6mm SWA into CU for hot tub

[jimuthy2]

Hi
I want to install a 6mm SWA cable from the house CU to an external isolator to provide a 32amp supply.

At the CU connection, do I need to first bring the cable into an insulated adaptable box, then run the core into the CU, or can I simply gland the SWA straight into the CU?

 

[SUNRAY]

Hi
I want to install a 6mm SWA cable from the house CU to an external isolator to provide a 32amp supply.

At the CU connection, do I need to first bring the cable into an insulated adaptable box, then run the core into the CU, or can I simply gland the SWA straight into the CU?

If it's a metal CU then just gland it straight in, in the normal way.
If it's plastic, gland it but ensure the cable isn't stressing it, and use a piranha nut for the earth connexion.

Additional boxes are not normally required or even helpful, that is of course unless the position of the cable requires a very sharp bend etc. and the additional box alleviates the access problem.
Such as this example:

Read more: https://www.diynot.com/diy/threads/a-real-moan-tonight.552183/#ixzz7NAeTUnba
the white conduit on the left would ideally have entered the side of the CU but there was an obstruction below the conduit.
The black SWA is glanded straight in.

 

[jimuthy2]

Hi Sunray, thanks for your reply - I read that thread and that makes sense, so I will gland straight into the CU. The cable will drop down from the CU, then needs to turn 90 degrees to pass though the wall, before going into the ground - would you recommend a junction box / IP box either side of the wall?

 

[SUNRAY]

Hi Sunray, thanks for your reply - I read that thread and that makes sense, so I will gland straight into the CU. The cable will drop down from the CU, then needs to turn 90 degrees to pass though the wall, before going into the ground - would you recommend a junction box / IP box either side of the wall?

Oh I wasn't trying to point to the thread, I only used part of the picture to show the white conduit box as an example of where a box would be an advantage.
The additional box on the right side of the CU happened to present a significant risk to safety but it wasn't the box that caused the risk, (and indeed an additional box is highly unlikely to cause such a risk) it was a rare external fault and (in my opinion) poor practice that created the hazard.

I would try to avoid any junction boxes in the new circuit. Try to drill the hole as steep as possible to reduce the bends. (pointing down towards the outside)

 

[ericmark]

With a hot tub you need to consider earthing if any, the problem is many houses are supplied with a TN-C-S supply, which under fault conditions can result in the DNO supplied earth not to be equal to real earth. In the main out doors we use class II equipment which does not require an earth. This includes some hot tubs.

However as has been seen with EV charging points having a TT and TN-C-S supply in the same property can cause problems, and the person designing the installation has to decide how to proceed. In Wales any supply to the garden needs registering with the LABC so either a scheme member electrician or inspected by the LABC.

I have seen the method used with EV charging where the voltage is monitored and if it stays out of the range 207 to 253 volt the supply then the earth is auto disconnected, this is claimed to ensure the voltage between the two earth systems will not exceed 70 volt, however that is rather high for a hot tub, the normal limit is 50 volt.

Loss of PEN is rare, and this is in the main what causes the volt difference between the two earth systems, but if you see the regulations for earthing a cattle shed this is really what is required for a hot tub if it requires an earth. So I would say this is not a DIY job to design the supply to a hot tub unless subscribing to the Darwin theory.

 

[jimuthy2]

Oh I wasn't trying to point to the thread, I only used part of the picture to show the white conduit box as an example of where a box would be an advantage.
The additional box on the right side of the CU happened to present a significant risk to safety but it wasn't the box that caused the risk, (and indeed an additional box is highly unlikely to cause such a risk) it was a rare external fault and (in my opinion) poor practice that created the hazard.

I would try to avoid any junction boxes in the new circuit. Try to drill the hole as steep as possible to reduce the bends. (pointing down towards the outside)
View attachment 263690

Thanks again Sunray - much appreciated!

 

[jimuthy2]

With a hot tub you need to consider earthing if any, the problem is many houses are supplied with a TN-C-S supply, which under fault conditions can result in the DNO supplied earth not to be equal to real earth. In the main out doors we use class II equipment which does not require an earth. This includes some hot tubs.

Thanks ericmark, but I don't have an EV charging point and I'll be sticking to petrol for as long as I can. Also, my earthing arrangement is TN-S not TN-C-S, so I don't think this concern applies?

 

[mikeey84]

If this is a new circuit, I'm pretty sure it's notifiable work if you weren't aware...

 

[ericmark]

my earthing arrangement is TN-S not TN-C-S

In that case easy, how you know it is TN-S I don't know.

 

[bernardgreen]

If the "earth" connected to the water in the hot tub is the "earth" that arrives at the house via the local supply network then there is a risk of electric shock for anyone standing on the Ground and reaching into the water in the hot tub.

The water in the hot tub will not be distilled water and therefor it will be conductive, the water will be in contact with various pipes, pumps and other items which are connected to the "earth" from the local network.

The only way to ensure a person standing on the ground by the hot tub with their hands in the water will not be at risk of significant electric shock is to ensure the pipes pumps and other items are at Ground potential. that Ground being the Ground under and around the hot tub.

Loss of PEN is rare,

Total loss of the PEN may be rare but Neutrals being far from Ground potential are not rare,

 

[flameport]

my earthing arrangement is TN-S

It may look like that, but it won't be.

TN-S only exists in situations where the entire cable from the property to the transformer is known to be SNE, and it will be kept like that forever. The only realistic scenario for that is where a transformer supplies a single installation.

 

[jimuthy2]

It may look like that, but it won't be.

TN-S only exists in situations where the entire cable from the property to the transformer is known to be SNE, and it will be kept like that forever. The only realistic scenario for that is where a transformer supplies a single installation.

The cable is lead sheathed and wrapped in an oil tape with a metal clamp round the cable linked to the MET, but I take it you mean the high likelihood of parts of this SNE cable (between me and the transformer) inevitably having being replaced at some point with CNE? That sounds reasonable, so it would be safest to treat it as TN-C-S?

I am thinking there is a need for an earth spike near the hot tub to ground the armour of the SWA and the reinforcement of the concrete slab it sits on, with the armour connected to the earth in the CU at the other end?

 

[ericmark]

Yes it may look like TN-S but it may really be TN-C-S. The problem is when moving from TN-C-S to TT the two systems need to be far enough apart to ensure you can't touch both at the same time. With caravans which are not permitted to be supplied from a TN-C-S supply there is also fire regulations requiring them to be 2.7 meter or more from a building, so unlikely both earths can be touched at the same time.

However the caravan rule is for caravans being used, and not simply stored, and I have seen caravans stored closer that 2.7 meters to house, and there was a pair of cupboards built into the wall with gas and electric meters in them, so one could touch the meter and caravan at the same time, and the caravan was powered to keep batteries charged.

However no access to general public, and so did a risk assessment and decided better to supply caravan with the TN-C-S earth, maybe I was wrong, but would be wearing shoes, I would not be wet or naked.

The same problem with EV charging, one can install a 32 amp socket for using a welding set, then use the same socket to charge a car, there is a danger, but again unlikely to get in or out of a car without wearing shoes, it would not comply using the 32 amp socket to charge car, if the socket was installed to charge car, but if installed for some thing else hard to stop some one using it.

Hot tub however we are looking at wet people, with out any shoes or other insulating material so about as vulnerable as they can get, some hot tubs are class II, but not all, and also looking at what the pipe work is connected to, it may be fed with plastic pipes, but not always, so the person connecting the supply has to assess the risks, hot tube in an integral garage is very different to at end of garden, so there is no one system suits all.

Personally my garden is big enough to have the hot tub or EV far enough from house to safely have a 32 amp TT supply for either use, but this would not have been case with last house.

So not saying do or don't do, or how to do it, what I am saying is one needs to assess the risks, and design the installation not throw it together. Personally I think TN-C-S should be banned, but that's not going to happen.

 

[jimuthy2]

Hot tub however we are looking at wet people, with out any shoes or other insulating material so about as vulnerable as they can get

Indeed, water, electric and bare feet are what bother me. I have dealt with some dodgy contractors, so want to have a good idea how this should be done before I let someone loose on it (no offence to anyone on here ). I am glad I asked on here, as it now clear that my TN-S should be treated as a TN-C-S. That being the case, there is, as you say, a potential difference between earth and true earth. There seems to be a couple of ways to address this, but I'm thinking earthing the CPC and armour at the CU end to the MET, with a 30mA RCD and 32A MCB type C. Meanwhile, the hot tub, the CPC and armour at the tub end to either an earth rod or mat to compensate for the potential difference between earth and true earth.

 

[bernardgreen]

Meanwhile, the hot tub, the CPC and armour at the tub end to either an earth rod or mat to compensate for the potential difference between earth and true earth.

You could export the "earth" from the house to the ground around the hot tub using the SWA armour and/or a conductor in the SWA cable. This would create a potential gradient in the ground surrounding the earth rod.

The ground rod and the soil in contact with it will be at the potential of the "earth" exported from the house.
The soil a few feet from the earth rod will be at natural ground potential.

Where there is no fault these two potentials will be very close. If there is a fault ( or significant unbalance between phases in the local network ) then the "earth" potential in the house could be many tens of volts different from the natural potential of the soil thus creating potential gradient. People standing in bare feet in the area of the potential gradient may feel an electric shock from foot to foot.