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

 

[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)

 

[davesparks]

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

60A supply isn't pushing it for a house at all.

And 50A for a domestic hob is bit overkill don't you think?

 

[shanky887614]

60A supply isn't pushing it for a house at all.

And 50A for a domestic hob is bit overkill don't you think?

what im saying is a lot of people are going for electric showers, induction hobs etc now so they are using a lot more electricity than they used to


at Christmas with hob, oven and electric show going plus heating could trip iit

 

[HandySparks]

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!

In which case, providing it's deep enough, lay a suitable size of duct in with appropriate backfill and warning tape. Take photos and measurements to show your electrician. Then he can come along later and pull in whatever cable is required.

 

[jonA]

First, my apologies - I copied and pasted my last from Word and it all came out wrong. Makes it difficult to follow so I will try posting it again.

Meantime, for 13A RCBO, please read 16A!

Also, the 'house' is more an upmarket shed and will have minimal power demand. There won't be an electric shower or hob, just a few power and lighting circuits. 63A will be more than enough!

 

[davesparks]

what im saying is a lot of people are going for electric showers, induction hobs etc now so they are using a lot more electricity than they used to


at Christmas with hob, oven and electric show going plus heating could trip iit

Why do people always harp on about christmas dinner when discussing cookers?

Diversity applies on the 25th of December exactly the same as it does every other day of the year!

 

[shanky887614]

Why do people always harp on about christmas dinner when discussing cookers?

Diversity applies on the 25th of December exactly the same as it does every other day of the year!

on christmas your more likely to have meat slow cooking in the oven and things on the hob while people are in the shower and playing games so demand is higher for a lot of people than any other time of the year

 

[telectrix]

on christmas your more likely to have meat slow cooking in the oven and things on the hob while people are in the shower and playing games so demand is higher for a lot of people than any other time of the year

good point. i have a shower at xmas, whether i need one or not. it's a bugger xmas day as the takeaway is closed and the cooker has to be turned on.

 

[davesparks]

on christmas your more likely to have meat slow cooking in the oven and things on the hob while people are in the shower and playing games so demand is higher for a lot of people than any other time of the year

Diversity will be the same as any other day of the year though.
The oven and hob elements will go through exactly the same cycles as they always do.

 

[ImpededLoop]

good point. i have a shower at xmas, whether i need one or not. it's a bugger xmas day as the takeaway is closed and the cooker has to be turned on.

That's where you're going wrong. Save the shower until Boxing Day when you're eating cold leftovers. That's how diversity should work!

 

[Octopus]

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

Sensible advice:

1. Post your location

2. Get 3 or 4 LOCAL compentent sparks to quote

3. Select best option (not always the cheapest)

4. Get work done


Simples.