Hot Tub Cable Rating - 6mm vs 10mm

[Richard Dabell]

Hi Guys

I'm about to install a hot tub in my garden, but need some advice regarding the required cable size.
I have already checked with two local electricians, and both have offered different advice, so hence asking on here.
Also, depending which I use, online calculators suggest either 6mm or 10mm

The cable run to the hot tub is about 45m, I already have a 3 core 6mm SWA cable feeding the garage (next to tub) and I am wondering if I can use this or if I need to run a second feed using 10mm cable.

The tub specs show three power options
15A, 23A, 29A
The lowest setting disables the heater whilst pumps are running, the middle allows the heater when one pump is on, and the highest allows everything to be on simultaneously.
So 29A is worst case I guess.

I'm obviously hoping to use the 6mm as that saves expense on running more cable, but if its no good then I'll get some 10mm I guess!

Thanks in advance!

 

[bernardgreen]

While 6 mm SWA cable installed buried would be safe

Required Cable Size 6 mm
Voltage Drop 9.31 Volts.
Percentage Drop 4.0%
Current Load 30.4 Amps
Max Cable Load* 39.2 Amps

you would be wasting almost 300 watts in heating the cores in the cable ( 9.3 x 30.4 ) and the heaters in the tub would have 9 volts less than the supply voltage.

There would be less wastage if you use 10 mm

 

[Richard Dabell]

Thanks Benard!
Can you let me know what the wastage would be on 10mm?

 

[Richard Dabell]

I've ordered the 10mm cable, but just out of general insterest would like to know loss on 10mm
Thanks!

 

[ban-all-sheds]

https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

Specify 2-core PVC, even if it's 3-core, or XLPE.

 

[bernardgreen]

Required Cable Size 10 mm
Voltage Drop 5.5 Volts.
Percentage Drop 2.4 %
Current Load 30.4 Amps
Max Cable Load* 73.0 Amps

Wastage in the cable circa 170 watts

 

[endecotp]

So by using the thicker cable, you save 130 W i.e. about 2p per hour. How much did the thick cable cost you?

 

[bernardgreen]

I agree the saving on wasted electricity is small but a 9 volt voltage drop might affect the operation of the hot tub and might has well have as much as possibel to get the water heated as quickly as possible.

Of course there is the advantage of slightly warming the soil above the cable ( if buried ) and maybe having a path of noticably green grass to mark the cable route.

 

[JohnW2]

I agree the saving on wasted electricity is small but a 9 volt voltage drop might affect the operation of the hot tub ...

Well, as you go on to say, it would inevitably mean that the water would take a little longer to heat up ... but, to put things in perspective, a 9V voltage drop fades into relative insignificance in comparison with the 36.8V permitted spread of supply voltages!

Kind Regards, John

 

[ban-all-sheds]

Of course there is the advantage of slightly warming the soil above the cable ( if buried ) and maybe having a path of noticably green grass to mark the cable route.

I'm sure that would mean that the cable was nowhere near deep enough.