SWA to Garden

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I'm the process of reworking my garden and as part of this will be running an SWA feed.

Initially this will feed just a twin outdoor socket and a switched FCU for a water feature, however I want to over-spec the cable in case I want to extend the circuit to a shed or workshop in the future.

The cable will be XLPE insulated SWA so I'm referring to Table 4E4A of the regulations. This table accommodates 2-core cable for single-phase use, and 3-core or 4-core cable for 3-phase use. However, I'm exporting the earth from the house so intend to use 3-core cable for a single-phase supply...

Am I missing something?
 
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Use the 2-core single-phase figures. If you choose to use a third core as earth instead of just using the armor, it isn't classed as a current-carrying conductor, since it will carry current only under fault conditions.
 
The cable will be XLPE insulated SWA so I'm referring to Table 4E4A of the regulations.
Don't forget to refer to Note 1 in that, as it's almost certain that nothing you connect the cable to will be rated for 90° operation.
Very valid point, and not one that I'd taken into account. I was thinking when I looked at 4E4A that the ratings seemed higher than I had in my head, which is kind of the reason for my post.

I guess the ratings from 4D4A for 70° cable would be a more suitable reference point.
 
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So I'm going to go with 4mm cable. At present I'm only feeding a twin socket, and a switched FCU for a water feature, so I'll put this on a 20A breaker. Longer term though I might want to extend the cable into a shed/workshop and increase the load to 32A which won't be supported, so 4mm gives me that headroom.

I have a couple of questions about the installation method though.

1) When the cable reaches the socket I can take this in at the bottom using a standard gland. I plan to mount the FCU directly alongside the socket, however I'm not sure how to extend the power from the socket to the FCU. Rather than taking SWA back out of the bottom of the socket, looping it underneath and back into the FCU again, I presume I could use the side entries of the two boxes and wire straight through - however how do I prevent water ingress into the enclosures? What's "best practice" to achieve what I'm after?

2) When the cable reaches the house it needs to arrive at the consumer unit (or maybe at a 20A switch so I can isolate the outdoor wiring if I want to - I don't like relying on MCB as a means of isolation) - am I able to bend the SWA to take this through a hole in the outside wall, and then terminate in an adaptable box inside, or does the bend radius of 4mm SWA not make this a practical solution? Would I be better terminating into an appropriately rated enclosure using an outdoor gland, connecting to T&E with terminals and running this through the outside wall? Again, what's "best practice"?
 
So I'm going to go with 4mm cable. At present I'm only feeding a twin socket, and a switched FCU for a water feature, so I'll put this on a 20A breaker. Longer term though I might want to extend the cable into a shed/workshop and increase the load to 32A which won't be supported, so 4mm gives me that headroom.
Enough headroom? How long would the cable be when extended?

If you use 6mm² when 4mm² would have done, you've wasted ~70p/m. If you use 4mm² and then later find you need 6mm² you've wasted ~£2/m.


Rather than taking SWA back out of the bottom of the socket, looping it underneath and back into the FCU again, I presume I could use the side entries of the two boxes and wire straight through - however how do I prevent water ingress into the enclosures?
The best way is to use bottom entries, as water rarely flows uphill. If these accessories are going to be in a location where that loop of cable needs to be armoured then they probably shouldn't be there.


2) When the cable reaches the house it needs to arrive at the consumer unit (or maybe at a 20A switch so I can isolate the outdoor wiring if I want to - I don't like relying on MCB as a means of isolation)
Make it a 32A one then you won't need to change it if you increase the circuit capacity.


am I able to bend the SWA to take this through a hole in the outside wall, and then terminate in an adaptable box inside, or does the bend radius of 4mm SWA not make this a practical solution?
I don't know how much space you have available, how far above ground this will be, whether you can accomplish the bend by using more than one plane, etc. If the cable emerges from the ground, would it need much of a bend anyway, given that the hole in the wall should run upwards, outside to in?


Would I be better terminating into an appropriately rated enclosure using an outdoor gland, connecting to T&E with terminals and running this through the outside wall? Again, what's "best practice"?
Best practice is to have as few joints as possible. But it's also to recognise the (probable?) unwieldiness of 4/6mm² SWA indoors, so terminating outside might make sense. You could do it in another socket - might come in useful. Or put your isolator outside. Again, using bottom entries rather than rear will be the best way to avoid water ingress.
 
ban-all-sheds said:
Enough headroom? How long would the cable be when extended?

If you use 6mm² when 4mm² would have done, you've wasted ~70p/m. If you use 4mm² and then later find you need 6mm² you've wasted ~£2/m.
In reality, I don't think I'll be living here for long enough to actually want to extend the circuit anyway. The cable would be around 20m if I was to extend it, so within voltage drop limits for 32A. Should it be over that limit anyway, I don't foresee a need for more than 20A anyway so could probably live with that limitation. Obviously my foresight might be wrong however my current needs could be satisfied by 1.5mm cable on a 20A breaker, so moving to 6mm seems excessive.

ban-all-sheds said:
The best way is to use bottom entries, as water rarely flows uphill. If these accessories are going to be in a location where that loop of cable needs to be armoured then they probably shouldn't be there.
Agree with the bottom entries. I'm not sure what you're getting at WRT the location - both accessories will be positioned on a retaining wall halfway down the garden so will be as exposed as they're likely to get and also susceptible to damage so I would prefer to use SWA for any accessible cabling. I imagine your point is more that they ought to be somewhere less exposed however if they're outdoors it would still be preferred to use SWA over exposed T&E, rubber cable, hi-tuf, etc?

ban-all-sheds said:
Make it a 32A one then you won't need to change it if you increase the circuit capacity.
Valid point. I'm going to connect to an already existing 20A circuit, hence the 20A isolator, however as you say no reason not to use a 32A switch now.

ban-all-sheds said:
I don't know how much space you have available, how far above ground this will be, whether you can accomplish the bend by using more than one plane, etc. If the cable emerges from the ground, would it need much of a bend anyway, given that the hole in the wall should run upwards, outside to in?
After the cable emerges from the ground it needs to run along the house, horizontally, before entering, so it would need to move from a horizontal run, clipped to the brickwork, to go through the wall. Maybe I'm overthinking but surely to get the cable into a plane to go through the outside wall, at a slight incline, it's going to need a fairly tight bend, or a loop which brings it away from the brickwork an inch or two in order to accomplish that bend?

ban-all-sheds said:
Best practice is to have as few joints as possible. But it's also to recognise the (probable?) unwieldiness of 4/6mm² SWA indoors, so terminating outside might make sense. You could do it in another socket - might come in useful. Or put your isolator outside. Again, using bottom entries rather than rear will be the best way to avoid water ingress.
I'd prefer to keep the isolator indoors. The isolator will be under the stairs so the unwieldiness of the cable isn't really a concern. It's more getting the cable from it's present path, through the wall, that's posing the concern.

Terminating in another socket is a good idea - the additional cost over an outdoor enclosure is next to nothing anyway and it probably would come in useful, so I'll probably go with that approach. I'm still interested in the best way of routing the cable should I decide to bring it inside...
 
The cable would be around 20m if I was to extend it, so within voltage drop limits for 32A. Should it be over that limit anyway, I don't foresee a need for more than 20A anyway so could probably live with that limitation.
OK - it was just that you said workshop, so if you have things with motors you need to consider starting surges. VD can cause a sag when a motor tries to start that b****rs up the starting...


Agree with the bottom entries. I'm not sure what you're getting at WRT the location - both accessories will be positioned on a retaining wall halfway down the garden so will be as exposed as they're likely to get and also susceptible to damage so I would prefer to use SWA for any accessible cabling. I imagine your point is more that they ought to be somewhere less exposed however if they're outdoors it would still be preferred to use SWA over exposed T&E, rubber cable, hi-tuf, etc?
My thoughts are that whilst you might need SWA for the general run of the cable, if in the immediate vicinity of the socket and FCU there is such a risk of damage that a small loop of cable from one to the other can't be, say, TRS, then the accessories should probably be elsewhere....


After the cable emerges from the ground it needs to run along the house, horizontally, before entering, so it would need to move from a horizontal run, clipped to the brickwork, to go through the wall. Maybe I'm overthinking but surely to get the cable into a plane to go through the outside wall, at a slight incline, it's going to need a fairly tight bend, or a loop which brings it away from the brickwork an inch or two in order to accomplish that bend?

Dunno if these views make sense.

screenshot_776.jpg


Where it goes in/out of the wall (I've drawn sharp angles, not radiused bends) there would need to be a bit of accommodation for bends, but not too bad.
 

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