Wiring a 24,000 BTU AC unit

[dynomo]

I have recently purchased a 24,000 BTU Split AC unit. The supplier has advised that I need an isolated feed from the consumer unit fused at a rating of 20A, and this should connect direct to the external unit of the AC system.
I have a brand new house and don't really want to be chasing wires into walls etc etc as the consumer unit is at the front of the house and i need my AC unit at the rear (conservatory).
I have direct access to the supply before it enters the garage via this weatherbox (this is situated as the wire exits the house and before it heads underground): http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A340303&ts=54960&id=44658
Inside the weatherbox is a 30A terminal block with the obvious, live, neutral and earth.
My question is, can I take a live feed from this weatherbox, after 3m, drill into an exterior wall and bring the live indoors (conservatory), offer some sort of circuit protection to comply with the 20A requirement whilst indoors and then continue to the other side of the conservatory where the supply will then exit the house into the external AC unit. Im not sure this is the right method. Another would be to take a live feed direct from the garage which has one of these inside: http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331827&ts=56032&id=63138
(well not the exact model but very similar).

Im sure there are several options but the right one is the one im after.
I am competent enough to carry out the basics myself, and will be calling in a professional to examine the work prior to connecting to the supply. I am also aware that this work is subject to notification

Thanks guys, i trust you to help me on this one as you've always done.

 

[RobFurn]

I have installed a couple of AC units recently a Mitsubishi ant Tohiba both installation guides state they have to be placed on there own circuit (MCB) so spuring form your garage feed is probably not the best. I assume the whetherbox is just a junction box where the cable from your CU meets the Underground SWA to your garage?
It would be better to go from your garage CU to the conservatory, however make sure the cable from your Main CU and the Underground cable can take the additional load.
Also as your adding a new circuit its notifiable work .

 

[bernardgreen]

The start up current of the compressor needs to be considered when planning the supply as well as the normal running current. The high current drawn by the motor before it reaches normal speed has to be used in calculating voltage in the supply cables and equipment. Too high a voltage drop and the motor will take longer to come up to speed. This can affect motor / compressor life time and have other effects such as noisy start up.

 

[dynomo]

cheers
wonder where 'securespark' is this time round? confused? i hope not,

I did assume the latter and am actually planning to go forward with the feed direct from the CU in the garage as opposed to the junction in the weatherbox. As you rightly state Rob, this is the junction between supply from main CU and cable running underground into garage.

If there are no vacant fuses in the CU inside the garage, what is the best way of connecting to the mains via a 20a/30a circuit breaker. Can I possibly split the feed into the garage so that it enters a seperate CU, i.e this one: http://www.screwfix.com/app/sfd/cat/pro.jsp?id=29659&ts=20130#

....And connect the AC unit direct to this??

 

[RobFurn]

For that price I would just change the Existing Garage unit to something with more ways. It would be just as easy and a neater job all round. "The Electric Center" do a 5 way one with RCCB incomer for about £45
however I bet someone on here could recommend a cheaper source.

 

[dynomo]

that does sound alot more convenient now, lol
I'll do some research now to see what I can find.
I take it something like this can be installed into a garage:
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331831&ts=22975&id=94258
If so, it is ideal.

Oh, one question Rob, for the wiring that will run outdoors (3m), which conduit is recommended, will this one suit http://www.screwfix.com/app/sfd/cat/cat.jsp?cId=A331821&ts=23501
I may aswell get started with the basics for now and deal with the nitty gritty later...

cheers bud

 

[RobFurn]

The cable that goes underground to your garage is likely to be SWA (check this first) so your better off with a matal clad consumer unit for your garage as insulated (plastic) consumer units are usually too thin to hold the SWA gland. Either that or use in conjunction with a metal adaptable box to gland off your SWA.


The conduit is fine as long as you dont bury it (it must be visable i.e. attached to a wall).

 

[dynomo]

thaks mate
anyone who can suggest the best CU for the garage to replace the exisiting, please do.

cheers

 

[dynomo]

Rob,
I have finalised plans now...
the existing CU in the garage is precisely this: http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331831&ts=40837&id=26018
I have the 2way system at the moment consisting of a 6a mcb and 16a mcb. I will be replacing the old CU for the 5way CU and shall be adding in one of these babies:
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331832&ts=40953&id=54816

The AC unit has got a max current demand of 19A at worst condition according to the specs. The cooling input current on system is 13.5A and heating is 14A. Length of cable to be run is approx 11m with no breaks and I am thinking of using 2.5mm SWA or 4.0mm SWA cables to be run via steel conduit around wall and partially underground (approx 5m). Overall voltage drop is working out at 3.13V with 2.5mm SWA cable and 1.985V with 4mm SWA cable, i think!! I am still a little sketchy on some of this but hope you can guide me if I make any mistakes. The max volt drop between origin and equipment mustn't exceed 9.2v right? Which is the best suited cable for the job, I am assuming it is 4mm to lower the voltage drop value.

Let me know what you think buddy, I think I have covered everything here!!

 

[ColJack]

swa doesn't need to be in metal conduit and doing so will actually de-rate it's current rating..

 

[r.bartlett]

Is this not part P ?

if so and he contacts B.C.O before he carries out the work, is he liable for an inspection fee ?

Cheers

Richard

 

[RobFurn]

Dynamo

2.5 will be ok and your volt drop is within limits so long as the your main CU feed is large enough (check the CSA) fom your main CU to external junction box and also check the SWA from junciton box to garage) also record the lengths then work the volt drop out as a whole. Your allowed to drop 4% as you say which is 9.2v.

Couple of questions. The feed from your Main CU at the front of the house is that on the RCD side of the board? Remember your garage sockets need RCD protection. I only ask as the Metal clad CU you have pictured has no RCD fitted. I suppose you could fit an RCBO to cover the sockets but check this anyway. As ColJack says you dont put SWA in conduit the metal armour provides the EARTHED and mechanical protection. I would use 3 core SWA (Phase, Neutral and Earth) if you use 3 core you must still earth the armour via the bango.

Finally you have selected a B type breaker this should be ok however as Bernard says the large startup current may disconnect this occasionally try it and if the MCB keeps tripping change it for a C type. I had this issue with an AC unit recently.
Finally (again) notify your Building contol office, as its notifiable work, the fee is about £85-100 then there is an inspection fee on top.

 

[dynomo]

Rob,
I thank you for your help, seriously, I wish I had your knowledge!!!
Anyway, yup the garage does have RCD protection via the main CU, which is brill. I will be notifying my BCO later today, and handing in my proposal no doubt. Overall voltage drop comes way under 4%, this is a figure direct from the main CU. That fact that 2.5mm SWA doesn't need steel conduit for exterior is ace, it makes life alot easier. Im actually looking forward to digging the trench etc lol. dont ask why!! (and yes I know the official procedure, dont worry..)
Great stuff regarding the type B CB, i'll pay close attention to that no doubt.
All that is left for me to do now is to get the AC lads in and get em to put up the blimmin thing!!! ITS HUGE!!!

 

[dynomo]

Rob,
Seeing that you are an AC expert!!
Can I incorporate one of these isolator switches into the ciruit for exterior mounting:
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=15054&ts=15424

I've seen them on various setups but am not sure what purpose they serve. I have a mate who has a split air 12,000 BTU unit but he didn't connect any such device or carry out any extensive electrical work prior to installation. His electrical connection was direct to the ring mains via a 13A plug from the interior unit and then two feeds that inter-connect the two units (interior and exterior). I take it there is a feed for each of the operations i.e. cooling and heating???

Sorry bud, I'm directing all this to you..

Anybody else wana jump in, please feel free!!!

 

[Steve]

professionally installed units have the feed to the outdoor unit.

This isn't a 3 phase unit is it?