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2 pole 20 ampere breaker

    • 3 posts
    June 19, 2019 10:45 AM PDT

    very simple and basic but it I want to know

    an AC motor with 18 ampere which is shown in name plate.  can I use 2 pole 20 ampere breaker?

    • 160 posts
    June 19, 2019 11:22 AM PDT

    Installing a dp 20 amp breaker on an 18 amp motor would be a mistake and would probably cause the breaker to trip.  Overload protection that is usually built into the motor is designed to be 125% or more of the nameplate amp, depending on the service factor.  The overcurrent protective device can be sized up to 250% for an inverse time breaker. In your case a 45 amp breaker or less would be compliant.  I would probably use a 40 amp overcurrent protective device. 


    The overload in the motor will protect the conductors, sized at 125%, from overloading so the circuit breaker is there to trip on short circuit or ground fault only- not overload

    • 15 posts
    June 19, 2019 12:56 PM PDT

    Dennis is correct. A 20 amp breaker would not be able to handle the start up amperage of the motor.

    • 72 posts
    June 19, 2019 1:33 PM PDT

    There are some hard start motors. 

    Breaker size: 18A*225% = 50A

    • 29 posts
    June 19, 2019 1:34 PM PDT

    The maximum size breaker for single pole motors would be 250% of the nameplate FLA. For 18 amps, you should use a 40A breaker.  Unless you can find a 45A breaker!

    • 75 posts
    June 22, 2019 3:35 PM PDT

    Slightly different take on the issue. (Following NEC 2017, art 430)

    For sizing the CB, use the Tables, not the nameplate current.  Since this is single phase, that would be table 430.248

    430.52.C.1, Ex 1 for a TM CB allows 250%, next size up.

    Example:  115V, 1.5 hp motor

    Table 430.248 says, FLC is 20A

    2.5 x 20 = 50A

    Table 240.6.A says that is an available standard size so that is the max

    Example:  208V, 3hp

    Table 430.248 FLC is 18.7A

    2.5 x 18.7 = 46.8A

    Next size up, 50A is okay.


    Minor cautionary note - Which all already know:

    The CB and conductors are sized from the tables FLC

    Overloads are sized from the Nameplate FLA

    • 160 posts
    June 24, 2019 8:00 AM PDT
    Carl Coulter said:


    Minor cautionary note - Which all already know:

    The CB and conductors are sized from the tables FLC

    Overloads are sized from the Nameplate FLA


    Carl I would have to dispute this.  I don't see anything in the table or section 430.52 that requires us to use the FLC from the tables at the end of article 430.  IMO, The flc x 250% is based on the nameplate FLC

    • 29 posts
    June 24, 2019 9:39 AM PDT

    I agree with Dennis.  The Canadian Electrical Code (CEC) and NEC are very closely aligned, and nameplate values are to be used whenever they are available.  FLC tables are used only when nameplate values are not known.

    • 75 posts
    June 24, 2019 12:23 PM PDT

    Dennis, Derek - 


    430.6 Ampacity and Motor Rating Determination. 

    (A) General Motor Applications. For general motor applications,
    current ratings shall be determined based on (A)(1) and

    (1) Table Values. Other than for motors built for low speeds
    (less than 1200 RPM) or high torques, and for multispeed
    motors, the values given in Table 430.247, Table 430.248,
    Table 430.249, and Table 430.250 shall be used to determine
    the ampacity of conductors or ampere ratings of switches,
    branch-circuit short-circuit and ground-fault protection,
    instead of the actual current rating marked on the motor
    nameplate. Where a motor is marked in amperes, but not
    horsepower, the horsepower rating shall be assumed to be that
    corresponding to the value given in Table 430.247, Table
    430.248, Table 430.249, and Table 430.250, interpolated if ....

    (A)(2). says to use the nameplate for the overloads

    Previous cycles were not always as clear



    • 156 posts
    June 24, 2019 1:57 PM PDT

    I not part of NEC but I confirm Carl 50A. 

    If you check below screen shot, you have no 45A. it also confirms 50A. 

    • 160 posts
    June 25, 2019 9:41 AM PDT

    I guess I stand corrected.  I can only say that every exam I have seen I believe they use the nameplate for overcurrent protective device. 

    And then there is this so now I am a bit confused


    Exception No. 2: For equipment that employs a shaded-pole or
    permanent-split capacitor-type fan or blower motor that is marked with
    the motor type, the full load current for such motor marked on the
    nameplate of the equipment in which the fan or blower motor is
    employed shall be used instead of the horsepower rating to determine the
    ampacity or rating of the disconnecting means, the branch-circuit
    conductors, the controller, the branch-circuit short-circuit and ground fault
    protection, and the separate overload protection. This marking on
    the equipment nameplate shall not be less than the current marked on
    the fan or blower motor nameplate.

    • 75 posts
    June 25, 2019 10:51 AM PDT

    430.6.A.1, Ex 2

    I don't know what that applies to.  I don't even know what a "permanent-split capacitor-type" motor is.  "Shaded pole" sounds like ceiling fans.  So I lost here as well.

    430.6.A.1, Ex 3

    I think this one applies to sears compressors that have a "6HP" rating, yet have a 115V, 15A cord cap, or a Big Box shop vac rated for 5HP (and a 115V, 15A cord cap) 

    They are both strange stuff that have not come up in my work.

    As for the exams using nameplate, in the past the code has not been clear.  However I recall discussions where "FLC" related to the tables and "FLA" related to the nameplate.  It seemed reasonable at the time, but I m no longer clear on the reasoning.

    I do recall one class (Far enough back that rocks were still young) Paraphrased:

    The motor currents listed in the tables are conservative.  The issue is that if a motor is changed out and the new motor FLA is a bit different from the original.  If the nameplate FLA is used to size the conductors and CB, it could make a change, either a required larger conductor or a lower rating CB.  By using the conservative table amperages, set by the motor horsepower, the conductors nor CB ratings change.

    Since overloads are set pretty close and are easy to change, using the nameplate is reasonable.  Consider that if the motor is changed out to an energy efficient and has a significantly lower FLA than the original.  One might consider setting down the OL.

    I can honestly say that yes I have changed motors  and don't recall ever checking the overloads.  Not exactly true - might check to insure the OLs were not wrong from the get-go.  Or checked that there had not been a dead-of-the-night swap to get a motor up an running and that is why the motor failed BBR instead of tripping the overloads when the pump started dragging.