Hi Im building a garden office 30m from the house , need to run some armoured cable up to it Going to run water heater 9.5kw skirting board heating 3kw 2 x double sockets down lights macerating toilet etc Can I run 10mm 3core and have an 80amp breaker at the house end ? Cheers baskey
No! If its a PVC SWA, a minimum of 25mm. If it is an XLPE thermosetting type, you could go 16mm, but there are then further considerations to make. I would go for 25mm. Can you not reduce your load by getting rid of the 9.5kw heater with something smaller? I assume this is an instantaneous type - Cant you opt for a 3kw unvented type? This sort of load is also quite substantial when considering adding it to an existing installation.
I make it just within using 16mm but for a 80A moulded breaker as you will not get a MCB that size it will cost quite a bit and then likely the whole supply is only 100A so your trying to run house off 20A. I would guess 60A would be enough but still too big for a MCB so still moulded breaker and I am not sure about using a moulder breaker on domestic. In the main the moulded breaker has adjustments and these are not permitted to be assessable by an ordinary person. I would find the biggest MCB which will fit in your consumer unit and then reduce demand to be within that limit. You can get under sink water heaters with a 7 litre tank with either 3kW or 1kW elements which would be ample for washing hands.
If it was a 80amp submain, I would be going with an 80amp switch fuse, not an MCCB. If it was a 60amp submain, I would also go with a switch fuse, although MCB's are available upto 63amp, depending on brand. I clearly make 16mm only suitable if it is XLPE, not PVC. You need to take into account conductor temperature then, to ensure the terminals can cope with the higher cable operating temp of XLPE over PVC. I would be opting to reduce the loading on this install.
Be it 16mm or 25mm it is really still not a good idea to have a water heater requiring 9.5kW and I think all are agreed it needs a re-think on the water heater. My program take into account the correction factor which is the likely reason my calculations were just within the limits. But in real terms working right on the edge is not a good idea and I would agree really need 25mm. I am sure once the poster sees the price of 25mm he will modify his plans. Something like this is what I would use. 2kW is ample with a 10 litre reserve.