Seeking advice on SWA cable choice and routing garden wiring

I'm planning to have wiring extended from an existing external socket into a small garden for the purpose of lighting, water features and use in a garden room, and would appreciate any advice on cable choice and routing. I put a rough sketch with some dimensions and layout at http://64.191.63.245/garden-wiring.gifSo far there is a 20m run of 2.5mm SWA cable from the CU to an external socket. The electrician put a 32A RCB on the circuit, which is also covered by the RCCD that some other house circuits are on. I was thinking of SWA running from the end of the existing circuit, under a path to one corner of the garden, with a junction box to run to the room corner (D) and up to the far corner (C). Cable runs for LV circuits could fan out from the corners and shouldn't be excessively long. Lighting will be LED's with possibly some LV halogens, computer controlled (the house already uses DMX), and along the garden periphery, in trees etc. Pumps will be LV if possible around areas P1 and P2 but we might need a mains feed for those (or for any UV unit). I'd also considered a short run from either another junction box to point A or from the planned junction box to point B for LV circuits back along the path and trees.

I'm not sure what the max likely current demand would be, but as there's 20m of 2.5mm already from the CU, when calculating voltage loss (based on worst case) I figured that 4mm or even 6mm SWA would be prudent so that there'd be at least a 13 amp circuit for the room. The house supply is TN-C-S and I'd assumed that extending the earth from the house would be ok and that there'd be no need to have any complications of a separate earth.

There's a debate about how to feed the room. One suggestion is first to take the feed to point D directly into the room through the concrete base (not yet laid), to the circuit in the room and then out from the room to an external socket at the back or side of the room. That socket would be used for any garden power tools. The LV control gear could be in the room and feed out separately to circuits on that part of the garden. Another idea was to take the feed to point D first to an external junction box, and then to feed the room as a spur from the junction box in that corner; basically just a decision of where to first take that mains spur.

As the garden is small, I'd also wondered whether it would make sense simply to extend the mains feed only into the room, with an external socket as spur from the room, and then house the LV control gear for the whole garden either in the room or outside at corner D and fan out all LV wiring from that point. But as LV cable runs could then be up to 15 to 20 meters or so, I'd thought taking mains to at least two opposite corners would be prudent.

So to sum up, I'm not sure about cable choice although am leaning towards 6mm, and also any suggestions and thoughts on routing the mains would be welcome as we've ripping up the garden with diggers now :)

Nick

Reply to
Nick
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Nick,

There are a few aspects of taking power outside like this that can get a bit complex. You may find having a read through this:

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help.

Nick wrote:

If this run is inside the house, why is it in SWA?

I assume by RCB you mean MCB...

You are going to have to take care here since it sounds like a design decision has been made that could have implications that may come and bite you later!

Circuits need two types of protection: overcurrent and fault current. The former is designed to clear a sustained overload that results in too much current being drawn, and the latter is designed to operate when you have an outright fault like a short circuit that results in a massive fault current flowing. Normally both types of protection are provided by the MCB at the head of the cable. However this does not appear to be the case in your existing setup. I would guess the designed was relying on the 32A MCB to provide only the fault current protection for the SWA cable, and was relying on the fuse in the plug connected to the external socket to limit the maximum current since the 32A MCB is of a too high a rating to do this itself.

This is generally a poor design choice. A fault in the garden will trip power in the house, and also make nuisance trips more likely since garden circuits are more likely to give rise to problems.

See the nuisance trip section here:

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was thinking of SWA running from the end of the existing circuit,

I would suggest a SWA cable submain to the outbuilding, and a consumer unit installed there to supply any other circuits required.

You may want to consider running a larger cable through the house and then converting to SWA for the external run.

No - absolutely not! This is just the situation you really don't want to extend a TN-C-S earth outside. If it were just going into the outbuilding then it may have been a acceptable solution if implemented correctly. However since you also need feeds into the garden itself for lights and water features itself, using the TN-C-S earth is technically not feasible, and could pose a serious safety hazard if you did it.

This is what you need to do. The outbuilding wants to be a TT install with its own earth spike and RCD protection.

Reply to
John Rumm

Hi John

Many thanks for the reply.

back again in the light of some of your comments.

A good question indeed given the relative cost and inflexibility compared to the obvious alternative. I had the place rewired as part of a major refurb, and at the time it wouldn't have been so hard to sling the cable in, but with hindsight and now that I've audited and looked more closely at the job that the electricians did, that would be one question that I'd ask along with querying a number of discrepancies between the schedule of test results and reality. They weren't cowboys and were helpful with some unusual things that I needed done at the time for the lighting, but now that I've had a closer look, some of the wiring on the lighting circuits was sloppy and careless.

My typo.

Right. A 32A breaker seems very high given what the circuit is for; I'm not planning to run a few kilns!

Good point. As we're not having a chest freezer or anything that you don't want tripped this didn't seem too bad, but if there were trips from the garden then that could become much more than just a nuisance. I feel that it should be possible to have a garden install where a trip was a rare, but if it didn't turn out that way then...

Thanks.

I'd definitely do this if it were feasible but due to how the wiring needs to route in the building it would mean ripping up a large floor area in several rooms and cost =A3=A3=A3=A3. I could route a new SWA out the=

front of the building and then either around the house on the wall, which would be unsightly, or bury it for some of the way, but really neither would be good.

Thanks! I do need to get a cable laid to the outbuilding soon as the concrete base is going down (probably tomorrow), and planned to take the cable up through the concrete base so that it'll be there to feed into the building when that's built. I'll go for a beefy SWA cable to minimise losses on the run into the outbuilding, and would 2 core SWA be fine for that or should it be 3 core?

Nick

Reply to
Nick

In fact my bad possibly...

Just looking at table 4E4A in BS7671, the current rating for two core SWA if it has a thermosetting insulation[1] capable of withstanding a 90 degree temperature (i.e. XLPE) is actually 36A (and 31A for three core). So it would be adequately protected by the breaker you have (although you may want to change that for a 32A HRC fuse to better discriminate with downstream MCBs).

[1] Most SWAs are like this, however PVC insulated versions also exist and the max current ratings for these are lower at 28A/25A (2/3 core). To be sure check the markings on the sheath of the cable.

It should be possible - certainly in the outbuilding anyway. However there is greatly enhanced scope for problems, and as you say, if you are unlucky it might prove to be a PITA. If you have a split load CU in the house and there is space, I would be inclined to move the existing SWA to the non RCD protected side. As a minimum I would include a double pole switch such that you can completely isolate the garden feed.

Your limitation with the existing cable is going to be one of voltage drop. At full load it is already out of spec at over 12V drop at the far end (19mV/A/m). (obviously if it only has a socket on it at the moment you can't get it to full load and it is ok for its current use)

No scope for using the existing cable to pull through a new one?

2 Core is fine. You would need to connect the armour to earth in the house end, but then isolate it from the CU in the outbuilding. There was a recent thread that covered the issues here quite well:

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Reply to
John Rumm

Just a note. XLPE insulated (BS 6943X) SWA has max CCC for 90C conductor temperature. Where cables are connected to equipment/accessories designed to operate at 70C, the current ratings need to be derated to 70C such as by use of CCC specified in BS6004 BS6346.

Reply to
js.b1

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