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  • wiring for adding a mini-split

    I am going to use a ductless mini-split:

    Wall Mounted 9 - 15,000 BTU Hi SEER - Fujitsu Ductless Mini-Splits

    ... for a small addition. I'm doing this to achieve some zoning, to avoid running new ductwork, and because of the extreme efficiency (25 SEER for the 1-ton unit). When my current main-house unit gives up the ghost (a 15 year-old propane dual-fuel split-system heat-pump, 11 SEER ?), I'm thinking to add another mini-split (probably 2 tons) to my great-room. Then I'll have a very efficient all-electric system (propane is so much more expensive than electricity in my area that it's about break-even even for electric-resistance heat). These units actually do not require any backup heat (at least in my climate).

    Anyhow, I'd like to avoid running another circuit for the mini-split; I'd like to run it on the same circuit as the current heat-pump. The recommended fuse size for the mini-split is 20 amps, and for the heat-pump it's 35 amps. The running currents are 5.5 and 16 respectively. The circuit is actually a 60 amp circuit (from the ultra-inefficient gas-pack originally in the home), supplying a 35-amp fused disconnect for the existing heat-pump - so the circuit can easily handle both units.

    So I'm wondering the best way to do this. One option would be to put a blind junction box in the crawlspace and run another 20-amp fused disconnect off the same circuit. It seems cleaner to run a sub-panel off the 60-amp circuit, with breakers for heat-pump and mini-split; the issue here is making sure I meet code for proximity of the disconnect to each unit. Either way, when I add another mini-split (later when the heat-pump dies), the outdoor unit will go near the current heat-pump and mini-split's outdoor units. I'd appreciate any suggestions.
    Last edited by RustyShackleford; 10-01-2012, 04:07 PM.

  • #2
    Re: wiring for adding a mini-split

    If you want it to work, it should be fine. If you want it to pass code, I think it needs to be on a dedicated circuit. Also, remember I am not an electrician, but you will be exceeding 80% of the breaker.
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    • #3
      Re: wiring for adding a mini-split

      Hopefully the electricians read some of these threads.
      AllurePlumbing.com
      • leak detection
      • drain cleaning
      • utility locating
      • conductor fault locating
      • and other specialties.

      Greensboro NC, Winston-Salem NC, High Point NC, Thomasville NC, Kernersville NC

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      • #4
        Re: wiring for adding a mini-split

        Originally posted by Gettinit View Post
        If you want it to work, it should be fine. If you want it to pass code, I think it needs to be on a dedicated circuit. Also, remember I am not an electrician, but you will be exceeding 80% of the breaker.
        I am not an electrician either - but am electrical engineer with some house-wiring experience, mainly here to do legwork for a much-more experienced but very busy friend who's helping me.

        I do not think I am exceeding the 80% thing. When the recommended breaker size for a unit is X, then you're supposed to use an X-sized breaker. That's the case of the 35-amp and 20-amp recommendation (for the existing heat-pump and new mini-split, respectively). Note that the "running current" for both units (16 and 5.5 amps, respectively) is WAY below 80% of the breaker size. Unless you're supposed to allow another 80% margin for a breaker that's feeding the 35-amp and 20-amp breakers (or disconnects); but I don't think so. Or unless each unit needs its own circuit, as you suggest. If that's the case, I'm pretty sure putting in a sub-panel will deal with it. I think the main issue there is that the sub-panel must be within the specified proximity (for a service disconnect) from BOTH outdoor units - which is doable I think.

        I hope some electricians here read this too. Please don't fear you're gonna help me burn my house down by responding. I'm not a moron, just not clear on the vagaries of code. And this WILL be done with my friend, and inspected by the county.
        Last edited by RustyShackleford; 10-01-2012, 06:36 PM.

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        • #5
          Re: wiring for adding a mini-split

          Building inspector came by just now for on-site consult (for this and the other wiring I'm doing in my addition).

          He said it was fine to run two fused disconnects off the 60 amp circuit - the existing one for my old heat-pump and a second one for the new mini-split. But he said it's quite difficult to join 6 gauge wires and I probably need some special connectors. However he said that I need to look at the specs on the old and new HVAC units carefully and if it says "max breaker size" (and not "max fuse size") , then I've gotta have breakers instead of fuses. I checked and this will not be an issue for me, but I thought I should note it.

          Instead (and not much more expensive because of the special connectors), he said I could run a sub-panel. Since it's in the same building as my main panel, it does not need a main breaker (the 60 amp breaker in the main panel serves this function). As I had hoped, since I'm only running 240v equipment off this subpanel, and that equipment doesn't have separate neutral and ground connections, it is ok for the sub-panel not to have separate neutral and ground; it would just have a ground bus.

          As I'd hoped, with either option I do not need to run a new cable from the main panel over to the HVAC area; the existing 6-2 (plus ground) wire will suffice. Also, the sums of the "minimum ampacity" for the two units cannot exceed 60 amps (I thought it would have been the sums of the "maximum fuse size" but it's not); and apparently the "running current" spec is insignificant for purposes of code and breaker sizes.

          Thanks to all who helped with this.

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          • #6
            Re: wiring for adding a mini-split

            So what happens when (anad it will) both units start at nearly the same time. You'll have max draw on both for a few seconds. Wouldn't this have to add up to less than trip current of your sub-feed breaker back int he main panel which is 60A IIRC.

            One instance where this could happen from a momentary outage caused by a storm when both units were running before power was lost. When the power comes back there will be no staggered start, the units and not that smart though they may take different times to go through a restart cycle.
            Last edited by Bob D.; 10-19-2012, 03:58 PM.
            ---------------
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            “If I had my life to live over again, I'd be a plumber.” - Albert Einstein
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            • #7
              Re: wiring for adding a mini-split

              Originally posted by Bob D. View Post
              So what happens when (anad it will) both units start at nearly the same time. You'll have max draw on both for a few seconds. Wouldn't this have to add up to less than trip current of your sub-feed breaker back int he main panel which is 60A IIRC.
              Darm good point. I'll be ok anyhow, since the spec'ed "max fuse size" (and the size of the fuses or beakers I will use in my disconnects or subpanel) are 35 amps and 20 amps, respectively. But it does make me wonder about the inspector saying the sum of the MCA's (minimum circuit ampacity) can be as high as 60 amps. But I'm not sure of definitions - maybe the starting surge current is guaranteed not to exceed the MCA.

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              • #8
                Re: wiring for adding a mini-split

                I was told a long time ago to figure 110% for start up....don't know why that number sticks out? If you use fuses you may need slow blow.

                I would have a similar post in the electrical section. Seems like they don't stray much from their threads.
                AllurePlumbing.com
                • leak detection
                • drain cleaning
                • utility locating
                • conductor fault locating
                • and other specialties.

                Greensboro NC, Winston-Salem NC, High Point NC, Thomasville NC, Kernersville NC

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                • #9
                  Re: wiring for adding a mini-split

                  Did he say the MCA can be as high as 60A? Are those his words?
                  I wold think it would be the other way around, and that the he said the MCA must be 60A or greater.

                  MCA is the minimum current rating (in Amps) for the wiring and any other components that make up the circuit.
                  Did he maybe say MOP (Max Overcurrent Protection) can not exceed 60A? Your breaker or fuse needs to be sized
                  to prevent the unit from drawing more than the MOP rating.

                  A good place to ask electrical questions is Mike Holt's forum.

                  http://forums.mikeholt.com/showthread.php?t=59757
                  ---------------
                  Light is faster than sound. That's why some people seem really bright until you hear them speak.
                  ---------------
                  “If I had my life to live over again, I'd be a plumber.” - Albert Einstein
                  ---------
                  "Its a table saw.... Do you know where your fingers are?"
                  ---------
                  sigpic http://www.helmetstohardhats.com/

                  Comment


                  • #10
                    Re: wiring for adding a mini-split

                    Originally posted by Bob D. View Post
                    Did he say the MCA can be as high as 60A?
                    Yes, he said that if the "minimum circuit amapacities" are less than 60 amps I should be ok, if they are less than 48 amps I am golden (and the latter is the case, since they are 21.3 and 10.5 amps, respectively).
                    MCA is the minimum current rating (in Amps) for the wiring and any other components that make up the circuit.
                    Yes, so if the circuit can handle 31.8 amps I am ok; in fact it can handle 60 amps, since it's a 60 amp breaker and 6 gauge wire.
                    Did he maybe say MOP (Max Overcurrent Protection) can not exceed 60A? Your breaker or fuse needs to be sized
                    to prevent the unit from drawing more than the MOP rating.
                    Regardless of if I go with a subpanel or two fused disconnects, each unit will be controlled by a breaker or fuse equal to the "maximum fuse size" for that unit, 35 and 20 amps respectively. I believe the idea of this is to make sure the fuse blows if the thing draws more than that many amps, as you say, because something is wrong if it draws that many amps; I think some people like to use smaller fuses/breakers, in order to be aware if the thing is performing suboptimally.
                    Last edited by RustyShackleford; 10-21-2012, 03:29 PM.

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