Wiring a hot tub with SWA

[chrisb2340]

This is my first post so lets see how it goes.
Can anyone advise me on the following installation.
Have been asked to install wiring to a garden hot tub (24A max demand).
Its nearly 70m from the house and the customer also requires fan heater in the shed and some lights. Max demand in total about 38A. Cable calcs have given 10mm cable as voltage drop is getting close to 5% max.
I intend to fit a Henley block on the incoming tails to a new C.U.
This will have a 40/50A, 100mA RCD and a 50A Type C MCB.
In order to meet disconnection time I need to install an earth rod and make the new install a TT system.
Thats ok but should I use 2 core SWA, bond the steel armour the the house earth and use the TT system for the protection for the rest?
Was going to fit a 40A 30mA RCD with 32A, 16A and 6A MCB's in a CU in the shed.
Comments would be appreciated.

 

[truckster]

I'd use a 50 B type mcb, 100mA delayed rcd in the house and the rest as you say although if the VD is tight I would go up to 16mm to allow for additins in the future. I prefer TT outside of the equipotential zone, (I know others disagree) dont forget the bonding in the shed and ensure that the PME earth from the house is not accesible at the same time to the TT earth

 

[PC Electrics]

Don't forget 3% max vd for lights. Re-calc needed?
You haven't said what the earth type is at the house.
Why fit two RCDs? with a 100mA at supply end and 30mA at use end you'll have no discrimination. But why do you feel the need for a 100mA at supply end? If it's a TN you don't need RCD protection for swa. If it's TT then you'd need an S type - but why bother, if your 30mA is going to cover all the circuits just put it at the house end anyway !

 

[chrisb2340]

Thanks for the replies.
The house system is TN-S with a measured Ze of 0.17 ohms.
The maker of the hot tub stated 'C' type as presumably there is a high in rush current for the pump.
Very good point about the 3% VD for lights, I had worked on 5%.....
I think it will have to be 16mm cable.
I thought that the RCD at the house end would protect the cable but as it doesnt need RCD protection I think I will use a 30mA RCD at the tub end to protect all the circuits.
Thanks again.
Chris B

 

[nickblake]

Thanks for the replies.
The house system is TN-S with a measured Ze of 0.17 ohms.
The maker of the hot tub stated 'C' type as presumably there is a high in rush current for the pump.
Very good point about the 3% VD for lights, I had worked on 5%.....
I think it will have to be 16mm cable.
I thought that the RCD at the house end would protect the cable but as it doesnt need RCD protection I think I will use a 30mA RCD at the tub end to protect all the circuits.
Thanks again.
Chris B

its a long way so i think it will have to be

 

[cornburn]

Why not a 50A RCBO at the house end, protecting the 16mm sub main, only connect the earth at the house end. Then a small CU with breakers at the far end, SWA earth disconnected and a rod in the ground? I can't see a problem with that and more straight forward. Or a breaker in the house and a main RCD CU at the far end, doesn't make much difference either way.

If you put RCBOs at the far end though thinking about it you could have some discrimination between circuits, meaning that if the tub ever trips the RCD they'll still have lights, and so won't be stumbling around in the dark with wet hands all over the CU trying to reset the RCD!