Loft & Aircon circuit

[HDRW]

I want to put in a new circuit (let's take Part P as read) to supply a new split Airconditioning unit in my bedroom, and sockets in the loft.

The Aircon has an indoor unit high on the wall of the room, and an outside unit on a stand on the ground outside. I presume because of the latter that the whole thing needs RCD protection. Its maximum input rating is just under 1 kW, but the instructions say to feed it from a 16A breaker, presumably to allow for motor-start surges.

This is what I'm thinking:

32A /30mA RCBO in main CU, feeding a 4mm² T&E cable that runs up to the loft. Cable length will be about 12m, and at its most insulated it will be in a metal conduit of 15mm internal diameter.
Small CU in the loft with:
16A MCB for the Aircon
16A MCB for the sockets

2.5mm² T&E single to a 13A socket high on the wall of the bedroom below, to plug in the Aircon unit (cable run about 2m, some of which will be in insulation).
2.5mm² T&E ring for three double 13A sockets in the loft.

The loft sockets will only be used for low power devices like aerial amplifiers most of the time, and the occasional electric drill or vaccuum cleaner, so the 16A limit on the sockets will be plenty.

The wiring between the inside/outside Aircon units is specified in the instructions as "1.6mm" which is about 2.01mm², so I'm going to use 2.5mm² Multi-flex: http://www.tlc-direct.co.uk/Products/MF2dot5slash3.html

Does this all sound OK?

The thing I'm a bit iffy about is the small CU having two MCBs but no incoming switch or RCD (I'm thinking of strapping them together on the incoming side with the 4mm²). Would this be seen as a Bad Thing?

Finally, is there a good way to mount a CU in a loft where there aren't any vertical surfaces, or can I just make it up as I go along?

Cheers,

Howard

 

[Adam_151]

4mm is on the small side, it should be 6mm at the least, you might as well make it 10mm for future proofing.

The loft sockets, if you have a 16A breaker, you don't need to create a ring, just a radial will be fine, a ring will allow you to use a 32A breaker

EDIT: yes, your mini-cu without an isolator sounds dodgy to me, I don't know whether or not its allowed or not, but I doubt anyone would consider it good practice to put it like that

Why not just have a single 32A circuit from the main cu, with one circuit for all four sockets, if its a fair distance from the cu, a 6mm radial will probably be your best bet as opposed to a ring.

 

[mapj1]

Hmm
16A, 1.5mm sq sounds like the instructions have been translated from the continental where unfused 16A sockets are the norm. Its not Italian or German by any chance?
If so you should aim to provide an equvalent level of protection from UK standard parts, unless you particulary want to wire up to the Italian or German regs

In both those countries modern practice is to wire up all sockets and lights on any one floor (of area <50sqm) onto one 16A C type breaker. (the ones that screw in to replace "bottle fuses" have a surge trip of 8* the thermal rating +/-20%, which is also similar to a C type rating) The cable is then 1.5mm sq with fully insulated fullcross-section earth. (on an RCD the slightly higher earth impedance of UK cables is not an issue)
N.B. In the UK for such a radial, the floor area is 25msq, not 50 (is your usable loft floor area bigger than 5m*5m? )
This has a very similar characteristic to a UK 13A fuse backed up by a 32A B type MCB in practice. (the B type cuts off at instantly at say 70-150A or so, and the fuse melts after half an hour at 20A , the C type trips instantly at 80- 160A or so, and after 20 mins at 20A)
In this example, unlike a workshop or room where people will be active and could require space heating, I disagree with the future proofing argument - you don't really want to give anyone the option of setting fire to your loft. If its being rebuilt in 10 years as a loft extension the wiring should be redone properly then, and it wont be bousing a wsahing machine and dryer, ever !
If you are sure your total loft load is <4KW (and unless its huge or you are adding a fanlight vent to get rid of the heat build up it probably will be !) I'd suggest even a 16A RCBO in the CU downstairs and running it all as a radial, in 2.5mm unless voltage drop is an issue would be OK.
If you want an isolator for the air con in the loft (why not a fused spur by the aircon unit ?) the MCB route is fine, but will need labelling, as it will not be obvious where to look if it pops off one day.
Or for up to 7KW of loft loading, use a 32A RCBO or MCB in CU, and make a normal socket ring with Air -con as a 13A fused spur, no mini CU needed.

 

[ban-all-sheds]

I want to put in a new circuit (let's take Part P as read) to supply a new split Airconditioning unit in my bedroom, and sockets in the loft.

Did you do much product research? What make/model did you go for, and why, and ditto supplier?

I was looking at some the other day, and saw 3 different makes/models, with 3 different BTU/h ratings, yet they all claimed the same room volume for suitability, which seemed odd (the variations in BTU/h were substantial).

 

[HDRW]

Hmm
16A, 1.5mm sq sounds like the instructions have been translated from the continental where unfused 16A sockets are the norm. Its not Italian or German by any chance?

It's not 1.5mm², it's specified at 1.6mm diameter (yes, really!). The unit is made by Hitachi, but I'm not sure where. It comes with instructions in six European languages, including English. They all specify the same cable spec between the units, and specs. for the cable feeding it by length - up to 15m it's 2.5mm² "in consideration of the blocked rotor current", so I assume they mean if the compressor seizes that the cable has to cope until the fuse blows / MCB trips.

If so you should aim to provide an equvalent level of protection from UK standard parts, unless you particulary want to wire up to the Italian or German regs

In both those countries modern practice is to wire up all sockets and lights on any one floor (of area <50sqm) onto one 16A C type breaker. (the ones that screw in to replace "bottle fuses" have a surge trip of 8* the thermal rating +/-20%, which is also similar to a C type rating) The cable is then 1.5mm sq with fully insulated fullcross-section earth. (on an RCD the slightly higher earth impedance of UK cables is not an issue)
N.B. In the UK for such a radial, the floor area is 25msq, not 50 (is your usable loft floor area bigger than 5m*5m? )
This has a very similar characteristic to a UK 13A fuse backed up by a 32A B type MCB in practice. (the B type cuts off at instantly at say 70-150A or so, and the fuse melts after half an hour at 20A , the C type trips instantly at 80- 160A or so, and after 20 mins at 20A)

Well for protection it says "16A time-delay fuse", but I assume a B16 MCB will do (or should I consider a C16?)

As for floor area, if you mean the bit you can walk around (without crouching down under the purlins) then yes it is just about 5m x 5m.

In this example, unlike a workshop or room where people will be active and could require space heating, I disagree with the future proofing argument - you don't really want to give anyone the option of setting fire to your loft. If its being rebuilt in 10 years as a loft extension the wiring should be redone properly then, and it wont be bousing a wsahing machine and dryer, ever !

Indeed - and it would be hard to put in a thicker cable than 4mm² because the conduits that lead from under the stairs to the upstairs underfloor aren't really big enough, and because of the route it would be ridiculously hard to do anything about it. The work to turn the loft into a room (putting in deeper joists and rearranging the rafters) would be orders of magnitude greater than running a new cable - and it isn't any easier now than it would be then.

If you are sure your total loft load is <4KW (and unless its huge or you are adding a fanlight vent to get rid of the heat build up it probably will be !) I'd suggest even a 16A RCBO in the CU downstairs and running it all as a radial, in 2.5mm unless voltage drop is an issue would be OK.
If you want an isolator for the air con in the loft (why not a fused spur by the aircon unit ?) the MCB route is fine, but will need labelling, as it will not be obvious where to look if it pops off one day.
Or for up to 7KW of loft loading, use a 32A RCBO or MCB in CU, and make a normal socket ring with Air -con as a 13A fused spur, no mini CU needed.

All understood - the load will certainly stay under 4kW.

Thanks for the advice...

Cheers,

Howard

 

[ban-all-sheds]

It's not 1.5mm², it's specified at 1.6mm diameter (yes, really!). The unit is made by Hitachi, but I'm not sure where. It comes with instructions in six European languages, including English. They all specify the same cable spec between the units, and specs. for the cable feeding it by length - up to 15m it's 2.5mm² "in consideration of the blocked rotor current",

Also in consideration of the fact that 1.6mm dia is 2mm², so the first size that meets that is 2.5mm²..

N.B. In the UK for such a radial, the floor area is 25msq, not 50

Where is that defined?

Well for protection it says "16A time-delay fuse", but I assume a B16 MCB will do (or should I consider a C16?)

A C16 is more likely to be appropriate than a B16 for this type of device (compressor motor). I'd be tempted to buy both, and try the B16 first, though.

As for floor area, if you mean the bit you can walk around (without crouching down under the purlins) then yes it is just about 5m x 5m.

Even though it's not been converted, you could probably justify working the floor area out using the same calculation that you would to find out what floor area you would be allowed if you did convert and did not make any changes to the roof profile..

 

[Steve]

a "16A time delay fuse" would be interpreted as a C16 in my opinion. Makes sense too, large starting current of a big compressor etc.

 

[HDRW]

Did you do much product research? What make/model did you go for, and why, and ditto supplier?

I was looking at some the other day, and saw 3 different makes/models, with 3 different BTU/h ratings, yet they all claimed the same room volume for suitability, which seemed odd (the variations in BTU/h were substantial).

I had a look at a number of them for sale on eBay, checked the specs on manufacturers' web sites, and also had a look at the ones that B&Q warehouse (sorry!) are selling. It's made by Hitachi (model numbers RAS-25HY4 / RAC-25HY4 for the indoor/outdoor units respectively) and the features I liked were that it's a reversible heat-pump, so it heats as well, using less than a third of the electricity that an electric heater would. It will also de-humidify alone, if the temperature is OK but it's damp.

It uses a DC motor in the compressor, so instead of just turning on and off it ramps the speed down as it approaches the temperature you've set, so you get better efficiency and less hunting around a temperature. And hopefully less of the annoying start-up noise that you often get.

The rating of this unit is 2.5kW of cooling (about 9,000 btu/h, I think) and this is probably going to be too much for the size of room, but leaving the door open means it will cool the whole upstairs somewhat, and may even affect downstairs a bit (cold air sinks). And anyway, that's about the smallest you can get! I'm much less worried about the oversizing because of the DC aspect I mentioned above (which they call "DC Invertor", rather strangely!).

It gets an "A" energy rating, incidentally.

I bought it in an eBay auction, from Phoenix Air Conditioning, in Cheshire. They had a number of them and I bid on several until I got one for the sort of price I wanted to pay. The only disadvantage is that it needs a proper refrigeration engineer and his equipment to commission it (unlike the B&Q ones, which are user-plug-together) which isn't cheap, but overall I'm happy with what I'm getting for my money.

Cheers,

Howard

 

[ban-all-sheds]

Thanks.

 

[Steve]

I'm not sure about those B&Q models, I picked up a CD with an installation guide. Apparently you need a 1M drop between the indoor and outdoor unit. Is this normal? I wanted one in my bedroom, but the only external wall is the front of the house, obviously not the most aesthetic place! Was going to put the outdoor unit in the loft (well ventilated!), but with these i cant. Also, the connecting pipes are inside a plastic jacket (0-shape), and the guy on the video reccomends fixing this to the wall with drainpipe clips, not very neat.

Careful with the ones on ebay - some need 3 phase supply.

 

[HDRW]

I'm not sure about those B&Q models, I picked up a CD with an installation guide. Apparently you need a 1M drop between the indoor and outdoor unit. Is this normal?

Well since they have the pipework pre-made, it may be something to do with that. I don't know about the refrigeration pipes, but on mine the condensation drain needs to run downhill all the way, because it's not pumped. That may be an issue on the B&Q ones.

I wanted one in my bedroom, but the only external wall is the front of the house, obviously not the most aesthetic place! Was going to put the outdoor unit in the loft (well ventilated!), but with these i cant.

I'm not sure this is a good idea - even if it's really well ventilated, the timbers are going to get some very strange temperature-cycles, not to mention the damp. And the noise - you may find you can't sleep even if the room is cool enough!

Careful with the ones on ebay - some need 3 phase supply.

It's OK, I established that first! Mine is certainly single phase, and can run from a 13A plug if needed.

Cheers,

Howard

 

[HDRW]

What make/model did you go for, and why, and ditto supplier?

I was looking at some the other day, and saw 3 different makes/models, with 3 different BTU/h ratings, yet they all claimed the same room volume for suitability, which seemed odd (the variations in BTU/h were substantial).

BAS: You might like to know they are selling another one of the type I got - see item 5980087852 on eBay (finishes tomorrow afternoon). There's a link on there to a calculator for room-size (and type of room) to find the kW of cooling needed.

Beware that the auction doesn't include any pipework (or cabling) and it has to be commissioned by someone who has the equipment and knows how to use it! And hiring someone in could easily cost more than you pay for the unit.

Cheers,

Howard

 

[Steve]

Aren't refrigeration engineers like sparks? - they only like working with their own stuff that they like? I have a feeling they wont like installing cheaper models that you've bought on ebay.

 

[ban-all-sheds]

Beware that the auction doesn't include any pipework (or cabling) and it has to be commissioned by someone who has the equipment and knows how to use it! And hiring someone in could easily cost more than you pay for the unit.

Indeed. This from the item on eBay:

Q: approx How much would it cost to install this. Also does it come with the wall brackets?? Thanks.
06-Jun-05
A: Installation cost (by us) typically £650-£850 + VAT depending on location in country & distance between indoor & outdoor units. (outdoor unit) Wall brackets are not supplied as standard with the unit


Hmmm - where's the DIY aircon forum?

 

[plugwash]

a "16A time delay fuse" would be interpreted as a C16 in my opinion. Makes sense too, large starting current of a big compressor etc.

type C breakers have a higher fast trip point but once that point is hit they still trip out insanely fast.