Ampere Rating & Interrupting Capacity for Motor Disconnect (Over 1000 Volts, Nominal)

[xptpcrewx]

Ampere Rating & Interrupting Capacity for Motor Disconnect (Over 1000 Volts, Nominal)

Can someone tell me if/where the NEC has Ampere Rating & Interrupting Capacity requirements for disconnects on motors operating above 1kV nominal? Also, just to throw a wrench in the works, consider motors larger than 500 HP. Thanks in advance!

 

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Follow-Up

Follow-Up

Can someone tell me if/where the NEC has Ampere Rating & Interrupting Capacity requirements for disconnects on motors operating above 1kV nominal? Also, just to throw a wrench in the works, consider motors larger than 500 HP. Thanks in advance!

Anybody???

 

[Jraef]

Well that isn't really a "wrench in the works" because it's somewhat rare for MV to be used on motors UNDER 500HP...

I think you are looking to the NEC as a design guide for equipment, it isn't. 110.3 actually covers it; "Suitability for installation and use in conformity with the provisions of this Code". that covers the Short Circuit rating, because if you install something that is not rated for the available fault current, it is not suitable for the use.

Then Part XI, starting with 430.221 covers MV installations and again, in GENERAL, it calls for "suitably sized" components; the term is used in both the CB (430.223-C.1.a) and Fused Disconnect (430.223.C.1.b) sections. Then finally in 430.226 it says;
"Rating of Motor Control Apparatus. The ultimate
trip current of overcurrent (overload) relays or other
motor-protective devices used shall not exceed 115 percent of
the controller's continuous current rating. Where the motor
branch-circuit disconnecting means is separate from the
controller, the disconnecting means current rating shall not
be less than the ultimate trip setting of the overcurrent
relays in the circuit."

So 115% minimum, the same way it is described for LV.

 

[xptpcrewx]

Jraef,

Thanks for taking this one on...

Well that isn't really a "wrench in the works" because it's somewhat rare for MV to be used on motors UNDER 500HP...

Wrench in the works because there are no FLC currents listed in Table(s) 430.247, 430.248, 430.249 or 430.250 for over 500HP.
Note Table(s) 430.249, 320.250, and 430.251(B) list several MV motors under 500HP.

I think you are looking to the NEC as a design guide for equipment, it isn't. 110.3 actually covers it; "Suitability for installation and use in conformity with the provisions of this Code". that covers the Short Circuit rating, because if you install something that is not rated for the available fault current, it is not suitable for the use.

Not as a design guide but as an installation requirement. 110.3 is actually a pretty good point. :thumbsup:

Then Part XI, starting with 430.221 covers MV installations and again, in GENERAL, it calls for "suitably sized" components; the term is used in both the CB (430.223-C.1.a) and Fused Disconnect (430.223.C.1.b) sections.

You lose me here. I am on 2011 code.

Then finally in 430.226 it says;
"Rating of Motor Control Apparatus. The ultimate
trip current of overcurrent (overload) relays or other
motor-protective devices used shall not exceed 115 percent of
the controller's continuous current rating. Where the motor
branch-circuit disconnecting means is separate from the
controller, the disconnecting means current rating shall not
be less than the ultimate trip setting of the overcurrent
relays in the circuit."

So 115% minimum, the same way it is described for LV.

What is "ultimate" trip setting? I've heard it before but don't understand if this has a specific meaning.
Also, this is not really the same as described by LV because we don't have FLC values...