Help with electrics needed

[Ford Prefect]

Hi all

Just had the quote back from my electrician for installing 16 sockets and 2 lamps in my obsy, he has quoted for 16mm 3 core armoured cable, does this see overkill to anyone? I thought iIhad read most people use 4mm.

Any help gratefully received

Cheers Jon

[Doc]

Not much help Jon, but when we had our shed wired we had 16mm armoured run up from the house to the shed.

[jgs001]

I think it probably depends on where it's got to go Jon... is it all interior cabling, or does some of it run outside ?

[yesyes]

Well, 16 sockets is quite a lot. Are they all going to be fed through one cable?

I'm not exactly up to scratch with cable sizes but 4mm seems a bit thin for 16 sockets.

Are you going to run the cable overground or underground? If underground you might get away with using "normal" cable (not the outdoor armoured variant) and run it inside a hose pipe that has a metal mesh layer. make sure both ends of the hose pipe are indoors and that the hose is in one piece. Not sure if that works out cheaper but might be worth checking.

[Ford Prefect]

Hi guys

Its a 70m length over ground, ( I didnt fancy digging a trench that long )

[yesyes]

Ah, there might be different regulations when running it over ground. So you don't accidentally prune it.

[jgs001]

Seems fair to be armoured to me ...

[Ford Prefect]

I was haapy with the fact its armoured ( I dont what my puppies chewing through it) I just thought 16mmm was overkill but maybe its not.

[jgs001]

I guess it depends on how it's measures... my Armoured cabling is 16mm in diameter.. that's the outer diameter, not the core though...I can't remember what it's rated for in terms of current capacity... quite high I think... and that's going to be the key dependency for that many sockets

[yesyes]

Just out of interest...

If you don't run it underground, how do you run it? Along a fence of something like that?

Just interested as I will have to do something like that soon (hopefully) ;-)

[yesyes]

I guess it depends on how it's measures...

I thought the standard was the cross section area of each core in mm^2?

[Ford Prefect]

Thanks John

I will be running it along a fence boundry, as long as its armoured its per regulations, or so Im being told

[jgs001]

So did I, but 16mm cores would mean an enormous cable...

The armoured cable I had put in, really wasn't cheap... surprisingly expensive on a per m basis. Just had a quick look and 4mm core cable is £2/m from Screwfix Direct.

[yesyes]

If the 16mm^2 is the cross section area, then the diameter of the core would be about 4.5mm.

Maybe one states the diameter and the other the area and they are in the end the same cable?

[jgs001]

That sounds about the size of the cores in the cable I have.

[martin_h]

With 16 sockets you have the potential of drawing 16 x 13A = 208A !!! and over a 70 mtr run it could get a tad warm!

[haitch]

What sort of current are you going to be drawing? Lots of sockets doesn't always mean lots of power use.

Could you run it all off 1 or 2 13A extensions? (Obviously with RCDs and suitable weather proof stuff where necessary) That way you don't even need an electrician.

Edit... Crikey lots of responses since I last looked before posting. Hmm, at 70m I think I'd look at ta professional job too.

[haitch]

With 16 sockets you have the potential of drawing 16 x 13A = 208A !!! and over a 70 mtr run it could get a tad warm!

Wrap it round the tubes on the way into the obsy to prevent dew.

[geppetto]

Yep, definitely needs an electrician as it likely falls

under the Part "P" regs as it may be notifiable

[geppetto]

With 16 sockets you have the potential of drawing 16 x 13A = 208A !!! and over a 70 mtr run it could get a tad warm!

Nope...

It will be fed from a fuse in the consumer unit which will

dictate the maximum current you can draw.

Most main incoming fuses to the house are only 100A:)

[martin_h]

Nope...

It will be fed from a fuse in the consumer unit which will

dictate the maximum current you can draw.

Most main incoming fuses to the house are only 100A:)

I only said the potential to draw that current!

I use a 16amp RCBO in tha fuse box and 2.5mm cable to the obs.

[barkis]

Your sparkie knows what he's about Jon. and he won't compromise on safety. That cable needs to be Ultra safe out in all weathers.

Ron.

[decoyp]

Hope I can help here a bit. I am a Sparky and do this sort of thing regularly. There are a couple of questions, first is regarding load. When you say 16 sockets, is that 16 double sockets or sixteen places to plug in something. The regs regards a double socket as one outlet. So what some people would commonly call sixteen sockets could be actually 8 double socket outlets. Still, its quite a few. If its for an obsy, I can't imagine you,re running anything huge except maybe a 2KW heater for those winter nights. It's a mistake to add up each socket as a 13 amp load. The regs allow for diversity which means that they know that not every outlet will be pulling maximum power all the time. You can see this in your house wiring because your ring circuit will likely be protected by a 32A breaker but you probably have 10 or more 13A outlets on that circuit.

The second point is that you state the cable run is 70m. The most limiting factor in cable selection is not load capacity but volt drop. The regs has a minimum permissible vold drop from the source (your fuse box) to the point of utilisation (the socket or whatever). This is 3% for lighting and 5% for all other applications. So if you plan to have lighting on your new circuit, the maximum volt drop must be 6.9v based on a 230 supply. On a long run this is probably where the 16mm figure has been calculated. The longer the run, the bigger the cable needed to me the volt drop requirements. So you are probably not being ripped off by the sounds of things. Your contractor seems to know what he's at in this case.

Sorry to go on a bit, I get carried away. Hope this helps clarify the situation. BTW it needs to be RCD protected. Armoured cable is not essential if its not buried but it will need to be in a conduit if not so its probably the neatest solution. It is the way I would recommend.

Cheers

Richard

[Ford Prefect]

Many thanks for the advice. You are correct that it is 8 double sockets. Each socket will have a RCBO.

cheers Jon

[Peter Drew]

Cables are a bit like mountings, you're always glad you went for the larger one! We have some very long runs at the Astronomy Centre and had to go for heavy cables to avoid voltage drop, not cheap!