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