Domestic Air Conditioning Unit Questions

[Damian30]

Hi,

I'm about to get an air conditioning/inverter unit installed by a refrigeration engineer. We are going to use a temporary flex/plug to commission and then I'll get an electrician to put a permanent feed in.

It would really be helpful to get some views on the best way to install the power supply, it will help me sound like I know what I am talking about when shopping around for a price for the work.

The unit has a starting/running current of 4.4A and power input of 0.9kW. It needs a 10A breaker.

It is going to be really difficult to give it a separate feed from the CU, it is about as far away as it could be. The nearest mains electricity is an inside socket which is on the upstairs sockets ring which is also protected by a RCD at the CU (13 years old).

Questions are:

1. I know it is not good practice but is it acceptable to take the power from this ring. I can't see the load on the upstairs socket ever getting anywhere near 32A, so hoping this will be okay?

2. If I can use the the ring, whats the best way of connecting in. I was thinking that a spur through the wall into a MK Masterseal Plus FCU (10A) would be the easiest approach. I'm not sure though if SWA cable would fit into this, although another cable type protected by flexible conduit might work (the outdoor unit is round the side of the house so low risk of interference).

Another alternative I though was to put a switched FCO inside next to the socket and bring a spur through to a rotary isolator with SWA then to the unit. The internal FCU would be highly visible with access by young hands but although a rotary isolator isn't essential (apparently) may be a more substantial option. I can't think of any other alternatives.

3. If connected into a spur from the RCD protected ring, would this provide RCD protection by default. I'm guessing being an outdoor unit, RCD protection is essential?

4. Mitsubishi optionally recommend providing a 6A switched protection between the outdoor and indoor unit. I'm guessing that this is so that the indoor unit can quickly be isolated, although in my situation it would be nearly as quick to go to the CU or switch for the outdoor unit. Does anyone have any thoughts whether I need to put in a switch here, I believe most installations don't have these.

5. Finally, the outdoor/indoor units are connected with a 4 core SY cable. I'll get an electrician to sort out my work (see attached photo) but do I need to get some SY type glands or could I just earth sleeve the braid and just tidy up how I've wired it so far?

I'd be very grateful of any views on the above ramble.

Many thanks,

Damian

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• Damian30
• 12 Sep 2014
• 1

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[flameport]

1. You can, it's only 900W
2. Either, but the means of isolation is normally required to be lockable and accessible/in view of the outside unit.
3. Obviously if the circuit already has an RCD, then whatever is connected to it will be protected by that RCD. However being outside does not mean an RCD is required.
4. Probably useful for when the inside unit is cleaned or whatever.
5. Proper glands are required, as is fixing the cables securely with appropriate cleats or other fixings.

Generally the sequence of installation is
1. Electrician installs the required new circuit for the device,
2. AC engineer installs and commissions the system.

Attaching some 'temporary' lash up with a plug does not help anyone, and is a total waste of time since it will have to be properly installed afterwards anyway.
It's also unlikely that any decent AC engineer will want to install a system where the mains electrical supply is not properly provided.

 

[eveares]

Hi Damian, here is my answer.

Providing you are not burring cable underground and mechanical protection is not needed, this is what I would do.

First of all, if the socket the spur is coming from is already RCD protected then no need for a another RCD, other wise would put a RCD switched fused spur in.

I would then use the switched fused connection unit on the inside with a 5 amp fuse in it taking a 2.5mm T&E spur from the socket to the fcu or by extending the ring final circuit to the fcu. (whats ever easier for you)

From the FCU, I would then 1.5mm T&E through the wall to a sutiable IP rated box with suitable glands where I would then take 1.5mm 3 core NYY-J to a 20A TP&N rotary isolator (also with suitable glands) and from that to the outside compressor unit. For the outside units glands, I would use them if it can accommodate them else use the strain relief built into the electrical connection cover.

Cable clipped to the outside wall to secure it or run in glue sealed plastic conduit also secured to the wall.

I would also use NYY-J cable from the outside unit to the inside unit, if not to late. For the picture you provided, I would put the two earth wires in crimped eyelets with vibration proof washers like the sheath of the cable going to the indoor unit.

Lastly the NYY-J cable can be replaced for SY cable, although not all SY cable is UV rated and is suitable to be used outside permanently. SY cable also has to be properly glanded with special galnds for SY cable what are rather expensive.

Regards: Elliott V

 

[Damian30]

Hi

Thanks both for your replies.

Elliot - it might be possible to change the interconnecting SY cable for NYY. I can see the benefits but don't understand why everyone talks SY in air conditioning. Thinking about it, most of the cable is in trunking outside apart from the last metre - might be easier for me to leave as SY but out the last bit in a 20mm flexible conduit?

When you come off your inside FCU. Can't you just go straight into a rotary isolator, rather have a second unit outside?

Thanks again.

Damian

 

[eveares]

When you come off your inside FCU. Can't you just go straight into a rotary isolator, rather have a second unit outside?

Yes, providing the rotary isolator has provisions for cable entry in the back of it; although not all rotary isolators do though, so be careful to make sure it does have a rare cable entry point otherwise you will need a second box as I originally mentioned.

 

[bernardgreen]

The unit has a starting/running current of 4.4A and power input of 0.9kW. It needs a 10A breaker.

The starting current is likely to be higher than the running current. If it is 4.4 Amps why specify a 10 amp breaker.

I know it is not good practice but is it acceptable to take the power from this ring.

provided the appliances supplied by that ring can cope / tolerate the transient voltage fluctuations as the air con motors cut in and out it should be OK

The earth connections to the frame of the unit look a bit "primitive".

Also no obvious means of clamping the cables, necessary to prevent the conductors under the terminals vibrating relative to the terminal and thus working loose over time.

For the safety of anyone working on the unit there should be a lockable isolator adjacent to the out door unit that cuts power to both indoor and out door units.

It is advisable to run the unit on its own supply from the CU via a double pole isolator so that in the event of damage to the outside unit the supply can be fully isolated from the unit to allow the RCD to be reset.

 

[Damian30]

Hi,

Thanks for the comments.

I've checked the spec again and the starting current is definitely stated as 4.4amps, with running current a maximum of 4.4amps (heating and 4.2amps (cooling).

I'll speak to the electrician and saes if it is worth giving it a go from the upstairs sockets ring, putting a fused spur connection inside and feeding into the rear of a 20amp rotary isolator (assuming I can find one with rear entry).

I'll pick up a couple of SY cable glands and earthing nuts for the interconnecting cable and connect the braid with a earth fly wire.

Thanks again.

Damian

 

[JohnW2]

I've checked the spec again and the starting current is definitely stated as 4.4amps, with running current a maximum of 4.4amps (heating and 4.2amps (cooling).

Fair enough - but, as others have said, that's rather odd. One would normally expect the starting current to be considerably higher than the running current.

Kind Regards, John

 

[Adam_151]

Sounds like its an inverter drive unit, John, as are most except the really low end units these days

 

[JohnW2]

Sounds like its an inverter drive unit, John, as are most except the really low end units these days

Ah, yes, that makes sense. Thanks.

Kind Regards, John