Loft & Aircon circuit

[Lectrician]

If you are running new cables - why not just run a 4mm radial for the couple sockets (or even 2.5) on a 32 or 20amp respectivly.

For the aircon, no RCD required. Fit 20amp IP56 isolator adjacent external compressor, and wire to this in 4mm on a 20amp breaker (C type), again, depending on run, 2.5 may be ok but would need calculating.

What is this conduit you talk of - 15mm internal diameter?? And where is it run??

Air con pipes that travel from inside to out need main eq bonding - most units come with a stud ready for this on the internal or external unit - rarely used - but should be!

 

[HDRW]

Lectrician, Thanks for the advice...

If you are running new cables - why not just run a 4mm radial for the couple sockets (or even 2.5) on a 32 or 20amp respectivly.

I want to limit the number of cables I run because of the difficulty of the route (see below). If the 4mm² cable isn't really enough for the Aircon and the sockets together, I'll scrap the sockets idea - it was just a convenience thing "while I was there".

For the aircon, no RCD required. Fit 20amp IP56 isolator adjacent external compressor, and wire to this in 4mm on a 20amp breaker (C type), again, depending on run, 2.5 may be ok but would need calculating.

I'm not sure this is necessary - because of the "DC Invertor" aspect of this unit, it's powered from the indoor unit, and I'd much rather isolate the whole thing than just half of it - and there are four conductors (plus Earth) connecting the two units, so I'm not even sure if there is mains between them, let alone which ones to switch/protect! The instructions make no mention of any switching or breaker between the units, just the on the feed to the indoor one.

What is this conduit you talk of - 15mm internal diameter?? And where is it run??

This house was built in 1937 and wherever cable runs are buried in walls they are in thin-walled steel conduit, either about 10mm or 15mm interrnal diameter - probably actually 3/8" and 5/8", considering its age. The mains comes in under the stairs, and since the top three steps are winders, there is no way to go directly upwards from the understairs cupboard to the underfloor upstairs. So when they built the house they installed six of the larger sized conduits between the ceiling of the cupboard, upwards and through the wall that seperates the bathroom from the top of the stairs, into the underfloor in the bathroom. They're about two feet long and they are the only way to run cables from the CU to upstairs. When the house was built there were only four circuits (lights, immersion heater, up and down rings) so I'm rather glad they saw fit to put in six conduits! But I now have only one left unused, so one 4mm² cable is pretty much the limit for this project.

Air con pipes that travel from inside to out need main eq bonding - most units come with a stud ready for this on the internal or external unit - rarely used - but should be!

That's something I hadn't thought of, and isn't mentioned in the instructions. Isn't the fact that they are connected to the units themselves at each end, and these are earth bonded, be good enough? When you say there is a stud (I haven't looked yet), does this mean you bond the pipes to it, or bond it to the house Earth? And where do you get Earth clamps for 1/4" and 3/8" pipes?

Cheers,

Howard

 

[Lectrician]

Most a/c units have the feed to the outdoor unit, and 3 or more cores to the internal unit - yours is the other way, so a DP 20amp moulded switch will be fine - don't use an FCU or plugtop - the 13amp fuse will blow

The bonding is required as you have an extraneous part introducing a potential from outdoors. 10mm bond to the CU required, although very often overlooked by installers. There is usually an M8 or similar stud on the chasis of either of the units - bond to one of them.

If you can give cable length etc, we can determine the min size of cable needed.

 

[ban-all-sheds]

The bonding is required as you have an extraneous part introducing a potential from outdoors.

Not necessarily? Could not the external component be of Class II construction?

 

[Lectrician]

umm - most a/c units I have seen have copper pipework from inside to out - although if this is a self fit system, it could very well have push fit plastic I suppose??? Never seen it.

Is this the case???

 

[ban-all-sheds]

No idea - just wondering if it's a possibility.

 

[HDRW]

umm - most a/c units I have seen have copper pipework from inside to out - although if this is a self fit system, it could very well have push fit plastic I suppose??? Never seen it.

Is this the case???

No, the pipes are copper, so the two units are definately connected that way. But there is also an earth core in the cables connecting them, so they're bonded to each other that way too.

Cheers,

Howard

 

[HDRW]

Most a/c units have the feed to the outdoor unit, and 3 or more cores to the internal unit - yours is the other way, so a DP 20amp moulded switch will be fine - don't use an FCU or plugtop - the 13amp fuse will blow

Why is that? The maximum input rating is a tad under 1kW - I know motors have a heavy startup surge, but is it as much as that?

The bonding is required as you have an extraneous part introducing a potential from outdoors. 10mm bond to the CU required, although very often overlooked by installers. There is usually an M8 or similar stud on the chasis of either of the units - bond to one of them.

OK, I'll have a look. I take it this in addition to the earth connection that's part of its supply wiring?

If you can give cable length etc, we can determine the min size of cable needed.

From the main CU to the proposed loft-CU will be about 7m, from the loft-CU to the indoor A/c unit will be about 2m, between the A/c units about 5m.

Cheers,

Howard

 

[mapj1]

If the start up surge really blows a 13A fuse, you'll have trouble holding in a C16 or B32 breaker. I'd be very surprised if it was that bad. A UK 13A fuse is really quite slow to blow.
If you plug a short fat 13A fused lead into a B32 protected ring main, and slice it with someone else's oldest curtters, the B32 will trip and the fuse will live to fight another day. This is really not a test to try at home, or at work, but is really true (I have the cutters with the hole in to prove it !)
After looking up the curves of current and time I am no longer surprised. OK its only true for a frisky B32, or C16, but worth bearing in mind. As I said earlier, 16A C type is very similar to 13A fuse plus B32 MCB - we are closer to Europe than we think.

 

[Lectrician]

I have seen a a/c unit rated at 10amp blow 13amp fuses repeadidly - the a/c engineers spurred it from the closest ring main (which was a C32 I recall).

Why do you want a Loft CU howard?? Are you still wanting sockets or sommet??

 

[HDRW]

I have seen a a/c unit rated at 10amp blow 13amp fuses repeadidly - the a/c engineers spurred it from the closest ring main (which was a C32 I recall).

When you say "rated" do you mean it was said to need a 10A supply, or it was actually 2.3kW input or thereabouts? If the latter, that's about 28,000Btu/h of cooling, which is a hefty beast - enough for a small house. Actually, about seventy square metres - not a very small one!

Why do you want a Loft CU howard?? Are you still wanting sockets or sommet??

I would like to if possible, just to save the hassle of running an extension lead up through the hatch when I want to hoover up there, or use a lead-light in the darker corners. I'm now thinking of going for a 20A MCB in the main CU supplying the 4mm² to the loft CU - that's well within the cable's rating, and if it nuisence trips it would only be because the start-up surge of the Aircon happened at exactly the same moment I started the hoover - that's a level of nuisence that I'm prepared to risk! (I believe the technical term for this is that I'm taking account of diversity...)

Cheers,

Howard

 

[Adam_151]

I've seen the 13A plugtop fuse and B32 Breaker thing as well.

When water found its way to into the electrics of our kettle, it worked until this water boiled (it was on the underside of the element, the element was not in the water container part of the kettle, but rather below the metal plate that forms the base of it) then with the water must have caused a short, and took out both the 13A plug top fuse as well as the 32A ring main breaker (actually think it might have been type 2, not B, not sure how that fits into things)

 

[mapj1]

I have seen a a/c unit rated at 10amp blow 13amp fuses repeatedly - the a/c engineers spurred it from the closest ring main (which was a C32 I recall).

sorry - we may be at cross purposes - I'm not surprised a 2KW air con has trouble with 13A fuses at start-up, but I suggest it might well have had trouble with a C16 or B32 breaker as well.
However, I thought this was only a 1KW air con, with makers recommending a 16A slow blow fuse (perhaps more or less equivalent to soemthing like a C type MCB) recommended, in which case, I wanted to suggest, a 13A fuse backed by a B32 trip would be very similar, and then it could be a fused spur of a 32A radial, that also feeds the loft sockets, and the complexity is vastly reduced as ther is ohnly one circuit.
Of course, it is always possible I have grabbed the wrong end of the stick again.
Howard - have you tried plugging it in on the ground yet ? (before you get it up there and find there is not enough oomph in the supply.)

As regards the floor area for a radial circuit - I am wrong again.

here http://www.tlc-direct.co.uk/Book/6.3.3.htm
has

Two types of radial circuit are permitted for socket outlets. In neither case is the number of sockets to be supplied specified, so the number will be subject to the constraints of load and diversity. The two standard circuits are:

1. - 20 A fuse or miniature circuit breaker protection with 2.5 mm² live and 1.5mm² protective conductors (or 1.5 mm² if m.i. cable) feeding a floor area of not more than 50 m². If the circuit feeds a kitchen or utility room, it must be remembered that a 3 kW device such as a washing machine or a tumble dryer takes 12.5 A at 240 V and that this leaves little capacity for the rest of the sockets.

2. - 32 A cartridge fuse to B888 or miniature circuit breaker feeding through 4 mm² live and 2.5 mm² protective conductors (or 2.5 mm² and 1.5 mm² if m.i. Cable) to supply a floor area no greater than 75m².

Quite why the expected demand for power in an area served by a 32A ring main versus a 32 A radial are not the same I am unclear.

Mind you - these guys //www.diynot.com/pages/el/el011.php
disagree, but they dont say 25msq either

Radial circuits can therefore only serve a smaller area. Using 2.5mm2 cable combined with a 20amp fuse/MCB an area of 20 square metres (24 square yards) is permissible. For 4mm2 cable combined with a 32amp MCB or a 30amp cartridge fuse (a re-wirable fuse is not allowed) an area of 50 square metres (60 square yards) is permissible.

Ah well, someone has to be wrong! (I wonder if it was once 25 yard sq and then got changed in a bid to harmonise...)

 

[ban-all-sheds]

Mind you - these guys //www.diynot.com/pages/el/el011.php disagree

What do they know?

 

[HDRW]

...
However, I thought this was only a 1KW air con, with makers recommending a 16A slow blow fuse (perhaps more or less equivalent to soemthing like a C type MCB) recommended, in which case, I wanted to suggest, a 13A fuse backed by a B32 trip would be very similar, and then it could be a fused spur of a 32A radial, that also feeds the loft sockets, and the complexity is vastly reduced as ther is ohnly one circuit.
Of course, it is always possible I have grabbed the wrong end of the stick again.

No, I think you've got it about right! The problem I see in what you suggest is that a 4mm² radial in a conduit doesn't make it to 32A (if I'm reading the tables properly) so I'm now thinking of a 20A MCB for it.

Howard - have you tried plugging it in on the ground yet ? (before you get it up there and find there is not enough oomph in the supply.)

No, utterly unfeasible to do this, because it needs proper refrigeration-commissioning equipment (vacuum pump, manifold, gauges) to connect up and condition the pipes before you can run it up. That's something I'm planning on getting a professional to do, only once! Messing about with the wiring is trivial in comparison. If it turns out there's not enough oomph for it and the sockets. I'll just scrap the sockets idea.

As regards the floor area for a radial circuit - I am wrong again.

I understood this lot until you explained it!

The area of the whole house is less than 50m² and the useable loft area is about half that, so I'm pretty sure I'm within all of the area limits.

Incidentally, I'm planning to use LSF cable (BS6242B) because I happen to have some - apart from being a bit safer in a fire, is there anything else I need to know about it? Does it handle the same as PVC?

Cheers,

Howard