Hot tub volt drop

smity · Apr 24, 2021

Can i get away with 6mm SWA on a 50m run for a 32a supply to a hot tub? Cable will be burried.

I believe its very close to being over the 5% max Permitted?

Whats your thoughts, should i go up to 10mm?

Thanks

SJD · Apr 24, 2021

Checking e.g. Superlec cable prices, for 50m, the cost difference between 6 and 10mm2 SWA 3-core is less than £30, which in the overall cost of the install and tub must be small change. So I'd just use 10mm2.

smity · Apr 24, 2021

SJD said:
Checking e.g. Superlec cable prices, for 50m, the cost difference between 6 and 10mm2 SWA 3-core is less than £30, which in the overall cost of the install and tub must be small change. So I'd just use 10mm2.

Yeah its not just the cost aspect, i already have a drum of 6mm and it would make the job easier as its running through the loft first (clipped up off the insulation).

50m is probably slightly over estimated, but i don't want to pull it in and have problems due to volt drop.

My calcs say under 50m is less than 5% but just wondered if you eould err on the side of caution?

Thanks

loz2754 · Apr 24, 2021

[ElectriciansForums.net] Hot tub volt drop

timhoward · Apr 24, 2021

I noticed that's in fact for 90 degree SWA. I ran it again for 70 degree as it's maybe more likely the OP has that?
You get 5.078% or 11.68V
If it's just 1m shorter it's within 5%.
Also I know we have to use nominal voltage but at a more likely 240v it's 4.87%

Personally I'd pay the minimal extra for 10mm but I wouldn't shoot anyone installing this in 6mm either.

loz2754 · Apr 24, 2021

Of course it also depends on the actual current draw of the hot tub. It's probably significantly less than 32 amps.

brianmoooore · Apr 24, 2021

Customer would rather heat their hot tub than the soil between it and the house if explained to them, and 5% drop is the current 'fashion'. 2.5% for most om my career, and, for the sake of a few pounds, is what I still prefer to work to. Better for the pump motor starting performance, and the electronics might appreciate the reduced spikes.

rapparee · Apr 24, 2021

brianmoooore said:
Customer would rather heat their hot tub than the soil between it and the house if explained to them, and 5% drop is the current 'fashion'. 2.5% for most om my career, and, for the sake of a few pounds, is what I still prefer to work to. Better for the pump motor starting performance, and the electronics might appreciate the reduced spikes.

Think of it as undersoil heating. So you can walk from the house to the hot tub in your bare feet.

DefyG · Apr 24, 2021

timhoward said:
I ran it again for 70 degree as it's maybe more likely the OP has that

70 deg PVC - Can you still get that?

My software with a correction for being buried is a 'no' for 6mm, 32A at 50m. Are there any other derating factors to consider?
I thinks it is too 'close to call' and with relatively small price difference its 10mm for me, and no worries.

pc1966 · Apr 24, 2021

6mm 70C PVC in duct is easily 32A and for 90C XLPE SWA you could go thinner, but it will be the volt drop that makes the difference. Having said that, I doubt very much that a hot tub will suddenly fail at 5.5% drop!

SJD · Apr 24, 2021

If I was doing this for my own hot tub and happened to have a spare drum of 6mm2 SWA, I'd perhaps consider using it. But if this was for a customer, with a risk in the future of an observation on an EICR that the cable didn't comply, and having to explain why I'd not installed a big enough cable, ... I don't think so.

brianmoooore · Apr 24, 2021

Hot tubs run up a fair old electricity bill. In normal, non Covid times, I run two, so I should know.
I wouldn't want to spend 5% of that cost heating the ground, just to save a few pounds on installation.

pc1966 · Apr 24, 2021

brianmoooore said:
Hot tubs run up a fair old electricity bill. In normal, non Covid times, I run two, so I should know.
I wouldn't want to spend 5% of that cost heating the ground, just to save a few pounds on installation.

That is a good point, wonder what the cross-over point is when it pays for the bigger cable?

pc1966 · Apr 24, 2021

Cable difference is about £90 (inc VAT) which is about 475 kWh at current domestic prices, so if going from 5.5% to 3.3% you are saving 2.2% then you have to spend £4k on electricity to cover the cable costs (around 21.5 MWh).

Possible over a year or so, but I have no idea what the running cost of a stable heated sex pond is.

Soi disant · Apr 24, 2021

DefyG said:
70 deg PVC - Can you still get that?

Whether you can or not, unless the accessories etc to which it's connected are rated to run at 90°, 70° is all you can take it up to.

Hot tub volt drop

smity

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SJD

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smity

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loz2754

timhoward

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loz2754

brianmoooore

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rapparee

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DefyG

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pc1966

SJD

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brianmoooore

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pc1966

pc1966

Soi disant

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