Single motor disconnecting means

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playfarmer

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In a single motor circuit, does the disconnecting means need to be rated for locked rotor? Understand it must be rated for horsepower and FLA per 430.109 and 430.110 (A) respectively. Article 430.110 (C)(1) mentions locked rotor, but that is for combination loads. What is the specific code reference which makes the requirement if it must be?


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playfarmer said:
In a single motor circuit, does the disconnecting means need to be rated for locked rotor? Understand it must be rated for horsepower and FLA per 430.109 and 430.110 (A) respectively. Article 430.110 (C)(1) mentions locked rotor, but that is for combination loads. What is the specific code reference which makes the requirement if it must be?


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Table 430.251(A) & (B) tells you the situations you must use the locked rotor current. If your single motor is not classified under NEC 430.110, 440.12, 440.41, 455.8(C), OR does not have the FLA or HP listed on the nameplate then the disconnecting means does not have to be rated for locked rotor current.
 
Thanks for replying Jeremy. 430.110 (A) speaks to all motor circuits 1000 volts or less. 430.110(C) goes further with combination loads. The text under tables 430.251 (A) & (B) notes for use with 430.110. So I gather your interpretation is yes, they would need to be rated for locked rotor?


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playfarmer said:
Thanks for replying Jeremy. 430.110 (A) speaks to all motor circuits 1000 volts or less. 430.110(C) goes further with combination loads. The text under tables 430.251 (A) & (B) notes for use with 430.110. So I gather your interpretation is yes, they would need to be rated for locked rotor?


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Sorry for the confusion. I was referring to the combination motor scenarios of each Article 430, 440, and 455. That is the only time you consider locked rotor current when sizing a disconnect. For a single motor you will use the FLA or HP of the motor to size the disconnect, so my answer is no. Table 430.251 (A) & (B) provides you with typical locked rotor currents specifically to use for sizing a disconnect that feeds a combinational motor load. A single motor's disconnect will be sized using the full-load current tables.
 
Good day Jeremy, and anyone else who may be following this. I've been made aware and just looked at 430.103 Operation, in the 2014 NEC Handbook. Specifically in the commentary, in which the 4th paragraph mentions - "If a motor stalls or is under heavy overload and the motor controller fails to properly open the circuit, the disconnecting means, which must be rated to interrupt locked rotor current, can be used to open the circuit." It now looks to me as a "yes".


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playfarmer said:
Good day Jeremy, and anyone else who may be following this. I've been made aware and just looked at 430.103 Operation, in the 2014 NEC Handbook. Specifically in the commentary, in which the 4th paragraph mentions - "If a motor stalls or is under heavy overload and the motor controller fails to properly open the circuit, the disconnecting means, which must be rated to interrupt locked rotor current, can be used to open the circuit." It now looks to me as a "yes".


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No.

The commentary is not code, and besides it does not say "rated for LRC" it says rated to interrupt.

Fire pumps are different, but most everything else used the HP of the motor.
 
playfarmer said:
In a single motor circuit, does the disconnecting means need to be rated for locked rotor? Understand it must be rated for horsepower and FLA per 430.109 and 430.110 (A) respectively. Article 430.110 (C)(1) mentions locked rotor, but that is for combination loads. What is the specific code reference which makes the requirement if it must be?


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Your answer is mentioned in your question 430.110(A) is the general rule:

(A) General.
The disconnecting means for motor circuits rated 1000 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.
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The exception that follows allows a listed switch to have a horsepower rating in lieu of the 115% rating:


Exception: A listed unfused motor-circuit switch having a horsepower rating not less than the motor horsepower shall be permitted to have an ampere rating less than 115 percent of the full-load current rating of the motor.
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As you mentioned (C) is for combination loads - the single motor on a switch is the simple way as the switch will typically have a horsepower rating, but a switch with motor(s) and other loads gets more complex as to what the maximum current could be when one or more of the motors is in a locked rotor condition.
 
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