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Thread: Air Conditioner Disconnect for Mitsubishi Mini Split

  1. #1

    Default Air Conditioner Disconnect for Mitsubishi Mini Split

    Been a while since I posted here but I always received good advise here.
    I am having a 36K Mitsubishi Extreme dual zone air conditioner and heat pump installed and had a few questions. The Ac will be done by a licensed contractor but I am responsible for the Ac and the Disconnect.
    The model is MXZ-4C36NAHZ

    http://www.mitsubishicomfort.com/sit...e.pdf?fid=1010

    From what I can see on their PDF it needs 208 / 230V,1-Phase, 60 Hz with the Maximum breaker size being 50 Amp.

    A few years ago when I redid the basement I put in a Murray sub panel that has nothing currently connected to it. It is wired to the main panel on a 60 Amp breaker and has passed inspection already. I installed it just for this purpose.

    So………..
    Can I just leave the breaker in the Main Panel at 60 AMP, put a 50 AMP breaker in the Murray sub panel, wire it to the Disconnect with copper Romex 4-3 NM-B = 70 amps or Romex 4-2 NM-B = 70 amps
    (Near as I can tell from the specs the inside unit is DC powered and doesn't need 110 so neutral would not be needed correct? )

    Also since I'm putting a 50 Amp breaker in the sub panel is there any need for a fused disconnect or will an unfused model be sufficient.

    Distance from Sub panel to outside is not more than 10 feet as sub panel is inside direct;y opposite where I antiquated the Air Conditioner would be installed

    Finally
    Would NM be the correct choice for the wire type or would THNN in conduit be better?
    Thanks
    John M

  2. #2
    Join Date
    Dec 2003
    Location
    Kent, WA
    Posts
    8,496

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    These are still a pain in most areas because inspectors haven't decided how to deal with them and each manufacturer does things a bit different and not always in a code compliant matter.

    There are 3 aspects to the circuit -- wire size, over current protection, and disconnecting means. This unit needs wire sized at 42A or larger (the unit's MCA rating). SO 6-2 NM cable would be acceptable. You put a 50A breaker on this so you could either change the breaker feeding the subpanel to 50, put a 50A circuit in the subpanel for this, or put a breaker in the disconnect rated at 50A.

    The disconnect must be within sight and no more than 50' away from the unit receiving the branch circuit. If the outside unit gets the circuit, then the disconnect needs to be outside. If the wire starts from inside and terminates in a disconnect on an outside wall, you can still use NM cable. If that wire travels outside in a conduit or on the surface, it must be UF cable or wet rated wire. If you put a 50A breaker in the subpanel, then you can use a disconnect box that has no over current protection (e.g. either a pull out or a switch that looks like a breaker but is really just a switch with no over current protection). Usually, you use a flex conduit whip from the disconnect to the unit with individual THWN wires inside the conduit.

    Now the tricky part. The inside unit needs a disconnect too. Some places allow the main unit disconnect to be the disconnect for the other half. Some do that, but require a label on the other unit indicating where the disconnect is. Others need a disconnect box for the other half (assuming inside unit). These many times are low power, so a regular switch can be used. But sometimes there a 3 wires, and always at least 2. The disconnect may need to disconnect all poles (so a 2 or 3 pole switch). In your case, if the inside subpanel is within sight of the inside unit, the breaker feeding the outside disconnect can be your inside disconnect.

    Check your instruction to see what is recommended. I didn't see this covered in your units documentation. If nothing there, ask your local inspection department how they want you to handle that. These have been around long enough now that you'd think the manufacturers would address it in the instructions.

    Finally, 2014 code requires an HVAC service receptacle (120V, 15 or 20A) within 25 feet of the unit. If you have an outside receptacle there you are done. Otherwise, you need to run a circuit for that too from your subpanel.
    Last edited by suemarkp; June 12th, 2015 at 12:10 AM.
    Mark
    Kent, WA

  3. #3

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    Mark
    Thank you for your response. Answers my question completely.
    John M

  4. #4

    Default

    So a little confusion. Thought I was done.
    The contractor installed the Air Conditioner but it did not come with an electrical whip to connect it to the disconnect.

    As indicated by Mark's reply indicated earlier the unit requires min SO 6-2 NM cable. It is on a 50A breaker in a sub panel connected to a non fused disconnect.

    So here's my question.
    The unit did not come with a whip and the largest I can find is a 3/4" flexible conduit whip with 3 separate 8 AWG THWN wires inside. It's 6 ft long.

    If I'm understanding this correctly the unit would require a 6 AWG whip as it's on a 50 amp circuit and the maximum amperage on 8 AWG is 40 amps. Anything larger than 3/4" 8 AWG does not seem to exist at least not pre made. If I make my own am I correct that I have to go to 1" conduit because 3/4" conduit is too small for (3) 6 AWG THWN?

    I do seem to recall reading somewhere that the requirements for Air Conditioners, Heat Pumps and motors differed from standard electrical requirements in terms of wire size.
    Do I use the 3/4" whip with the 8 AWG or build my own 1" whip with 6 AWG?
    Thanks again

  5. #5
    Join Date
    Dec 2003
    Location
    Kent, WA
    Posts
    8,496

    Default

    You need 2 wires of sufficient ampacity for the MCA of the unit and a grounding conductor that is #10 copper (grounds aren't as large as the ungrounded conductors once you get over 30 amps).

    Most AC disconnects have 75C rated terminations, as do most AC compressor units. Therefore, you are allowed to use the 75C ampacity column in the code book if you use 75C or greater rated wire. THWN wires, which are commonly available at Home Depot and Lowes, are rated at 75C in wet areas (outside is wet by definition, even inside a conduit whip).

    Therefore, you could use THWN #8 copper for this as it is rated at 50A at 75C. If the disconnect or compressor terminals were only rated at 60C (or not labeled), then you'd need to use #6 copper. NM cable is limited to its 60C rating, so you had to use #6 with that (and the larger wire is better for a long run anyway for voltage drop reasons).

    A 1/2" flex conduit can take 3 #8 THWN conductors and a 3/4" flex conduit can take 4 #6 conductors. So a 3/4" with #8's is fine.
    Mark
    Kent, WA

  6. #6

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    Thank again Mark.
    jm

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