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  1. #1
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    Electrical Connections for Domestic Inverter Unit



    Hi,

    I've recently had a Mitsi inverter installed at my domestic premises by a local refrigeration engineer (and some much appreciated input from yourselves on here). We just used a plug connection to get things commissioned and the permanent electrics are now to follow along with a number of other jobs.

    Appreciate this is probably for an electricians forum but wondered whether anyone had any experience here of the best practice for connecting up these units electrically.

    Max load of the unit is 4.4amps so I have agreed with my electrician to take the connection via an internal fused spur from one of the ring main sockets.

    The electrician hasn't connected up one of these units before and he was happy just to take a feed from the FCU directly to the air con unit. I suggested it would probably be useful to have a (rotary isolator) switch next to the unit and he said fair enough. I agreed to pick up the isolator and we agreed that we would sort things out when he comes back. However, I thought I'd check if anyone on here has any experience of connecting up these units as I wanted to double check that the following is an acceptable approach ton the connection:

    • 2.5mm T&E from socket to adjacent internal FCU (with 10amp breaker as per unit manufacturer)
    • 1.5mm 3 core NYY-J through wall from FCU and short distance up the wall into rotary isolator with suitable gland (hole in wall sealed with silicon)
    • Craig & Derricott 25amp, 2P, IP66 rotary isolator (or similar good quality isolator)
    • 1.5mm 3 core NYY-J from rotary isolator with suitable gland and into air con outdoor unit (probably about 0.5m cable length required


    I thought NYY-J cable would be acceptable as mechanical protection is not required (unit is looked away down side of property and minimal cable lengths required, with the cable offering UV protection.

    I wasn't sure whether I would be better using a short length of 20mm conduit and a 1 way terminal box to cover the wall entry (1.5mm T&E could then be used) would be better but there would be minimal distance required to the isolator.

    Any thoughts would be appreciated as I want to make sure that my electrician's thinking is similar to those with more experience with these units.


    Much appreciated and many thanks,

    Damian



  2. #2
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    Re: Electrical Connections for Domestic Inverter Unit

    First of all,
    What fuse rating is your ring main fed by? is 2.5mm cable enough to cover this?
    The cable from your ring main to your fused spur by the indoor unit need to be sized according to the ring main's fuse.
    Anything "down stream" from your fused spur need to be rated for the size of the fuse in the spur.

    Then; is your ring main fed via an RCD? If so it is likely that it will trip due to the workings of your new inverter. Motor rated RCD has a tripping current of 150-300mA whilst RCDs protecting ring mains normally trip at 30mA.


  3. #3
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    Re: Electrical Connections for Domestic Inverter Unit

    Hi,

    Thanks.

    Yes - it is a standard 32amp socket ring main, protected by RCD which I think trips at 30mA. 2.5mm from socket to FCU is right but obviously the tripping point is a concern!

    The unit is a 3.5kW Mistubishi (https://airconditioning.mitsubishiel...2-54da09afed0f), although I guess that doesn'T matter.

  4. #4
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    Re: Electrical Connections for Domestic Inverter Unit

    Normally the power supply would start at the outdoor unit as that has the largest electrical load.

    An isolator local to that should be provided.

    The internal unit power is then taken from the correct terminals of the outdoor unit via a local, to the indoor unit, isolator. Only switch the live and neutral not the signal core.

    Ideally the power source would be your distribution board with the correctly rated mcb fitted.
    Brian - Newton Abbot, Devon, UK
    Retired March 2015

  5. #5
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    Re: Electrical Connections for Domestic Inverter Unit

    32A on 2.5mm T&E???
    Remember that the ring main itself got 2 cables going from the dist board to any one point in the circuit but from your ring main to the fused spur you will only have one cable...
    But your spark should know all about this.


    Screen Shot 2014-12-17 at 01.35.51.png

  6. #6
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    Re: Electrical Connections for Domestic Inverter Unit

    OK,
    Let us summarise what Brian and I have told you so far:
    - Feeding a fixed mechanical device from a ring main is not optimal.
    - The norm would be to have your AC system fead on a independent radial feed from the distribution board.
    - Normally you would also have the feed going to the outdoor unit first.
    - Current legislation demands a means of isolation "within arms reach" of each fixed electrical device.
    - All cables MUST be sized to cope with the rated current of the fuse/MCB feeding them.
    - RCDs feeding an AC unit should have a trip current >150mA
    - All electrical installations must be undertaken by competent and experienced persons.

    But none of this should be news, or even something requiring a second thought, for a qualified electrician.


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