What size swa cable should I get?

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DonDonson

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hi all, looking for some advice on what size swa I should run from the house db board to the back of the garden for an building that will be part workshop and part living area.
The building will have an electric shower, a couple of electric rads or maybe electric underfloor heating, under cupboard water heater sockets and lights.
The outbuilding will be approx 35mtrs away from the house which has a 100amp supply.
I will only be laying the cable, not terminating it as we are doing various ground works and haven't sorted an electrian as yet but would like to get the cable in.
Any advice would be much appreciated.
 
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I would recommend sourcing an electrician out first, there are many things to consider and an on-site visit and details of the load demand, would be the only real way, to get a true picture of this project.
The distance of 35m is going to give a voltage drop that will require an increase in the CSA of the SWA above that of the standard calculations (without factoring voltage drop)
 
As Prentice boy has said get an electrician in to calculate the size of cable required. What you can do as part of the ground works is to install a duct for the cable to be pulled through later. 32 mm internal diameter At least 450 mm deep. Electrical warning tape 200 mm above it. No sharp bends

Install a second duct for other cables such as data, intercom, alarm and telephones

One source for suitable ducting is

http://www.polypipe.com/civils/products/cable-protection-systems/power/polyduct-power-class-3

There are other manufacturers and suppliers
 
Sounds like a fairly major project which definitely needs some professional input to make sure it is feasible- your outbuilding is going to have a potential load of 40 amps (shower), 8 amps (heating), 10 amps undersink water heater, 6 amps lighting and presumably 32 A power so a total of 96A.

Not sure if you're allowed to use diversity calcs on an outbuilding- assuming you aren't then you are looking at 25mm SWA as a minimum and even that is skating it- popular opinion at the moment seems to be to use 3 core SWA for a single phase circuit, the 3rd core being earth and just bond the armour- and I don't know which number to use from the current capacity tables for single phase in 3 core cable. And earthing can be less than straightforward depending on what your supply arrangements are.

If your house is on a 100 A fuse then the incomer is probably only 25mm so going bigger to your outbuilding isn't going to work. Plus if you stick a 100A breaker in your house CU then the diversity calcs on that are going to go to pot.

Get an electrician in to have a look and have a chat before you spank a load of money on a cable and a digger. If you already have the digger booked/trench dug then stick some ducting down the hole (3 runs, 75mm for power, 50mm for water pipe, 50mm for comms/alarm etc, as much separation as you can get between comms and power, 50mm minimum) and sort your cabling later. Min depth for power cable 700mm, cross section would be 100mm sand, ducts, 100mm sand, warning tape, solid fill- bricks, concrete slabs, anything to protect the ducts from mechanical damage- , more warning tape, soil, more warning tape and so on. And photograph or mark the trench accurately with reference to fixed objects.
 
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DonDonson said:
hi all, looking for some advice on what size swa I should run from the house db board to the back of the garden for an building that will be part workshop and part living area.
Click to expand...


The building will have an electric shower, a couple of electric rads or maybe electric underfloor heating, under cupboard water heater sockets and lights.
Click to expand...
Can't you have a gas supply installed?

In any event - forget the idea of electric UFH - waste of money to install, useless at heating rooms.


I will only be laying the cable, not terminating it as we are doing various ground works and haven't sorted an electrian as yet but would like to get the cable in.
Click to expand...
When you applied for Building Regulations approval, what did you say, or allow to be said on your behalf, or by default allow Building Control to assume, would be the way you would ensure that the electrical work would comply with the Building Regulations? That is actually very important, because if you have promised to do A and then you go and do B you could well find that they refuse to give you a completion certificate. So if you have said that the work will be done by a registered electrician, then that's what has to happen, and you will not be able to get one to rock up later on and certify that he did the things you've presented him with which you yourself did earlier.


Any advice would be much appreciated.
Click to expand...
If you want to do the design work, that's absolutely fine. But in that case it has to be you who does it, not some anonymous stranger on an Internet advice forum.

It has to be you who can justifiably sign this:

I being the person responsible for the design of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2015 except for the departures, if any, detailed as follows:

and you can't do that if the design was actually done by the aforementioned anonymous stranger on an Internet advice forum.

So you have to acquire the knowledge to become competent to do the design phase of this electrical installation project yourself.
 
oldbutnotdead said:
Sounds like a fairly major project which definitely needs some professional input to make sure it is feasible- your outbuilding is going to have a potential load of 40 amps (shower), 8 amps (heating), 10 amps undersink water heater, 6 amps lighting and presumably 32 A power so a total of 96A.
Click to expand...
If you added up all the potential loads (including an assumption that all sockets circuits were 'fully loaded') for the average house, you'd probably end up with a figure of around 200A.

Kind Regards, John
 
If you must have a shower, have a power shower. It contains a pump which will give you pressure from a gravity fed hot and cold feed. Then you can have a 3kw electric immersion heater. Much better for your supply than a 9kw electric shower.
 
Pulling 35m of 25mm SWA through ducting isn't going to be easy if there are any bends in the run
 
Or run the HW supply from the main house if feasible and use a Split Inverter AC unit to provide the heating for the outbuilding.
 
Has this all scared the OP off do you think?

Getting gas up to the outbuilding ((c) ban-all-sheds) sounds like the best idea to me, nice little combi boiler for heat and hot water and that'll drop the electrical load to lights and power only and at that distance 10mm SWA would do it, 16mm will give room for Future Expansion.

And maybe 75mm duct was conservative for pulling SWA through- 100mm with lovely swept bends?
 
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