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Branch Circuit Selection

    • 2 posts
    July 10, 2019 3:19 AM PDT

    For an A/C unit (heat pump), what is the required disconnect (rated at 208 V) for the following: Compressor Motor = 21.2 RLA, 96 LRA, Branch Circuit Selection Current = 21.2 A, Indoor Fan Motor = 6.8 FLA, Outdoor Fan Motor = 2.4 FLA. The unit also has a minimum circuit amps of 34.1 printed on the nameplate.

    • 157 posts
    July 10, 2019 4:28 AM PDT

    I am a bit uncertain what you have here.  Is this a mini split unit or is this a standard system with a heat pump outside and a blower unit inside that has its own duct system.  The reason I ask is because when you say the unit  is 34.1 amps nameplate are you talking about the heat pump itself?  

     

    Assuming a heat pump only this will depend on the wiring method.  If you are using a non metallic cable such as nm then a #8 is required however, if the wiring is in conduit you could get by with a #10 based on 310.15(B)(16) at 75C and then using 240.4(G)

    • 29 posts
    July 10, 2019 12:31 PM PDT

    I'm making the assumption that at 208V, you are in the North American market as this is typically a 60Hz voltage.  If your unit has an MCA (minimum circuit ampacity) rating on the nameplate, it should also have a MOCP (maximum overcurrent protection device) rating as well. Look closely for it. The disconnect frame size generally needs to match the frame size of the overcurrent device.  My experience is that the MOCP is generally higher than the MCA.  Given that the MCA is 34.1, your overcurrent device will exceed those 34.1 amps and as such, a 30A disconnect will be too small.  I suggest that the disconnect size needs to be the next frame size higher, or 60 amps.

    • 157 posts
    July 10, 2019 1:36 PM PDT

    The actual disconnect must be no smaller than 115% of the FLA.  =  39.2 or 40 amps.  The overcurrent protective device can be larger than the branch circuit conductor rating simply because the heat pump has overload protection built in.  The OL will protect the wire while the overcurrent protective device only protects from short circuit and ground fault.

     

    440.12(A)(1) Ampere Rating. The ampere rating shall be at least
    115 percent of the nameplate rated-load current or branchcircuit
    selection current, whichever is greater.

     

    I have seen it often where there is a #10 wire with a 40 or even 50 amp overcurrent protective device.  Generally the overcurrent protective device can be 175% of the fla because of the OL protection.  However if the unit has a max overcurrent protective device then you must use those numbers

     

    440.22 Application and Selection.
    (A) Rating or Setting for Individual Motor-Compressor. The
    motor-compressor branch-circuit short-circuit and ground-fault
    protective device shall be capable of carrying the starting
    current of the motor. A protective device having a rating or
    setting not exceeding 175 percent of the motor-compressor
    rated-load current or branch-circuit selection current, whichever
    is greater, shall be permitted, provided that, where the
    protection specified is not sufficient for the starting current of
    the motor, the rating or setting shall be permitted to be
    increased but shall not exceed 225 percent of the motor rated load
    current or branch-circuit selection current, whichever is
    greater.
    Exception: The rating of the branch-circuit short-circuit and ground fault
    protective device shall not be required to be less than 15 amperes.

     

    Of course the disconnect must also be rated for the HP of the unit.

    • 43 posts
    July 11, 2019 11:27 AM PDT

     The ampere rating of the disconnect switch must be at least 115% of the sum of the individual loads (440.12(B)(2). The individual loads generally equal the maximum overcurrent protection (MOP), which is shown on the nameplate. Your loads are the rated load current 21.2 + 6.8 + 2.4 = 30.4 x 1.15 = 34.9. A 60-ampere disconnect switch is required.

    • 157 posts
    July 11, 2019 11:42 AM PDT

    I agree a 60 amp disconnect is required since they don't make a 40 amp switch. 

     

    My first post was responding to the original poster whom I thought was asking about overcurrent protective device as a disconnect not the disconnect itself.  

    • 29 posts
    July 11, 2019 2:16 PM PDT

    Sorry gents, but I'm on the run these days.  Nice to be busy! 

    I haven't found the time to look up where I found this but, I recall from my last HVAC job that if the manufacturer supplied a MOCP and MCA rating on the nameplate, then we must adhere to that specification.  I recall doing the calculation that Ronnie showed just to prove to myself that the nameplate was right, but I came up with a slightly different result.  The numbers were borderline, but I ended up following the manufacturers spec because of what I read 'somewhere'.  

    • 157 posts
    July 11, 2019 4:07 PM PDT

    No question, the nameplate overrules the nec.