Swa install to hot tub

[Sparksfly2]

Hi, looking for some advice,
6mm swa allready installed to a hot tub
Client now wants additions and my calcs say the original swa is allready too small.
26a but its120m run, im advising 16mm 2core.

However the swa installed is 4 core so was thinking of doubling cores together to save on time/ costs and putting rod in.

Any thoughts or potential issues with this?

 

[buzzlightyear]

6mm swa allready installed to a hot tub
Client now wants additions and my calcs say the original swa is allready too small.

what are the additional works regarding the up grade of wiring .

 

[buzzlightyear]

26a but its120m run, im advising 16mm 2core

what is the loading of the hot tube .

 

[buzzlightyear]

Multicore 90°C Armoured Thermosetting Insulated Cables​

Tables Apply to: H6942XL, H6943XL, H6944XL, H6945XL, Tuff Sheath,

Voltage:230V

Load:5.98kW / 26A

Length:120m

Method of Installationirect in ground or in ducting in ground

Cable Size:16mm²

 

[Sparksfly2]

Hot tub is 6.5Kw 26a.
Addition is few general sockets and light in shed/ summer house.

 

[loz2754]

My calculations also indicate 16 mm² for the hot tub alone. Doubling up using the other 2 cores of the 4 core would result in the equivalent of 12mm² at best.
A new cable run is the only option I can see.

 

[Sparksfly2]

Feel bit sorry for bloke and doubt he will pay for upgrade, and im not planning on doing works.
Was just thinking if in theory by doubling the existing cores would improve the current installation.

 

[davesparks]

Hot tub is 6.5Kw 26a.
Addition is few general sockets and light in shed/ summer house.

Surely you'd want a seperate circuit for general use sockets and a light?

 

[davesparks]

My calculations also indicate 16 mm² for the hot tub alone. Doubling up using the other 2 cores of the 4 core would result in the equivalent of 12mm² at best.
A new cable run is the only option I can see.

Odd, I get 10mm for the hot tub.

What voltage are you basing your calculation on? The load is stated as 6.5kW 26A which only makes sense if that rating is at 250V.
Based on 250V I get 10mm

But if you adjust the load for a 230V supply (assuming the majority of that load is heating and so resistive) it will be running at 5.5kW and 24A and again I get 10mm as being suitable.

 

[Sparksfly2]

My initial plan was a submain to a DB in summerhouse on TT feed hot tub from there utilising the existing swa until I realised it was undersized.

 

[davesparks]

Feel bit sorry for bloke and doubt he will pay for upgrade, and im not planning on doing works.
Was just thinking if in theory by doubling the existing cores would improve the current installation.

Yes doubling up the 4 core will improve the situation, it may not make it perfect but it will improve it.

But does the hot-tub work currently? If it does then it suggests that the volt drop is not currently so excessive that it will cause a functional issue.
Does the hot tub have a minimum supply voltage rating? (rememeber the percentages given in the regs are intended for use when the voltage tolerance of a load is unknown, not necessarily a hard limit for every circuit)

Obviously the fact that something works in its current states doesn't guarantee safety, but it does indicate that the supply may be functionally adequate.

Have you measured the voltage drop on the hot tub circuit?

 

[loz2754]

Odd, I get 10mm for the hot tub.

What voltage are you basing your calculation on? The load is stated as 6.5kW 26A which only makes sense if that rating is at 250V.
Based on 250V I get 10mm

But if you adjust the load for a 230V supply (assuming the majority of that load is heating and so resistive) it will be running at 5.5kW and 24A and again I get 10mm as being suitable.

Let me do it again. I think I may have used 32 amps as the load...

Edit:
Recalculated using 26 amps, still get 16mm² due to volt drop.

Edit 2:
I used installation method D1 assuming an underground cable run.

 

[Sparksfly2]

220v 6.4KW 29a at closer inspection of label

 

[davesparks]

Let me do it again. I think I may have used 32 amps as the load...

Edit:
Recalculated using 26 amps, still get 16mm² due to volt drop.

What voltage are you using at 26A?

 

[loz2754]

What voltage are you using at 26A?

230

Volt drop calculation:

For 6mm²
7.9 X 26 X 120 /1000= 24.648 Volts

For 10mm²
4.7 X 26 120 /1000= 14.664 Volts

 

[Sparksfly2]

Im getting 16mm for both calculations, borderline 10mm

 

[davesparks]

230

Ah, that'll be the difference.

We have been given a load of 6.5kW and 26A which only makes sense if that is at a rated voltage of 250V.

So you either need to calculate at the rated voltage or adjust the load value for the voltage you are going to do the calculation at.
Adjusting the load for a voltage of 230V (assuming the load is resistive, or mostly resistive, heaters) it becomes 5.5kW and 24A at 230V.

 

[Markyd]

Feel bit sorry for bloke and doubt he will pay for upgrade, and im not planning on doing works.
Was just thinking if in theory by doubling the existing cores would improve the current installation.

Dont worry....he will get Dave from down the pub to do it

 

[loz2754]

Ah, that'll be the difference.

We have been given a load of 6.5kW and 26A which only makes sense if that is at a rated voltage of 250V.

So you either need to calculate at the rated voltage or adjust the load value for the voltage you are going to do the calculation at.
Adjusting the load for a voltage of 230V (assuming the load is resistive, or mostly resistive, heaters) it becomes 5.5kW and 24A at 230V.

Thanks for pointing this out, however I believe I fully understand the nature of resistive loads in this context.

However, whichever way I calculate it, even with a design current as low as 24 amps, it doesn't reduce the volt drop enough for a 10mm² cable to comply with maximum 5% volt drop.

I just noticed the OP has given new figures from the rating plate of 220 V, 29 amps.
Using these figures, volt drop is still the limiting factor, according to my reckoning.

 

[Sparksfly2]

Yes I can see the existing just being added to and him ending up with underfloor heated garden

 

[Sparksfly2]

I agree as ive tried various alterations to current voltage also just based on the hot tub without any additions but the length of run is the limiting factor.

 

[brianmoooore]

Hot tub will be far from a resistive load. There's likely to be a couple of 2hp motors, plus a smaller air pump, as well as the resistive load of the heater.

 

[littlespark]

120m away from the sounds like a big garden…. Will be freezing before you get back to the house.

 

[Sparksfly2]

Yeah its quite a run, long way to go for a top up, previous install looks pretty good

 

[buzzlightyear]

120m away from the sounds like a big garden…. Will be freezing before you get back to the house

a squrials nuts off.

 

[Spoon]

Just a reminder people, if you are using 90°C cable for the calcs, you have to make sure the equipment the cable is connected to is also rated for 90°C.

 

[brianmoooore]

If anything is going to run anywhere near 90 degrees in a hot tub install, then that thing needs to be immersed in the tub water. Tubs are expensive enough to run, without heating soil.
At the distance involved, voltage drop considerations will dwarf the current rating of the cable, so cable temp. is not likely to be a concern.