From the information given I could work out the volt drop calcs for a 2kw load over 14m length to start getting an idea of what sort of cable size may be needed had I been running direct back to the consumer unit. But without more information about the ring characteristics you are considering spurring off, such as what is the existing load on the ring, protective device, Zs at the point your connecting onto. How will the proposed extension alter the overall circuit characteristics, etc, etc. It is difficult to be precise, but my gut instinct is that spurring off the kitchen ring is probably not a wise design consideration. How long does the heater on this tub take to get to full temperature, I presume if staring from cold it could be drawing max current for a reasonable duration. How is this loading on the ring balanced with other loads around the ring if the customer also happens to have the dishwasher running and a few other kitchen appliances doing some food whilst waiting for the tub to warm up? This is where your diversity considerations have to be applied. What other large appliances are on the kitchen ring, or is there a separate utility circuit with all the laundry appliances. Please give us more of your calculations and others may be able to help with suggestions and tips as to the cheapest options to meet volt drop, disconnection times, Zs etc. without overloading the existing wiring.
Doc H.
Thanks for replying Doc Hudson. All valid points and glad you are interested in helping and not abusing me for my limited knowledge.
There is existing power running out to the garden to a shed where a tumble dryer is located and the new place of the proposed lay-z-spa. The existing run was done badly and needs replacement before connecting anything.
I know I have to do volt drop calcs and choose an appropriate cable to handle the expected load. My first stop was here as a sounding board to see if I was on the right road so to speak before I did anything else.
The spa/hot tub is a plug in appliance and therefore can't be more than 13A so it is similar to having a 2kw oven which according to the regs can be connected to a kitchen ring and since there is already an extra load connected to the ring via the tumble dryer I couldn't see and issues. There hasn't been any overloads or nuiscence tripping faults on the circuit because of this addition either.
If I could do it my way and in a perfect world, I'd have done it as a sub main similar to a garage straight from the cu and a dedicated MCB. But we are not in a perfect world and so we have to make do with what we have without compromising on any safety aspects.
Design current (Ib) 3kw (for arguments sake) = 13.04A
Cable is in PVC conduit clipped to outside wall, no considerations for insulation/grouping or temperature
Since I'm considering spuring off the kitchen ring then the ring has a 32A MCB
It = In = 32A, table 4D2A in the regs, 6mm will carry 34A
Volt drop = mV/A/m
= 7.3 x 13.04 x 15 = 1.42V (max allowable is 11.5V so well within)
There is the option of using my preferred method which is SWA, I could run 4mm SWA as that will carry 38A, clip direct
