I would like to run some 6mm^2 T&E to the garage or rather get a Part P person to do it for me ;)
My set-up is meter > switch fuse > CU.
I presume I change this to: meter > switch fuse > henley blocks > cu (house) and cu (garage)
I've never used henley blocks before; is there any advantage to using two SP blocks or one DP block?
The cable will exit the house under the stairs into the garage (no underground or overhead runs required) but because it will exit at about 1.5m above floor level, it will be at a height that it might get knocked by people or cars, so I think I should protect it in conduit. Are all conduits created equal or are some better than others? How thick is 6mm^2 T&E, does it require "fat" conduit? of course, IIRC conduit will reduce the max load the wire can carry.
Do you you have a spare way in your consumer unit? If so run the garage power from there, if not fit a small consumer unit with a single way and run the power from there. What are your earth arrangements and what are you wanting to run off this cable? Photos of the arrangement would help.
The CU is RCD'ed and I thought it was bad practice to run your garage/outdoor supply from an RCD'ed unit because if the RCD trips because of the lawn mower, the whole house goes dark.
I was going to fit a small CU like you describe but I thought I should site that in the garage not the house?
I will have to check the faq for the correct terminology
Garage light, outdoor PIR light, freezer, lawnmower, drill, saw, etc.
Let's call that switch-fuse 1 (SF1). Presumably it's there because the house CU is some distance from the meter.
You might not need the Henley blocks(s). If the output terminals in SF1 are big enough to accommodate two outgoing cables then you could split at that point without the need for any kind of junction box - a tidier arrangement. If you do need to split after SF1 remember that Henley blocks can only be used with single sheathed cable (6181Y), unless mounted in some sort of enclosure. (I'd use a DP block, unless there's any good reason for preferring 2xSP ones.)
Whoa! "The cable" here is the 6 mm^2 that will run to the a second CU in the garage? Before that you will need a 2nd switch-fuse (SF2) to 'fuse down' to (say) 30 A for the distribution circuit going to the garage CU. A 1-way Wylex unit fitted with a 30 A BS 1361 cartridge would be a suitable choice for this. Without SF2 your cable will only be protected by the fuse in SF1, which is presumably 60 or 100 A for the house.
OK. You haven't said anything about what circuits and equipment are going into the garage, but it might make sense to do the whole of the garage sub-installation in conduit. If all the wiring is enclosed you could wire in 'singles' which is much easier to use in conduit than T&E.
There are two grades of PVC conduit, standard and heavy gauge (most people use the latter), and there's steel. HG PVC would be fine for a domestic garage, is fairly cheap, is quite easy to work with and will give a tidy-looking job. Steel is obviously tougher, but involves more of a learning curve to master the necessary skills to use it and you'd have to hire bending and threading tools. Use PVC!
Sizes and ratings here
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mm^2 T&E will go through 25 mm PVC conduit OK for simple runs with only one or two bends. Don't even think about trying to use 20 mm.
Your earthing arrangements? Remember that 6 mm^2 T&E only has a 2.5 earth (CPC), which may be too small. You may need to consider running a separate 6 mm^2 earth for the distribution circuit.
Connect the CU tails and the T&E into the switch fuse if it will fit? I had not thought of that.
As you can tell, I have only just started to think about this. I had forgotten that one cable will be T&E not a "tail" cable (I presume that 6181Y means a tail).
which is presumably 60 or 100 A
Whoa indeed. Another refinement that I had missed in my initial thoughts. I presume I could run single sheathed cable from the Henley block to SF2 and the T&E from the other side of SF2 which would solve my tail to T&E transition problem. I must confess I have not seen 30A switch fuses; I thought they were 80A or 100A. I will have to pop to a electrical trade counter.
Not the 6 mm^2 T&E directly (into SF1) since you're still fused at 60 or
100 A at that point - but you might get 16 mm^2 single sheathed or T&E in, for the short run to SF2. It depends on what make and size SF1 is, but 100 A stuff often has 50 mm^2 terminal capacity, which would be sufficient. Check out the actual available wiring space, as well as terminal capacity.
Yes "meter tail type cable" readily available in cut lengths in 16 and
25 mm^2 for tails etc., but also available in the full range of sizes:
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Whoa indeed. Another refinement that I had missed in my initial
Exactly so.
It's a 1-way consumer unit really, e.g.:
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fit a 30 A cartridge fuse, not an MCB, as that will give you better downstream fault discrimination).
That's the second time I've made that mistake. I won't make it again.
Thanks. Where can I learn what all the codes, e.g. 6181Y mean?
IIRC 16mm^2 is good for 80A and 25mm^2 is good for 100A. The fuse at the meter is 100A so I hope the tails to SF1 are 25mm^2. The switch fuse is rated 80A and the CU is rated 63A so I think the cable from SF1 to the CU is only 16mm^2.
Since SF1 is rated at 80A would I be right to use 16mm^2 to SF2?
I've had a look at TLC and the Wylex range seem to be the only ones that can be used with either fuses or MCBS. The other products do not mention fuses. The price seems reasonable enough too. There is also a plastic version a little bit cheaper. Is there an advantage to using the metal or the plastic version? I guess metal is stronger so takes a few knocks but on the other hand, doesn't it pose a problem in that it must be earthed?
Hager also do HRC fuse carriers for their consumer units. They also fit Contactum and MK enclosures.
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they do come with a fuse fitted (still not found a use for the spare I ordered! ;-))
Assuming your not TT[1], then either should be ok. Earthing the metal ones is not difficult - they usually have the earth bus bar riveted to the metal of the box anyway, so it is hard to avoid.
[1] With TT installs its better (IIRC a requirement) to use insulated CUs since it lowers the possibility of a fault to earth occurring on the up stream side of your RCD, where the earth fault loop impedance will probably be too high to sink enough current to open the main cutout.
Actually it should be 6181YH these days, the final H indicating the new (harmonised) colours. These UK-specific codes originated in the cable manufacturers' association (CMA). They don't seem to have a website, or maybe don't even exist any more under that name. I found a couple of lists on the web though:
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IIRC 16mm^2 is good for 80A and 25mm^2 is good for 100A.
More or less - the clipped direct ratings for a single-phase circuit are 87 and 114 amps respectively.
Sounds OK.
16 will be fine. Actually you could probably use a smaller size since the downstream fuse in SF2 provides overload protection and the 80 A fuse upstream only has to provide s/c fault protection - but using 16 mm^2 will avoid the need for an adiabatic heating calculation.
What John Rumm said. Also the metal type will have round knockouts, so will be more suitable if you're going down the all-conduit road. Plastic is more suited to surface-wired T&E, where you can trim the cable openings to fit round the cables. Also look at MEM as another manufacturer offering HBC fuse carriers.
Thanks. That's a good point. I thought all the regs. demanded RCD protection now though?
I was going to get a 2-way garage CU but I think I now need a 3-way: lights, sockets, and freezer socket.
I don't know much about RCBOs. I think they are the way to go: at least they only switch off the faulty circuit rather than the whole system when there is a fault, but OTOH don't they just switch the live whereas a RCD's CU isolates both L&N when tripped?
The 17th edtn is not mandatory yet... you could also protect the circuit cable such that it meets the requirements for a non RCD protected circuit under the 17th edition.
However the simplest solution is a RCBO for that circuit. This does include a RCD - but it is dedicated to that circuit and hence unaffected by anything that may go wrong on other circuits.
No they switch both live and neutral - the neutral of the circuit has to pass through them as well as the live (makes sense when you think how they work). So they switch both. They normally hav a flying lead that attaches them to the neutral bus bar in the CU. The circuit live and neutral then attach to the RCBO.
Not necessarily, and most are single-pole switched.
That's a non-sequitur. Just because the neutral passes through doesn't necessarily mean it's switched.
Both single- and double-pole switched RCBOs are available. Generally speaking 2-module width ones are DP and 1-module width ones are SP, although that rule might not hold universally. Manufacturers' literature should make the situation clear though.
Apologies for making an assumption then. The only ones I had examined closely were two pole switched devices - but that was a while ago, and I don't recall the actual brand.
No indeed, very true.
The ones I looked at were the two module ones.
(although going back to the OPs question, even a single pole RCBO solves the discrimination issue for the freezer, and single pole RCD protection is still a significant advance on none (for situations that warrant it)).
What are the disadvantages of a single pole RCBO over a double pole one? Surely the important thing is that the live is cut in an emergency. What are the advantages of isolating the neutral too? The only one I can think of is if the Cu is wired back-to-front and I would hope anyone wiring a Cu would know enough not to do that!
Sorry to add to my own post but previously I was thinking of buying on of those "garage CUs" with two MCBS; one for sockets and one for lighting. I wasn't sure whether to "upgrade" to a 4-way CU and have one for lighting, one for sockets (a 32A ring?) and one for the freezer (possibly a 16A radial?). Bearing in mind that it would be supplied by 30A fused 6mm^2 T&E would this be sensible? Or is there a danger that someone might come along in the future, see a 4-way CU, fill the other MCB space and add to the rings and overload it all?
Not really, because anyone installing a new circuit would be responsible for ensuring the submain is adequate for the demand, and if it does get overloaded then the 6mm should be protected by the 30A fuse upstream.
I've forgotten what you're supplying but why don't you put in a 6 or even 8 way consumer unit with plenty of spare space, and supply at 45A with 10mm T&E? The additional cost now is marginal, and the spare capacity could come in very usefuy later
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