Cable size?

[kenny maccuish]

Hi guys I have a job next month where my customer has decided to split his house into 2, luckily there are CU's in the 2 halves and apart from a few wires needing moved for the hallway for lights etc the only real issue i have is a new supply cable to 1 of the CU locations which is about 20m away from a new DNO supply he has arranged.
So i was thinking 3 core SWA but i'm torn between 16mm & 25mm
its has 3 bedrooms with ensuites in each
living room dining room kitchen with gas heating so not much load wise at the moment but he did say an extension might happen in a year or 2 so who knows what he would put in but he did mention liking the idea of a sauna room so 25mm would future proof it i would hope.
I want this to be right first time so any help is appreciated.
thanks
Kenny

 

[Welchyboy1]

I would just go worst case 25mm then you won’t be wrong, it shouldn’t break the bank

16mm xlpe would be fine on an 80A switchfuse though

 

[SJD]

If I put 25m length (I always buy a few metres spare) into Superlec's web site, both 16mm2 and 25mm2, the price difference between the two (incl. VAT) is less than £50. So from a cost point of view, I'd opt for 25mm2 just in case, the difference will be peanuts compared to the overall cost of the work.

However, 25mm2 is more of a pain to work with than 16mm2 if you have to get it round tight corners and into constrained spaces like a switch fuse in a small meter cabinet. Only if that is a consideration would I think about the smaller size.

 

[kenny maccuish]

Thanks for backing up my initial thoughts guys

 

[Wilko]

If you can fit 25mm in, at least you won’t have some ”young whipper-snapper” questioning your design on his EICR in 5 years. Can I ask what fuse you will use?

 

[kenny maccuish]

If you can fit 25mm in, at least you won’t have some ”young whipper-snapper” questioning your design on his EICR in 5 years. Can I ask what fuse you will use?

I was thinking of using a 80amp switch fuse possibly going into a 63amp rcd make and model etc undecided. Does that sound ok?

 

[loz2754]

80 amp switch fuse yes, RCD no. Upfront RCDs would mean loss of power to all circuits in the event of a fault in any one circuit.
Also, any such RCD should have a current rating that can safely sustain the maximum demand for the installation, so it would probably be better for it to be 80 amp in this case. Though as I said, why would you need an RCD in this situation?

 

[kenny maccuish]

80 amp switch fuse yes, RCD no. Upfront RCDs would mean loss of power to all circuits in the event of a fault in any one circuit.
Also, any such RCD should have a current rating that can safely sustain the maximum demand for the installation, so it would probably be better for it to be 80 amp in this case. Though as I said, why would you need an RCD in this situation?

Thanks for the help. I was just thinking of extra protection for the cable to the CU. But going without makes more sense as you explained.