DIY Doctor

Wiring up an outdoor workshop/shed and hot tub

Postby Max77uk » Sat May 30, 2020 6:00 am

I have built an outdoor workshop which needs wiring, I also acquired a hot tub which runs 32A.

Please read through my wiring plan and feel free to pick up on any errors!

House CU (on 30mA 100A RCD) 40A MCB

Running 5 core 6mm Swa along outside house,
Cable buried at back of house at 500mm (buried electrical tape at 300mm)

Connected to Shed CU,
Shed CU 1x 6A MCB(lights) , 1x 16A MCB(sockets) , 1x 32A MCB(hot tub)

Hot tub connection has 45A rotary isolator switch 2.5m from tub.

Whole thing earthed via house PME

Does this sound right??

Note- I am part p registered so can sign off, however this is not the sort of electrics I deal with so need confirmation or advice!

Thanks
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Postby ericmark » Mon Jun 01, 2020 8:08 am

Personally I would not run any out building from the house RCD, I would want it on it’s own RCD even if it is in the house in the form of a RCBO, as don’t want to lose house power because of fault with outbuilding, if this can’t be done, then some double pole switch so both live supplies are switched both line and neutral.

Using house PME depends on distance, there was a now outdated article as part of the IET Wiring Matters https://www.theiet.org/Publish/WireRegs ... tdoors.pdf which explains it better than I can.
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Postby Max77uk » Mon Jun 01, 2020 9:02 am

ericmark wrote:Personally I would not run any out building from the house RCD, I would want it on it’s own RCD even if it is in the house in the form of a RCBO, as don’t want to lose house power because of fault with outbuilding, if this can’t be done, then some double pole switch so both live supplies are switched both line and neutral.

Using house PME depends on distance, there was a now outdated article as part of the IET Wiring Matters https://www.theiet.org/Publish/WireRegs ... tdoors.pdf which explains it better than I can.


I now have an additional RCD on workshop end.
House end RCD will be used solely for earth fault on armoured cable which is sheathed at MCU

Cable run is 15 metres so PME should be OK but I get what you're saying.

Is it worth tt'ing on a separate earth rod at workshop for belt and braces?
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Postby ericmark » Wed Jun 03, 2020 6:46 am

I don't think you understand how a RCD works, it does not actually detect current flowing to earth, it detects an imbalance line to neutral and it assumes if not the same, then some current must be going to earth, so you can't have a RCD which is solely for the SWA cable.

Personally I would not fit an earth rod to a PME supply, I have seen the result when the supply authority lost their neutral/earth, with a rod at 60 ohm not really a problem, but with a good earth, and with the case in question there were 4 earth rods connected with bare earth tape designed as the earth for a radio transmitter, when the supply earth/neutral was lost the whole street tried to use his earth, so likely swinging up to 300 amp, and the 4 mm earth cable melted.

It is OK when every house has water and gas earthed as the load is shared between all the houses, but if only one house is earthed that house takes it all, so let the DNO do any multiple earthing, either use PME (TN-C-S) or TT don't mix them.

The problem with TT is you need to test, there are two methods, one you use a loop impedance tester and compare your earth with the DNO earth, and second is you fit temporary test probes at 15 meters +/- 2 meters and 30 meters and test the earth using a proper earth test meter, both require expensive equipment to test, and the second needs access to the ground to fit probes. So for the DIY man it is an expensive exercise to use TT because of the testing involved.

Of course all installations should be tested, including the RCD, we all know you can't measure 40 mS with a stop watch so again should use a special tester, but in the main if the RCD trips at 30 mA then we just cross our fingers that it does it fast enough.
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Postby Max77uk » Wed Jun 10, 2020 5:06 pm

Thanks for you advice,

I've ditched the earth rod idea on your advice. DNO have informed that PME is sufficient for cable run length.

I have 40A MCB connected to RCD on main, if I have the issue of tripping the whole house, I will break mcb and reevaluate.

The reason I'd like to use Armour as earth is because I am running 2 Neutral, 2 Live lines as 6mm is cutting it fine with ampage required for hot tub. So my thoughts were to use sheathing to make up the loss while still using 1 earth core as well.

So to recap,

100A 30ma RCD on MCU
Feeding 40A MCB

sheathed 5 core, earthed from armour and 1x 6mm core, 2 Neutral 6mm, 2 Live 6mm

18m run (DNO approve distance on PME)

Cable run to CU.
Armour earthed along with 1x core
2 Neutrals to N busbar
2 lives to 40A RCD

6A for lights
16A sockets
40A run through earthed armour to Rotary isolator 40A

Armour run to tub

ARMOUR EARTHED with core at tub.
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