Cable sizing

[Salisbury]

I am about to embark on a project to convert the top floor of my garage into a small granny annexe. There are lots of considerations to make, but one of them is the electric supply.

The garage is about four meters away from the house. At present, it is served by a 6mm twin and earth single cable running in an old conduit underground to a separate fuse box with a small ring main fuse and a lighting circuit fuse. In the garage there are just two sockets, so obviously the 6mm cable is adequate for the existing purpose. I cannot be 100% sure because there are no markings on the cable, but its outer width is about 14mm across the flat pvc sheath, which I believe means that it is likely to be 6mm. I suspect that for my future purposes, this may not be large enough. Can anyone advise what cable size would be recommended to comply with regulations ?

My intention is to have a new fuse box / consumer unit fitted in the garage, with six or seven circuits - one for the 2 power points in the garage, one for the lighting in the garage, one for the power points in the granny annexe above (maybe 6 of them ?), one for the lighting circuit in the granny annexe, one for an immersion heater, probably, and one for an electric cooker, probably. The heating will be separate, either bottled gas or preferably oil (if I can find a small enough oil boiler which is difficult these days !) I am hoping to be able to fit a hot and cold piped shower, but if I cannot find a suitable small boiler, I may be forced to install an electric shower, so when sizing the underground cable, it should be taken into account possibly.

I am about to have the drive tarmaced which means that I will have to lay any utility cables now before it is too late !

While on that subject, I wonder whether I should also lay a TV antenna in screened cable in the ground to serve the granny annexe. We live in the country and the TV signal is very weak - I need a booster box. Therefore there will definitely be no signal in the annexe because the roof is far too low. The only conventional possibility would be to take a cable from the TV antenna on the chimney of the house, down the wall and underground into the garage and then to the annexe - that would be a long run, even for a booster probably. The alternative would be some sort of wireless transmitter from the antenna on the chimney to the granny annexe which would avoid the need for a cable underground. I do not have a TV myself, so I know absolutely nothing about these things - I stopped paying the BBC the licence fee when Jonathan Ross was being paid £millions, and I decided that if they had that much money to waste they did not need any money from me ! I got rid of the TV and I have not missed it for one moment; I have far too many interests to have the time to watch it. The trouble is obviously that a granny annexe will need a TV and therefore I will have to do something about it now.

So can anyone advise whether there are gadgets on the market that will simply broadcast the signal by wireless over that distance, or is it better to lay a TV cable now ? I do not want to have to dig up the new tarmac later.

Thanks for any advice on these two matters.

 

[telectrix]

i would say that before you even start this project, you need an electrician to assess the requirements and design a safe, compliant installation. it might be possible for you to lay cables under his direction.

 

[davesparks]

The design of the distribution circuit to feed the garage will need to be carried out by the electrician who will be installing it, they will be signing their name against the job and accepting the liability for the work so it would be unfair to expect them to accept liability for a design done by a random stranger on the Internet.
But I can say for certain that it won't be T&E buried in the ground!
I recommend you install two twinwall utility ducts to the building so that all the mains and low voltage cables can be installed once ground works are complete, that way you avoid the risk of the cables being damaged during the ground works and can wait until the loads are confirmed to install the distribution circuit.

I would also suggest that the minimum cable size you walked want for the distribution circuit would be one that has a 10mm copper protective conductor (or conductor offering equivalent conductance) so that main bonding can be installed suitable for any standard domestic supply and then avoid having to deal with the nonsense debates about exporting/extending earthing systems.

 

[Spoon]

I recommend you install two twinwall utility ducts to the building so that all the mains and low voltage cables can be installed once ground works are complete, that way you avoid the risk of the cables being damaged during the ground works and can wait until the loads are confirmed to install the distribution circuit.

I would also suggest that the minimum cable size you walked want for the distribution circuit would be one that has a 10mm copper protective conductor (or conductor offering equivalent conductance) so that main bonding can be installed suitable for any standard domestic supply and then avoid having to deal with the nonsense debates about exporting/extending earthing systems.

Hello OP. Welcome to the forum.
The above is the best solution for you. You did not mention any water pipes going to the garage/anexe. Are there existing ones? Are you installing new ones? Plastic or copper pipes? As dave has mentioned above, this is an important factor in the installation.

 

[Salisbury]

Thanks for the various replies. No other utilities are needed because the mains water pipe for the house already comes through the garage - I thought of that about ten years ago when the water main had to be replaced, so I deliberately routed it through the garage and from there underground into the house. Similarly, at the same time, the telephone cable was laid in twin-walled conduit and the 6mm cable was also laid in twin-walled conduit for the garage. The telephone conduit is large enough to take an antenna screened cable if necessary, but I fear there would be too long a run even for a booster (however, I have found other more suitable solutions to the TV problem since my original post including Freesat and Rasberry Pi Kodi systems).

But I am assuming a 6mm cable is not adequate for the annexe. Getting an electrician in at this time is pointless as it will be many months before I get to that point, which is why I hoped that I could get a sensible answer from someone about the likely sizing needed for a new cable. Once I know that, I can investigate whether the existing underground trunking is large enough to take a new cable after the existing 6mm cable is withdrawn. If it is not, then I will have to lay new conduit but that means finding someone with a digger. I am just trying to avoid the unnecessary extra need to having to dig up the new tarmac at a later stage. I am not actually installing any electrical cables at this stage. That would be premature.

Living in a very rural area, getting an electrician out here just to advise on the likely size of the cable is asking a lot when it would mean a fifty mile round trip. I have every intention of using a fully qualified electrician to do the work when the time comes, but I do not even have any room plans even finalised yet, so it is much too early. When that time comes, I will want to get several quotes, and so I am not going to ask one electrician to come fifty miles to advise on the cable and then go and ask other electricians to quote as well. That would be taking unfair advantage and I am sure all you qualified electricians would agree.

Some of the posters sound rather defensive. All I am asking is what size cable would be advisable so that I can decide whether to use the existing conduit or to install a new one, which means bringing in a digger at great expense. I am trying to do things properly, and suggestions that I get an electrician to come fifty miles is not very practical or fair. As the warning at the top of the Forum says "Please be reminded that if your reply can't help the poster, it may be seen as pointless. Please be nice and helpful or don't reply at all". When the time comes, I will get several quotes from qualified electricians to do all the electrical work and pick the best value, please rest assured. If I cannot find a suitable answer, I will have no alternative but to use my own judgment, but I would rather use the judgment of someone who knows the answer if possible. I do not understand why anyone would not want to express an opinion to ensure that the job was done professionally. I could ask a local handyman who would no doubt give me an answer despite not being qualified, which I do not want to do but I may be forced to do so.

Thanks.

 

[HandySparks]

What's the internal diameter of the existing ducting?

 

[telectrix]

as a rough guide a 10mm SWA cable will be good for a 50A supply. a 16mm will give you 80A. that is only approximate . selection of cable will depend on several factors . if your ducting is 2" I/D, then you should not have a problem.

 

[Octopus]

I am about to have the drive tarmaced which means that I will have to lay any utility cables now before it is too late !



So can anyone advise whether there are gadgets on the market that will simply broadcast the signal by wireless over that distance, or is it better to lay a TV cable now ? I do not want to have to dig up the new tarmac later.

Thanks for any advice on these two matters.

Hum.... you can afford to pay for an expensive drive, yet you won't pay a sparks an hour or so's work to confirm the design requirement for your submain and TV/phone considerations........

Just get your wallet out matey.... you know it WILL make sense in the long run - just imagine the costs to re-tarmac your drive

 

[Spoon]

Hello again OP. Again the above posts are your best bet. All depends in the internal diameter (ID) of your ducting. You may need a bigger dusting fitting.
I have no idea where you get your "Some of the posters sound rather defensive." bit from. All the posts have been very informative.
As you say, two of the items you are having fitted is an immersion heater and electric cooker. It would be good if you could tell us the power rating of these, as this will affect the cable size.

 

[Salisbury]

Thanks very much for the replies; that is just what I wanted to know. At this time, I do not know how big the ducting is as it has all been neatly sealed in the wall of the garage and it is impossible to tell without deeper investigation. The easiest thing will be to dig a small hole outside on the other side of the cable entry to expose the ducting and measure it there. If it is obviously too small, I need go no further and will have to replace it. From inside the garage, it looks to be about 2 inches but I cannot assume that it is the same on the other side. I should point out that the garage is built into a hill, and open only on the front, and therefore the cable entry comes through the wall at about five feet from the floor. So I can see the opening for the ducting clearly but I cannot look down the ducting as it has been professionally sealed with tar or something similar. I am assuming that the hole in the wall is representative of the ducting but I cannot leave it to chance, and therefore at the weekend, I will dig a hole outside to see what happens between the garage and the house. All I do know is that twenty years ago, I had the original cable pulled out and a new cable, which appears to be only 6mm, pulled back through the ducting. In those days, the top of the garage was just my office with a coke fired boiler and a few lights and power points, so the 6mm would have been adequate. It was not contemplated to get it turned into a granny annexe in those days. So fingers crossed.

Thanks for the information.

 

[Spoon]

Any info on the existing or new water pipes? Copper or plastic?

 

[TallPaul]

I can't help with the cable sizing, teletrix gives a good guide I was impressed you got that much detail - I find questions like this on this forum generally get answered with call someone in. If you want to do some maths yourself though you can use the TLC calculator:
https://www.tlc-direct.co.uk/Techni...230&length=12&submit=Calculate+Min+Cable+Size

But note that burying the cable de-rates the capacity.

Ultimately... your going to need to get someone in ;-) Or run big conduit so later whatever cable is needed can be pulled.

It sounds like you answered the TV cable question yourself, which I can help with - but coax of that length attenuated at TV frequencies is pointless - get a small sky dish on the side of the garage with freesat, or put a small USB dongle on a pi in the main house close to a good feed and steam to the remote location as you seem to have already found would be my suggestion. Other streaming solutions are available ;-)