Sub main to house

[jonA]

Hi Guys,

Question if I may for all the design-minded types here!

I am converting a former chicken shed to a very small small, studio-type house. At the outset – four years ago – I had power brought into a garage on the site which is about 30m from the house.

I fitted two meter boxes in the garage which were equipped as follows:

1[SUP]st[/SUP] box (EdF equipment):
- 100A fuse
- Meter.

2[SUP]nd[/SUP] box (my equipment):
Garage unit incorporating:
- 100A DP switch
- 16A 30mA RCBO supplying a 13Amp socket which I have been using for building work
- Spare way which I now wish to use to feed a CU/submain in the house through a buried SWA cable

The installation was deemed by EdF to be temporary, even though it wasn’t. Still, a TT connection was made and an earth rod is connected to the garage unit. EdF said that once the house was connected - with a TT connection - they would convert the garage connection to PME whereupon the earth rod would no longer be required there.

The issue now is how to connect up the house. I hasten to add, and before anyone jumps down my throat, I will not be doing this work myself since I have got to have it signed off as Part P compliant. However I would like to be able to specify the design properly and understand what will be involved before getting quotes. Otherwise every electrician that quotes will quote for something different!

My instinct was to specify a 63A MCB in the spare way in the garage unit and use this to supply the new CU in the house which would contain an isolating switch and one RCBO for each circuit.

Problem I see with this is that there would be no RCD protection for the supply to the house. That said, if I replaced the MCB with a 63A RCBO, if it tripped, I’d loose everything and have to go outside to the garage (probably in the dark) to reset it.

I’d very much appreciate your thoughts about this and perhaps your steer as to the best way to go.

Separately, I understand that 2 core SWA will suffice for the connection and that the armoured part should be earthed via the glands at both ends. Am I correct in believing this?

Thank you for your time reading this!

Jonathan

 

[yellowvanman]

Replace 100 DP switch with a time delayed 100mA RCD.

That protects your supply to house and in there you can put CU with RCBOs @ 30mA, in the event of a fault the RCBOs should trip first but if they don't then the RCD in the garage will trip as a 'backstop'.

2 core SWA is OK, you will need to put an earth rod at the house end if you have water in the garage - to cover your bonding requirements.

 

[jonA]

Thanks yellowvanman!

That makes perfect sense

I'm guessing that the time delayed RCD can also be used as an isolating switch. Does this one look okay to you?

100 Amp 100mA Time Delayed RCD 2 Pole - Wylex

I guess too that I could use the existing 100A DP switch in the garage unit as an isolating switch for the house CU. So I won't be throwing anything away.

To play it safe, I will specify two core SWA cable rated at minimum 100A.

Thanks very much for your help with this.

Needless to say, there is a job that needs doing here for someone that can issue a Part P certificate! The site is near Sevenoaks in Kent.

Cheers, Jonathan

 

[Carlosfandango]

If you put an armoured cable in there would be no need for rcd protection at the supply side. I'd be using 3 core armour using the third core as an earth. Anything over 16mm should do depending on your calcs.

 

[jonA]

Thanks Carlosfandango

I understand the connection to the house must be a TT connection so an earth rod will be required at the house. If this is correct then there is no need for a third core. The armour of the cable will be earthed through the gland at each end.

Cheers, Jonathan

 

[Engineer54]

Why are you ****ing around with a TT installations when the DNO have told you they will supply you with a PME connection?? What would you rather have at this small Studio type house, a crap 200 ohm or so TT system earth, or around a 0.35 ohm earth??

If the DNO are going to provide a PME connection, then you will need to provide a 3 core SWA cable to the studio house, unless you run in a separate bonding conductor from the MET in the garage.

 

[davesparks]

If EDF are going to connect PME once the house is completed then just run a 3 core SWA to cover your bonding requirements and have the PME connected when it's ready.

You can also leave the earth rod connected to the MET for an even better setup (assuming the earth rod has been installed properly)

 

[jonA]

My apologies, I am not an electrician and am probably a bit out of my depth.

The DNO provided a TT supply to the garage because they deemed it to be a temporary builders supply. They said they would convert this to PME once the house was completed.

For some reason I can't quite fathom, I recall them saying that the house itself has to have a TT connection. I think they consider it to be a 'caravan' though it is no such thing! Something to do with equipotential zones?

Of course I'd rather rely on the DNO to provide the earth for both garage and house. But as I understand it, they will only do so for the garage once the house, which is on a submain, has a TT connection.

I might well have misunderstood all this and whether I have or not, I would very much appreciate your clarification!

Cheers, Jonathan

 

[davesparks]

Why don't you just talk to the electrician who is going to design install and test the installation?

 

[jonA]

Davesparks, I don't have one as yet.

As a customer, my experience is that it's best to specify a job first and then get someone to price the job and do it. If I got say three professional electricians to quote for doing the job, each would come up with their own design and the cheapest quote probably wouldn't be the best.

I'll put my hands up here - I can't afford to pay a specialist M&E consultant to specify the job properly so I'm trying to work it out for myself!

Cheers, Jonathan