cable rating - spa / hot tub

[PaulGrant]

Hi All,
I am from the UK, currently living in Australia and am having a hot tub installed however i am concerned regarding electrical quotes to have it wired
The spa company states that it should be hard wired on a 25amp outlet but 2 electricians have submitted quotes specifying different sizes of cables.
The total run is 40m from the board, up into the attic, (above the roof insulation), down the outside of the wall in flexible conduit, and under the deck to the spa again in flexible conduit.
The first quote specified 4mm twin and earth whilst the second quoted 6mm twin and earth.
Any comments would be appreciated.
Regards, Paul

 

[sparkyspike]

Assuming you're talking 240v supply in the uk? I would have recommended 6mm twin and earth. 4mm is too small. Over 40m your voltdrop would be too high.

 

[PaulGrant]

Yes, its 240 v here just like the uk. From my basic understanding of electrics, i agree that 6mm would be better. Not sure what the voltage drop is likely to be, but 40m is a long run. Will have also to check what size of mcb they are suggesting to fit.
Thanks for post. Paul

 

[sparkyspike]

Volt drop for 4mm T+E is 11mV per Amp per metre. When calculating the volt drop, use the actual (design) current, not the fuse setting current. The current carrying capacity of the cable depends on your route. Not sure how you're getting from the fuseboard to your loft, but the rest of your route (in conduit) means that the 4mm can only carry 30A, which may be less than the fuse.

 

[PaulGrant]

Thanks Sparkyspike for your helpful comments. 30A sounds okay given the rating of a 25A supply. (The route from the distribution board to the attic is via the wall cavity - single storey house - thats the way they route all cables here. Not to UK standards, thats for sure). Paul

 

[securespark]

Not sure of Aussie regs. Figures for cables & volt drop allowances.

But if I was doing it here, I'd use a minumum of 6 mm², 10 if I had isulation issues. Is there insulation in the cavity?