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Thread: Sizing circuit breakers for motors

  1. #1
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    Sizing circuit breakers for motors

    I am a little confused as to how to correclty size a circuit breaker for protecting a motor. I am refering more specifically to continuous duty motors on a branch circuit from an MCC or or other type of bus. In sizing an overcurrent protective device or circuit breaker I'm not sure weather to follow NEC section 430.32 stating to select a device at 125% of motor FLA, or to follow table 430.52 which gives percentages (from 300 to 800 percent of FLA) for selecting a circuit breaker. Which of these rules to follow, or do I follow them both? How should I correctly go about sizing a circuit breaker on a motor branch?

    I would appreciate any information on this subject.

    Thank you

    mull982

  2. #2
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    430.32 is used for sizing the motor overload protection. 430.52 is used to size the branch circuit short-circuit and ground-fault protection. 430.32 directly protects the motor, whereas 430.52 is protecting the branch circuit.

    Based on your post, it is likely you are looking for the b.c. breaker size which would be the FLC of the motor times the 430.52 table value. Most likely 250%.
    Bryan P. Holland, MCP

  3. #3
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    You have to read what the paragrapg says'


    430.32 is is part III and it addresses overload protection.

    430.52 is part iv and it is Motor Branch Circuit Protection.

    If your question was sizing circuit breakers for branch circuit protection, then

    430.52 Table is the place to go

    in general 250% for inverse time breakers on AC

    Instantanious breakers are another story

    If you are concerned with running protection then apply 430.32

    (Read the small print under table 430.22A
    you mentioned continuious duty, if you are aware of this section then disregard)

    Charlie
    Charlie

  4. #4
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    Sorry Brian my page did not update befor you posted

    Charlie
    Charlie

  5. #5
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    Thans for you response.

    When you refer to overload protection, I assume that you are refering to a relay or something similar (which are part of the starter) that causes the motor contacts to open, and are represented by the two half circles on a single line drawing. Is this correct?

    Also I now understand that the overload protection is for the motor and the circuit breakers are for protecting the branch circuit however I have noticed on our single-line drawings that a lot of the breakers are sized at around 1.5 times the FLA of the motor. Does this seem correct, since you said that a typical value should be around 250% ?

    Thanks for the help

    mull982

  6. #6
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    Quote Originally Posted by mull982
    Thans for you response.

    When you refer to overload protection, I assume that you are refering to a relay or something similar (which are part of the starter) that causes the motor contacts to open, and are represented by the two half circles on a single line drawing. Is this correct?


    mull982
    I believe so

    "Also I now understand that the overload protection is for the motor and the circuit breakers are for protecting the branch circuit however I have noticed on our single-line drawings that a lot of the breakers are sized at around 1.5 times the FLA of the motor. Does this seem correct, since you said that a typical value should be around 250% ? "

    the nec is a minimum standard , if the person who drew the diagram has determined the motor will start with that breaker set at 150% then there is nothing in the NEC to stop you (in general)

    Charlie
    Charlie

  7. #7
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    Quote Originally Posted by mull982
    Thans for you response.

    Also I now understand that the overload protection is for the motor and the circuit breakers are for protecting the branch circuit however I have noticed on our single-line drawings that a lot of the breakers are sized at around 1.5 times the FLA of the motor. Does this seem correct, since you said that a typical value should be around 250% ?

    Thanks for the help
    The 250% is the max setting. You can use another setting if you want. If the setting is too low the breaker will trip on starting.

  8. #8
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    Bob,
    You are correct that a smaller circuit breaker may be use if it allows the motor to start, but 250% is NOT the maximum breaker allowed. 430.52 exception No. 2(c) allows us to go up to 400% of the FLC as determined by 430.248, 430.250, etc. if the 250% will not allow the motor to start.

  9. #9
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    I always just use my "Square D Motor Data Calculator". You pull the card out of its sleeve until you see the HP you want. Then you look under the right voltage, and it lists the standard size breaker for that size motor.

    Steve

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