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Thread: Compressor motor marked spl

  1. #1
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    Compressor motor marked spl

    I am currently wiring a 208-240volt air compressor. On the motor where the horsepower should be is the acronym spl. The motor also says just simply 15amps. On the front of the tank in big letters is: 3.5 running horse power. The Unit came with a 250volt twist lock cord end. a. How do I size the O.C protection and conductors for this air compressor if there is no horse power given?B. Is a twist lock cord sufficent as a disconnect. The motor is thermally protected and there is an on off on the presser switch.
    Thanks!:-?

  2. #2
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    I would just match the cord caps rating.

  3. #3
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    if you interpolate 250.248, or estimate the flc using the value for 3 or 7-1/2, flc= approx. 22 amps

    spl means split phase

    hp = 3.5

    conductors: if this was continuous duty(?) then conductor ampacity should be 125% x 22 = 27.5 minimum (430.22)


    ocp: for an inverse time breaker (250% - 430.52), the breaker size would be
    2.5 x FLA = 2.5 x 15 = 37.5 maximum

    I'm guessing motor has internal overload/thermal protection ? if not, and if you're not using continuous, then these answers are wrong (?)

  4. #4
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    Quote Originally Posted by josoglory
    I am currently wiring a 208-240volt air compressor. On the motor where the horsepower should be is the acronym spl. The motor also says just simply 15amps. On the front of the tank in big letters is: 3.5 running horse power. The Unit came with a 250volt twist lock cord end. a. How do I size the O.C protection and conductors for this air compressor if there is no horse power given?B. Is a twist lock cord sufficent as a disconnect. The motor is thermally protected and there is an on off on the presser switch.
    Thanks!:-?

    SPL stands for Sound Pressure Level

    Basic Motor Terminology and Theory


  5. #5
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    The marking "SPL" is a trick some manufacturer's like A.O.Smith use on their homeowner-grade air compressor motors. They'll advertise these as 5 horse air compressors, but that's "peak horsepower". The real horsepower is less than 5, so they put "SPL" in the horsepower spot on the dataplate for "SPeciaL".

  6. #6
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    Quote Originally Posted by josoglory
    I am currently wiring a 208-240volt air compressor. On the motor where the horsepower should be is the acronym spl. The motor also says just simply 15amps.
    430-6(A)(1) "where the motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be the corresponding to the value given in Table 430-248", that is 3 HP & 17A.

    Quote Originally Posted by josoglory
    ...a. How do I size the O.C protection and conductors for this air compressor if there is no horse power given?
    Size the OCPD as per 430-52 & table 430-248: 17A x 2.50 = 42.5, use a 40A breaker and #8 THHN cu branch conductors 310-15, (building wire not NM or SE, these will need to comply with 334-80 & 310-15), the equipment grounding conductor is sized as per 250-122.

    Quote Originally Posted by josoglory
    B. Is a twist lock cord sufficent as a disconnect. The motor is thermally protected and there is an on off on the presser switch.Thanks!:-?
    Yes the cord cap can be used as the disconnect but it needs to be sized as per 430-110(A) 17A x 1.15 = 20A. If the motor is TP rated you will not have to provide OL protection.

  7. #7
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    Quote Originally Posted by tryinghard
    If the motor is TP rated you will not have to provide OL protection.
    Doesn't that mean he could then use #12 wire?
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  8. #8
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    Smile

    Folks, as I understand it this is a factory assembly with a factory installed cord cap.

    Match the cord cap and all will be well. :smile:


    20 amp cord cap = 20 amp branch circuit

    30 amp cord cap = 30 amp branch circuit

    KIS = Keep it simple

  9. #9
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    Quote Originally Posted by iwire
    20 amp cord cap = 20 amp branch circuit
    Okay, but will a 20a breaker start the compressor?
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
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    Quote Originally Posted by LarryFine
    Okay, but will a 20a breaker start the compressor?
    Yes, just like my 15 amp circuits have no trouble starting my "5 HP" 125 Volt vacuum I bought from the big box store. (Marketing Hogwash)

    Regardless what other choice do you have?

    I would not replace the cord cap with a larger one on UL equipment and I would not place a larger OCPD on a branch circuit supplying a receptacle.

    There is no way I would have say a 50 amp OCPD supplying a 30 amp receptacle.

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