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

 

[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.

 

[smity]

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.

Thanks for your reply, its much appreciated.

i understand the groans and i have spent a lot of time reading the other threads and various other documents trying to find the best option, also spoke to other electricians and everyone seems to have a different view.

I agree with you on option 2 being the better option and most probably what i will go for TBH unless anyone has a reason why this wouldn't be acceptable.

Thanks

 

[Pete999]

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

What do your cable calcs tell you? if of course you have bothered doing any.

 

[smity]

What do your cable calcs tell you? if of course you have bothered doing any.

My cable calcs told me that the only limiting factor would be the VD (5.19%) at 50m using installation method 72 - D2 but as its only 45m it would come under the 5% for a power circuit.