New Hot Tub Install

[springcrocus]

I have been asked to provide an estimate to lay in the supply for a proposed hot tub in an existing customers back garden but they haven't yet chosen their new tub so manufacturers requirements are not available. I asked them, "How long is a piece of string" but the answer was, "Just give us a ballpark figure". Great. They also want to festoon the garden with a load of lighting.

Source is TNC-S with 100A fuse (someone's already pinched the seal) and current max demand about 50A. Hot tub is most likely going to be class II and will be filled by hand-held hosepipe so no bonding requirements that I can see and the tub will probably be a 6-8 person size.

I'm thinking split the tails into Henleys /ISCOs, 60A switch fuse, then 10mm SWA to IP65 CU (no shed to put it in) with two ways, out again with 10mm SWA to rotary isolator. The total run is about 25 metres and my best guess for the hot tub load is about 32A.

Now, three choices. If the hot tub goes onto wooden decking, I could use the TNC-S earthing with no real problems. If it goes on the ground, TT might be better but because there is no bonding required I could also use the TNC-S earthing supplemented by a rod near to the hot tub if I can get the resistance down low enough.

I would be grateful for anyones thoughts on this. I don't want to over-engineer this and lose the customers confidence because it's too expensive, nor do I want to underprice and end up way out of pocket.

 

[Wilpickard]

I'd TT it locally as you can get a potential deference between the earth around the hot tob and any metal work on the tub, if there's not exposed conductive parts there's still the heating element which will be submerged in the water some where,

Also most I've seen have a built in cupboard, you might be able to mount small rcd board and isolator in there.

 

[springcrocus]

I'd TT it locally as you can get a potential deference between the earth around the hot tob and any metal work on the tub, if there's not exposed conductive parts there's still the heating element which will be submerged in the water some where,

Also most I've seen have a built in cupboard, you might be able to mount small rcd board and isolator in there.

Good points, and I forgot to say the CU would have a 63A 30mA RCD main switch although I think some tubs come with an RCD built in but I don't know for sure. Regarding local earthing, as I stated it will likely be class II so there should be no ECPs. However, a 6-8 person tub might be too heavy for decking. What are we talking about when full - a ton, ton and a half?

 

[Wilpickard]

Prob a bit more, 1m cubed weighs a metric ton, think how much waters going to be in it

 

[Wilpickard]

The bit I don't get about them being class 2 is, the heater will be in the water somewhere, now the water and any additives surely can conduct, so wouldn't the element be grounded?

How many immersion heaters have you changed because they leak to earth?

 

[steptoe]

OK, Ive seen your other pointer, 

firstly, do you know if it is TNC-S or deffo PME ?

in either case Id be looking at a rod, if the supply is PME then simply extend and localise it, if not, then seperate it and TT the tub,

decking or not, its still possible for wet feet and hands to be in contact with 2 points of earth,

 

[kme]

Concur mate - I`d look at TT, tbh.

I didn`t get why you were having the IP65 unit though - it it IS going to be TT, and there IS a built-in RCD, you could use the supplied earth for the SWA, and drop it at the connection to the tub - i.e. just a rotary isolator.

If its for the RCD, then maybe mount it next to the rotary iso?

Or have I missed summat? Its getting late, and I`ve been on the happy juice 

 

[springcrocus]

OK, Ive seen your other pointer, 

firstly, do you know if it is TNC-S or deffo PME ?

in either case Id be looking at a rod, if the supply is PME then simply extend and localise it, if not, then seperate it and TT the tub,

decking or not, its still possible for wet feet and hands to be in contact with 2 points of earth,

Don't know for sure whether it's PME, all supplies are undergound at this location. I could probably ask at the local depot but when I've asked for info from them before it's always been, "We'll get back to you". Still waiting...

What am I looking for - better than 20 ohms or so? I'm assuming anything greater wont be worth the effort. The ground is clay over chalk so shouldn't be too bad.

 

[springcrocus]

Concur mate - I`d look at TT, tbh.

I didn`t get why you were having the IP65 unit though - it it IS going to be TT, and there IS a built-in RCD, you could use the supplied earth for the SWA, and drop it at the connection to the tub - i.e. just a rotary isolator.

If its for the RCD, then maybe mount it next to the rotary iso?

Or have I missed summat? Its getting late, and I`ve been on the happy juice 

Garden lighting from the same supply point.

 

[steptoe]

measure it,

its not mega hard to check for proper PME, you can even do it using a meter that doesnt do Ra, to a degree,

all you need is an extra earth rod, and if it isnt proper PME well you've just helped the TNC-S that is already there.

where is your supply in relation to the hot tub?

 

[springcrocus]

measure it,

its not mega hard to check for proper PME, you can even do it using a meter that doesnt do Ra, to a degree,

all you need is an extra earth rod, and if it isnt proper PME well you've just helped the TNC-S that is already there.

where is your supply in relation to the hot tub?

The job is the other side of the island, and I haven't got it yet so measurements can wait for later. The tub will be about 5 metres from the back of the house, 12 metres from the head but needs a 25 metre cable run. Supply cable comes in at the front so local rodding likely to be a long way away.

So I shall allow for a couple of 5/8" rods in the estimate, but not sure whether to go for TT or augmented TNC-S.

And now it's bed-time. :z

 

[steptoe]

The job is the other side of the island, and I haven't got it yet so measurements can wait for later. The tub will be about 5 metres from the back of the house, 12 metres from the head but needs a 25 metre cable run. Supply cable comes in at the front so local rodding likely to be a long way away.

So I shall allow for a couple of 5/8" rods in the estimate, but not sure whether to go for TT or augmented TNC-S.

And now it's bed-time. :z

island.

island?

what bloody island?

you in the carribbean somewhere?

allow for some 16mm earth to in there buddy.

 

[Wilpickard]

Why would the rod be far away? Wouldn't you rod it near the tub? Earth cable from rod to isolator but only connect to outgoing earth, earth the swa with house earth but don't continue to the tub?

 

[Wilpickard]

Sorry steps, didnt read properly, your suggesting using PME. I was thinking of TT

 

[steptoe]

Sorry steps, didnt read properly, your suggesting using PME. I was thinking of TT

if its PME incomer then I would localise it,

if its simply TNC-S then Id go with your suggestion of simply TT the tub,

 

[Apache]

He lives on the Isle of Man.

 

[steptoe]

He lives on the Isle of Man.

F'in 'ell,

he'll have problems with rods then,

its ALL slate,

about a hundred rods all 12" long may be needed!