly sure what the correct size of the MCP should be.

Any help or thoughts are appreciated.

 

[augie47]

pretty much exactly what you see and state. Per 430.250 the FLA of a 150 HP/480v ,motor is 180 amps/ 430.52 allows you to size and Instaneous Trip Breaker for a Design B motor at 1100% of FLA, or in your situation , 1980 amps.. Not sure of the availability of such an animal.
Make sure to read all the restrictions noted in 430.52(3)(3) most notably that Instaneous Trip protectors must be part of a listed combination motor controller.
A Inverse Time C/B would be 250% or 450 amps

 

[templdl]

Remember that motor over load protection is provided by a properly sized and set overload relay (OLR) which is part of a motor starter.
The MCP provides motor circuit protection (MCP) which is actually short circuit protection. As such between the OLR and the MCP you have thermal magnetic protection.
The MCP is intended to take the motor off line should the motor windings fail. Years ago motors often started on fire if the windings failed to ground so the MCP was invented I believe by Westinghouse.
If you look art art 430 it refers to ground fault protection which is actually means an MCP.
When a motor winding fails to ground it creates a ground fault. The magnetic (instantaneous) MCP, if set properly, should detect the fault current clearing the fault and limiting the damage to the motor.
If I remember correctly Art 430 does give the setting allowed for the MCP which is up to 13X the motor FLA, if the MCP trips on start up at 13X you are allowed to go to 17X max.
The objective is to keep the setting of the MCP as low as practical such that is doesn't trip upon motor start up that way if it does trip at another time there is a great possibility the there is a winding that has failed.
I believe that you will find that energy efficient motors will require settings above the 13X.

One thing that is very important is that you must pay attention to the rating on the MCP which must be greater than the FLA of the motor or you can burn it our as they do not have any way to protect themselves from over current. The 150A frames and smaller are particularly susceptible to being destroyed when applied incorrectly.
And there is no way that they can be tested in the field.