16mm SWA

[motorbiking]

A friend of a friend is building a garden workshop / play room. He has another friend who is a sparks and has advised him to fit 16mm2. SWA cable back to the consumer unit. This seems a bit chunky for a garden room. His house is a small modern 2 bed.

Why would a sparky suggest such large cable? I reckon he probably has a max (absolute max) of 20A.

Any ideas.

 

[Taylortwocities]

There are lots of factors.

The load is one. Distance is another.
Maybe your friend of a friend hasn't revealed the full story. Does he have future plans for a sauna, shower, mig welder, engine hoist maybe some heating??

In any event. This is a new circuit and notifiable, so I guess the spark will be the one who signs the document that says he has designed installed and tested the new circuit. So I'd leave it to him/her.

 

[ban-all-sheds]

 

[chivers67]

Yeah, find out the distance from the consumer unit in the first instance. It does sound overkill but without more details hard to comment!

 

[motorbiking]

12M from consumer unit to garden room/workshop. Its just two small-ish rooms within a 15m2 timber unit, insulated with 75mm rockwool, so U-value will be around 0.5-0.75 is my guess. So a couple of fan heaters, (say 4kw) TV and bits.. say another kw and lights. Max would be about 6-7kw, but that is with everything on at the same time.

 

[flameport]

probably has a max (absolute max) of 20A.

is completely different to

Max would be about 6-7kw,

The additional cost of a larger size cable over a distance of 12m will be trivial.
Digging up the garden to fit a larger one in the future because the existing one is too small is not.

 

[bernardgreen]

Digging up the garden to fit a larger one in the future

Installing a duct for the cable allows replacement without digging.

A second duct for circuits such as phone, alarm, remote control, etc etc and other ELV equipment can be ( should be ) installed when the trench is dug

 

[JohnW2]

[20A] is completely different to [6-7kW, aka ~30A]

I guess that depends upon what 'completely different' means - the latter is certainly about 50% more.

Although I agree with you that using larger cable will have little financial impact for such a short run, even if I take 'future proofing into account, if all other things are equal and there are not things we haven't been told (either of which may not be true!), 16mm² does seem a bit OTT for a 30A load. I wonder if there are perhaps extraneous-c-ps in the outbuilding and that the cable size was suggested so that it would be adequate for bonding if the house's earth is exported?

Kind Regards, John

 

[chivers67]

10mm should be more than adequate! Much easier to work with too

 

[JohnW2]

10mm should be more than adequate! Much easier to work with too

I suppose it's all relative, but I have to say that once SWA gets bigger than 4mm², certainly bigger than 6mm², it feels more like plumbing than electrical installation to me!

Kind Regards, John

 

[M1Chanical]

Why would a sparky suggest such large cable? I reckon he probably has a max (absolute max) of 20A.

Any ideas.

Maybe he or she is planning to take the feed off the main incoming tails at the main consumer unit, to feed a new consumer unit in the workshop.

 

[Taylortwocities]

Maybe he or she is planning to take the feed off the main incoming tails at the main consumer unit, to feed a new consumer unit in the workshop.

That's not the reason.
You cannot do that, without a switch/fuse at the origin.

 

[motorbiking]

it's being installed in to a spare module on the consumer unit. I haven't seen the current CU, but i'd expect it to have a max load of 100A. With at least 50 of that used up.

 

[ban-all-sheds]

I haven't seen the current CU, but i'd expect it to have a max load of 100A. With at least 50 of that used up.

I wouldn't.

 

[Taylortwocities]

Me neither