Hi, have a mate who is wanting to install some sockets in his shed... crude diagram.. The 3 core has to travel around 3m its going above ground along the fence. The question is, has he got the MCB Amp sizes correct? The supply unit with RCD was already there when he moved in, along with the 16A MCB for sockets (nothing connected to it) he wants to add the sockets in the shed and the lighting in the garage.
XPLE SWA can carry more current than twin + earth but I would need to check how much more and I simply can't face looking at the big bumber book of fun at the moment. If it's three meters the costs of using 4mm SWA (which IS ok on 32a breaker) is like about £5 more so it's not going to break the bank. I would use 4mm myself and be done with it. If the rest of the run is in t + e then 2.5mm on a 32a breaker is a no no.
Also the fence needs to be solid, like at least concrete gravel boards and fence posts, not some wooden thing.
That makes sense, thanks - He is going to use the SWA to do the radial circuit in the shed as well (using metal boxes with appropriate glands) so as it's the same cable - the 32A breaker would be ok correct?
If the cables clipped/cleated to the fence then its not in free air its clipped direct (reference method C). The CCC for 2.5mm SWA single phase reference method C is 28A. Reference method E (in free air or a perforated cable tray etc) the CCC is 31A. You have to use table 4D4A (70 degree C) even if the cable is XLPE because the tabulated values for CCC for XLPE are at 90 degree C. The only time you could use the table for XLPE would be if your switchgear etc was also rated at 90 degree C. Nearly all the bog standard stuff we buy from the wholesalers will almost certainly not be rated at 90 degree C. And as mentioned above under part P its notifiable work as its a new circuit so is your "friend" registered with a competent persons scheme or getting LABC involved?
I'm always surprised they know all the technical terms for everything but never how to actually do the job, its as if they have read about how to do it on the internet.
That's only true when you are using terminals capable of being run at 90 degrees. If they are 70 degrees then you derate for this (or for simplicity simply use the 70 degree, i.e. PVC, tables). The terminals are pretty much never rated for 90 degree operation though.
Hi nffc, it wouldn't but what I meant was the tables in the big yellow book for XLPE are based on a conductor operating temp of 90 degree. All the stuff we buy off the shelf from the wholesalers the terminals are only rated to what they are based on a conductor operating temp of 70 degree. Most if not all SWA brought from the wholesalers will be XLPE but you have to use the CCC table for PVC (4D4A) as this is based on a conductor operating temp of 70 degree.
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