Hot tub volt drop

[smity]

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]

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]

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]

 

[timhoward]

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]

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

 

[brianmoooore]

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]

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]

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]

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]

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]

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]

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]

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]

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.

 

[Soi disant]

My software with a correction for being buried is a 'no' for 6mm, 32A at 50m. Are there any other derating factors to consider?

But that will be a no for VD reasons, not because the current carrying capacity of 6mm² SWA is too low. And the VD will be calculated with the resistance of the cable at 70° or 90°.

Other derating factors would give the cable a lower CCC, but as I said, that's not the limiting factor here

There might be a converse to consider though - what ambient temperature does your s/w assume? If you can get the temperature down then the resistance comes down - it's about 25% lower at 10° vs 90°.

 

[Soi disant]

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

Depends on how many webcam customers you can get.

 

[DefyG]

But that will be a no for VD reasons,

Without stating the obvious, yes of course??

If you can get the temperature down then the resistance comes down -

If you can regulate the temperature in an attic void and underground?
We don't know the full facts and whilst the cable may be clear of insulation the attic void if south facing could be much higher than 30deg in the summer when the hot tub is most likely to be used.

If the OP can justify using 6mm then fine with me! but he has come on this forum for others views and is probably more confused than before!

 

[smity]

Thanks for all your replys and input ? its looking like its 45m which seems like it should be ok using 6mm with a 4.5% VD

Now i have unearthed another query with it being a PME system.

I have seen lots of conflicting information regarding how this should be tackled and I believe there are 3 options (probably more)

- 32a Type c from split RCD board

Option 1 - use the PME earth for the entire job as this isn't a swimming pool but a "built to product standard" hot tub so doesn't fall under section 702 - no exposed parts etc

Option 2 - PME earth covers supply cable to isolator but then is stopped there (Plastic gland and heat shrink armourings), Earth spike local to isolator and then use a TT earth from the isolator tail to the hot tub

Option 3 - PME earth covers supply to an external enclosure and 30mA RCD, TT spike from this point to isolator and tail but then you have 2 RCD's covering the circuit.

Anyone have any input on this?

Ive attached a document ive found on NICEIC site saying PME is fine but just wondered on your thoughts.

Thanks again ?

 

[timhoward]

I have seen lots of conflicting information regarding how this should be tackled and I believe there are 3 options (probably more)

- 32a Type c from split RCD board

Option 1 - use the PME earth for the entire job as this isn't a swimming pool but a "built to product standard" hot tub so doesn't fall under section 702 - no exposed parts etc

Option 2 - PME earth covers supply cable to isolator but then is stopped there (Plastic gland and heat shrink armourings), Earth spike local to isolator and then use a TT earth from the isolator tail to the hot tub

Option 3 - PME earth covers supply to an external enclosure and 30mA RCD, TT spike from this point to isolator and tail but then you have 2 RCD's covering the circuit.

Anyone have any input on this?

If you listen carefully you might hear a collective groan as this subject has been discussed a lot recently!
Brief comments:
Option 1 is compliant but has issues. I used to do this but wouldn't now. See recent thread on tingly feet when getting out if you want a long read. [Also IET Guidance Note 7 (Special Locations) does have stuff about hot tubs outside coming under Section 702, as a recommendation)
Option 2 - What I'd do.
Option 3 - Also Fine but I'm missing the gain over option 2 - complete pot luck which RCD would go? If you could avoid upstream RCD this would clinch option 2 for me.