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

 

[davesparks]

Sorry but if you aren't qualified and experienced in the design of electrical installations then you cannot be designing them. Are you going to sign the certificate for the design of the installation and accept liability etc?

You should just need to specify what you need installed in terms of number and location of points etc and let the electrician design the installation to suit.

Of course we all work differently, and the cheapest quote is rarely the best.

 

[Knobhead]

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

Carry on, feel free, no one will stop you.

No one with a modicum of professional integrity* on the forum will help when your DIY install has gone wrong.

Dave has given the best advice so far.

Sorry but if you aren't qualified and experienced in the design of electrical installations then you cannot be designing them. Are you going to sign the certificate for the design of the installation and accept liability etc?

You should just need to specify what you need installed in terms of number and location of points etc and let the electrician design the installation to suit.

Of course we all work differently, and the cheapest quote is rarely the best.

*I keep forgetting the Electrical Trainee element in here.

 

[Knobhead]

From a previous thread started by the OP

http://www.electriciansforums.co.uk/electrical-wiring-theories-electrical-regulations/17872-design.html
Hello .. great site and looking for some free advice !
I m a bit out of my depth here and need to speak to an electrician. Trouble is I hate spending money.



What you need is this to do the job yourself. Which we know it is what you intend to do.

You’ll have to reserve a copy, the Electrical Trainee’s have bought all the existing stock.

 

[Guest77]

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

Your a con man, you have said this before, I reckon you are doing all the work and getting a --- from the pub to part p it, you come on here for us to give you our free advice mocking us as you go along, you might fool most of the guys here but you do not fool me, off you go sonny, don't come back.

 

[jonA]

Crikey, some sensitive souls in here!

I’m sure MJD has many skills but reading my mind isn’t one of them. He is correct in believing that I am looking for free advice but I had already told him that.

I wasn’t aware that this was a crime indeed I thought it was the purpose of a public forum like this.

I am certainly not conning or mocking any of you.

For what it is worth, MJD is mistaken. I will not be doing the work myself but do want to get all my ducks in a row before getting anybody to price it.

Thanks again to all those who have helped!

Cheers, Jonathan

 

[Guest77]

Poppyc0ck, If your getting someone else to do it why are you not asking them to design and quote you, any electrician worth his salt would tell you to buger off, Why are you asking us strangers you have never seen to design you a system when you can get a professional around face to face and they can design for you, I saw your comment regarding they would all quote for something different, I doubt that very much, the fact is IMO you are doing the work yourself and you want us to tell you what to do so you can avoid getting an Electrician in to do the job.


http://www.amazon.co.uk/Home-Maintenance-Dummies-All-One/dp/0764570544

 

[Octopus]

Poppyc0ck, If your getting someone else to do it why are you not asking them to design and quote you, any electrician worth his salt would tell you to buger off, Why are you asking us strangers you have never seen to design you a system when you can get a professional around face to face and they can design for you, I saw your comment regarding they would all quote for something different, I doubt that very much, the fact is IMO you are doing the work yourself and you want us to tell you what to do so you can avoid getting an Electrician in to do the job.


DIY and Home Maintenance for Dummies All-in-One, UK edition: Amazon.co.uk: Jeff Howell: Books

Mike, you are being a little harsh. The op has posted the approx location and asked if one is interested so ......

post #3

 

[Guest77]

Mike, you are being a little harsh. The op has posted the approx location and asked if one is interested so ......

post #3

Um I saw it as there is a job for someone to part p it, not install it.

 

[HandySparks]

...I saw your comment regarding they would all quote for something different, I doubt that very much...

You're joking mate. Have you not noticed the diversity of opinion on this site?

TT or TN-C-S?
Duct or direct burial?
Upfront RCD or not?
Two core or three core SWA?
Fuse or circuit breaker?
Estimation of maximum load?
Margin for future additional load?
Voltage drop allowance for distribution circuit?

 

[Octopus]

Um I saw it as there is a job for someone to part p it, not install it.

Well.............. nobody in their right mind signs off a DIYer work do they?

 

[Guest77]

You're joking mate. Have you not noticed the diversity of opinion on this site?

TT or TN-C-S?
Duct or direct burial?
Upfront RCD or not?
Two core or three core SWA?
Fuse or circuit breaker?
Estimation of maximum load?
Margin for future additional load?

Um no not if a site visit occurs no, anyone qualified properly would access the earthing method first surely? then again I suppose we are talking about DIs as well so, okay your right lol :biggrin:

 

[Guest77]

Well.............. nobody in their right mind signs off a DIYer work do they?

You as a professional proper electrician Clive wouldn't of course, but your not a DI looking for a quick buck are you bud?

 

[Octopus]

You as a professional proper electrician Clive wouldn't of course, but your not a DI looking for a quick buck are you bud?

Like I said nobody in their right mind would sign off a DIYers work.

In this case I would let the OP dig the trench, then check the depth, and review the "infill" which would allow them to do the expensive ground works. I hate digging in my own garden, let alone a clients!

 

[PEG]

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

I am afraid to say,he is correct,either by accident or experience,in this regard. I could relate many,many occasions,where this occurs,and keeps on re-occurring. A lot of the tasks i get handed,come off the back of a shoddy or incorrect install or repair,carried out by certified,experienced,signed up tradesmen. So whilst i retain empathy with the general tone of not encouraging a DIY job,let us not pretend that ringing a dude in his local advertiser,with a big ad and credentials,will guarantee him anything. I have never,and would never advertise. The OP's best method of getting help,would be how my customers get me...and that should not require an explanation. :conehead:

 

[Octopus]

The OP may not like spending money, but he should consider the extra costs of doing the job twice!

 

[jonA]

Guys,

I didn't mean to stir up a hornet's nest here - promise!

What I meant in #3 was that whoever does the work must be Part P registered. Same with the gas supply, which is also proving a headache, they must be Gas Safe registered. Building Control won't sign the building off without the right paperwork and it really isn't possible to get this down the pub!

Handysparks, I recognise there are a lot of issues to be considered and that I am out of my depth on pretty much all of them. I'm just trying to learn to swim.

I come back to my starting point for all building work which is to work out what I want first and then find someone who can deliver it. Sadly, I have had too much experience being told what I want by so-called 'experts' and then finding out that it's not what I want!

Cheers, Jonathan

 

[jonA]

Guys,

I think some difficulty has crept in becausethe power supply is arranged back to front to the customary set up.

There has been a lot of debate here aboutexporting a supply from a house to an outbuilding and there doesn’t seem to bea consensus on this – specifically whether the earth in a PME installation canbe exported, and, if so, how.

In my case, the main fuse and meter arelocated in a garage. I have been using trailing leads from here to carry out buildingwork. The connection in the garage is currently a TT though the DNO is scheduledto convert it to PME shortly.

Building work is now pretty much completeand I want to connect up the house which is about 30m away.

There appear to be two alternatives:

1. Install sub main
   
   Garage consumer unit
   100A RCD 100mA timedelayed
   63A MCB for sub main tohouse
   13A RCBO for local powercircuit
   
   Sub-main to be 3 core16mm2 SWA with third core and armour bonded to MET at each end. As I understandit, the house and garage will then be in the same equipotential zone so acommon earth path is okay.
   
   House consumer unit
   100A DP isolating switch
   3 x 6A RCBO 30mA forlighting circuits
   3 x 13A RCBO 30mA forpower circuits
   
   
2. Split meter tails via Henleyblock to:

1. Garage consumer unit
   

100A DP isolating switch
13A RCBO 30mA for local power

1. Garage isolator switch

100A RCD 100mA time delayed

SWA cable and bonding to be the same as if sub main were installed.

House consumer unit
100A DP isolating switch
3 x 6A RCBO 30mA forlighting circuits
3 x 13A RCBO 30mA for powercircuits

I’d very muchappreciate your advice as to which of these arrangements you think would work best inpractice.


The most pressingissue for me is to get the right SWA ducted and installed in the trench whichhas already been dug. It's collapsing in this weather and is alsocompletely obstructing access to the site!


Last query, if Imay: I would like to use metal clad consumer units for the installation for bothaesthetic and robustness reasons. Recognising that they would have to beearthed, is there any reason why I shouldn’t specify them?


As ever, helpfuladvice would be much appreciated.

Cheers, Jonathan

 

[shanky887614]

i think you should stop before you hurt someone, a little knowledge is dangerous

for a start 13a? thats an odd size

what happens if they want a shower?

an 8kw shower is around 30odd amps, electric hobs etc are usually fed from 50amp mcb's

60amp supply is pushing it a little

 

[ImpededLoop]

Hi Jonathan,

I would turn this on it's head and invite 3 contractors to submit fixed prices for the entire package along with details of the submain size and earthing arrangements. You can then pick the one that best suits what you need (not neccesarily the cheapest).

 

[shanky887614]

its swa so doesnt need time delayed rcd either.

if they dont convert to pme bang an earth rod in next to the board.

(take a 3c swa)