Cable size changes over distance

[SelfBuild104]

I’m intrigued by cable size calculations for my garden. The distance from my consumer unit at the front of the house to the garden is 14m (20m cable route). Garden is 34m long to the front of a shed. There will be other sockets, lighting, irrigation control used in the garden as well. I am trying to work out where to put the outdoor distribution unit with the distro board and automation controller. From a landscape design point of view the best place would be on one side of the 6m wide garden half way down, 17m from the house and 17m from the shed. How does the position of the distro unit affect cable sizes, if at all? If I were to have one 13a socket in the shed, another on the side of the distro unit and one near the house (say 3kw at each socket for ease of calculations). Would that mean a 6mm swa would be run to the distro ( 9kw, 37m, run underground) and then the calculations for the sockets be done from that point? Which would mean for the shed socket 3kw, 17m, underground from distro mcb = 1.5mm or total length 44m, 3kw, underground = 2.5mm?

 

[buzzlightyear]

IMO I think you should get a quilifed spark to install and design the full shabag.with volt drop it could be any thing regarding cable size.and part p the work with the local building control.

 

[SelfBuild104]

IMO I think you should get a quilifed spark to install and design the full shabag.with volt drop it could be any thing regarding cable size.and part p the work with the local building control.

The question was more about the layout of the garden rather than the exact specification of the cable.

 

[davesparks]

Don't install a DB outdoors unless you absolutely have no other option, install it in a building where it is not subject to the weather.

 

[pc1966]

If you are looking at no more than 20A then you can get 69m length using 4mm cable as a quick guide, and in many cases it is simplest to select the once cable for all outdoor sections than to buy difference gauges for minor materials savings.

As davesparks has already said, put it indoors if you can, even "outdoor" equipment will last longer if protected from the worst of the weather, and if you have a shed at the end that is the most obvious location.

As buzzlightyear pointed out, a new circuit like this is Part P notifiable so your best plan is to get a sparky in to sort it out. But prior to that focus on what you want to use it for, not how it is delivered, as that will generally be adapted based on what the sparky finds in terms of the cable route, earthing requirements, etc.
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Is your plan to have the 13A socket outlets on automated control?

Or do you plan on a few always-powered for garden tool use and the automation will be stuff like lights and ELV pump for a pond, etc? If not automated the 13A sockets could be part of a radial circuit out to the shed for any automation control unit going back to other stuff.

 

[SelfBuild104]

Don't install a DB outdoors unless you absolutely have no other option, install it in a building where it is not subject to the weather.

Due to the shape and design of the house I would either need to try and hide the DB in the room facing the garden which is a clean lines type of design so would be quite tricky, or in a cabinet half way down the garden, or it could be mounted just outside the house in the (still to be built brick) weatherproof pump housing for a hot tub but would it be permissible for a DB to be only 1m away from a hot tub.

 

[davesparks]

Due to the shape and design of the house I would either need to try and hide the DB in the room facing the garden which is a clean lines type of design so would be quite tricky, or in a cabinet half way down the garden, or it could be mounted just outside the house in the (still to be built brick) weatherproof pump housing for a hot tub but would it be permissible for a DB to be only 1m away from a hot tub.

Yes it can go in a pump house, as long as there is space for it, the proximity of the hot tub is irrelevant in this scenario.
You really need an electrician involved in the design to avoid falling foul of the regulations or over-complicating something which should be very simple.


You said there was a shed in the garden, is that not suitably weatherproof?

 

[SelfBuild104]

If you are looking at no more than 20A then you can get 69m length using 4mm cable as a quick guide, and in many cases it is simplest to select the once cable for all outdoor sections than to buy difference gauges for minor materials savings.

As davesparks has already said, put it indoors if you can, even "outdoor" equipment will last longer if protected from the worst of the weather, and if you have a shed at the end that is the most obvious location.

As buzzlightyear pointed out, a new circuit like this is Part P notifiable so your best plan is to get a sparky in to sort it out. But prior to that focus on what you want to use it for, not how it is delivered, as that will generally be adapted based on what the sparky finds in terms of the cable route, earthing requirements, etc.
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Is your plan to have the 13A socket outlets on automated control?

Or do you plan on a few always-powered for garden tool use and the automation will be stuff like lights and ELV pump for a pond, etc? If not automated the 13A sockets could be part of a radial circuit out to the shed for any automation control unit going back to other stuff.

As the shed is at the far end of the garden my concern was that I could end up with a cable being run all the way down to a DB and then back up again to get to an outdoor socket near the house which I thought then would need to quite large cable. The power in the shed would be used for power tools such as circular saw etc.

The automation would be all low voltage, irrigation solenoids, LED lighting etc. The controller is a loxone extension which is din rail mounted.

The install will be done by a qualified electrician as the rest of my house was. I have no desire to put myself or others at risk. I’m just trying to figure out where I would like to locate things. As there are so many options I was trying to get an idea about how cable size changes when you introduce a DB into the system. Do cable sizes then get calculated from that point?
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Yes it can go in a pump house, as long as there is space for it, the proximity of the hot tub is irrelevant in this scenario.
You really need an electrician involved in the design to avoid falling foul of the regulations or over-complicating something which should be very simple.


You said there was a shed in the garden, is that not suitably weatherproof?

the shed is at the very far end of the garden. 34m from house to front of shed.

 

[telectrix]

you would size the cable from house CU (and the associated fuse/breaker) to the sub DB according to the total expected load on the sub. then you'd size the circuit cables and MCBs according to the loads on the sub. e.g the feed cable could be 6mm (40A) and the cables supplied by the sub would maybe be 2.5mm and 1.5 (or 1.0)mm dependent on load/s.

 

[pc1966]

As the shed is at the far end of the garden my concern was that I could end up with a cable being run all the way down to a DB and then back up again to get to an outdoor socket near the house which I thought then would need to quite large cable. The power in the shed would be used for power tools such as circular saw etc.

If they simply are permanently powered sockets and you don't need them to be controlled you could have the main feed as a radial set of sockets (so on, say, 20A RCBO at your home CU) daisy-chained from socket to socket up to the shed, and then tap off for your automation box.

But if you need more than 20A total, or have other requirements for independence of circuits in the shed, then going one main SWA cable run to there to a DB and back out on thinner cable would make more sense.

Ultimately the bigger effort is the route if it needs digging down, and a few tens of meters of cable extra is not going to make a huge difference to the cost!
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Just to add the issue of total current is more to do with the resulting cable size and difficulties of putting cable above 4mm doubled-up in to typical 13A sockets.
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Also to add that if you plan on much automation in the future, or changes, etc, it might be best to put in generous sized duct for the cables so you can pull other stuff through in the futures.

Normally you would not put low-voltage cables alongside mains cables but if the mains cables are SWA then it is safe to do so as even with a cable penetration accident it is safely shorted to the earthed armour.

 

[pc1966]

A further thought - don't underestimate what you want to use in the next couple of years. It is far cheaper to put in an over-sized feed cable just now than to replace it down the line!

So if you have aspirations for a hot-tub that might need, say, 30A on its own then you should plan to have something like 40A capacity to your garden DB wherever it is located. In turn that probably means having a fused-switch for feeding this and probably 10mm cable (if not for volt drop, to achieve acceptable disconnection times on a short circuit fault).

Also for that you might need more than the typical 2 circuits a normal "garage CU" offers as such a hot tub load would be on a dedicated ciruit and not along with your 13A sockets, so it might be worth getting a small home CU installed in the shed to have extra spaces, maybe also to have surge protection for any fancy controller(s) you install, etc.