Second Consumer Unit / Garden power advice needed

[rob29london]

Hi all, I’ve built myself a garden room and am currently after some advice regarding electrical work that needs to be carried out. I‘ve run the cables, and a sparky will be connecting everything. I’ve run a 35m length of 10MM SWA 3 Core from the house CU to the garden room. My House Consumer unit is a Dual RCD Wylex with a spare MCB space. The garden room will have its own (small) CU.


So I have a few questions::

1) How many amps will this allow me to have at the garden room Consumer Unit after voltage drop is accounted for?

2) Would it be suitable / possible to able to use a RCBO in my House Consumer unit (in spare MCB socket) so that if something outside trips it won’t take out half the house? Or would it be better to have a couple of RCBO’s in the consumer unit in garden room, Or some sort of secondary fused isolator (for a totally different supply) near the main CU indoors? What would be the best practice here?

3) There will be no Water/Gas Pipes in the Garden room, will an earth spike be required? Or would I use the earth from the House CU?


I will be getting a qualified sparky in to do this, but want to order all the parts in advance. I know I’ve asked a lot of questions, so I’d like to thank you in advance for your help!

 

[AndyPRK]

2) Couple of things to remember here. If you have 2 RCD devices in series there is no telling which will trip on a fault condition (infact both could)

RCBO's generally only disconnect the Line/Live conductor, so if there is a N E short the RCD would trip (and may not reset) even if the RCBO had.


I wouldn't bother with any RCBO's in your out house, just a RCD on the main incoming cable


As you have used 10mm cable it maybe more sensible to connect the supply up to the tails (prior to the house CU). (assuming your earthing isn't TT and therefore RCD protection would be required)

Alternatively a double pole isolator would enable you to isolate the outhouse supply from the house CU if you had problems.

 

[JohnW2]

Hi all, I’ve built myself a garden room and am currently after some advice regarding electrical work that needs to be carried out. I‘ve run the cables, and a sparky will be connecting everything. I’ve run a 35m length of 10MM SWA 3 Core from the house CU to the garden room. My House Consumer unit is a Dual RCD Wylex with a spare MCB space. The garden room will have its own (small) CU. ... I will be getting a qualified sparky in to do this, but want to order all the parts in advance

You really need to involve an electrician now (in fact, you should have done before you installed the SWA). You are talking about ‘notifiable’ work (a new circuit). A registered self-certifying electrician will be able to do that notification at minimal cost (but if you notified yourself it could cost ‘a few hundred pounds’), but (s)he would then have to certify that they had been responsible for all the design, construction and testing of the new work. Whether an electrician would be happy to, say, certify the installation of the SWA without digging and exposing it, only they could tell you.

How many amps will this allow me to have at the garden room Consumer Unit after voltage drop is accounted for?

Buried 10mm² SWA, per se, has a current-carrying capacity of about 60A. If there is to be any lighting in the outhouse, the guidance maximum voltage drop is 3% (6.9V), which, for a 35m run, would limit the design current of the circuit to about 44.8V. Your electrician will have to approve the design of the circuit.

Would it be suitable / possible to able to use a RCBO in my House Consumer unit (in spare MCB socket) so that if something outside trips it won’t take out half the house? Or would it be better to have a couple of RCBO’s in the consumer unit in garden room, Or some sort of secondary fused isolator (for a totally different supply) near the main CU indoors? What would be the best practice here?

For the reasons you mention, it is certainly best not to have the outhouse supplied via an RCD protecting other circuits in the house. Provided none of the cable between CU and SWA is buried in a wall, you would probably be best without RCD/RCBO protection at the house end at all (just RCD in outhouse CU). If your CU not only has dual RCDs but also some available ways that are not RCD protected, then you could use that (the same could be used for an RCBO, if you really wanted one) - but if it is ‘just’ a dual-RCD CU (with no non-RCD ways), then that wouldn’t be possible. In that case, the best course might be to get the tails to the CU split and a separate switch-fuse for outhouse supply installed. Your electrician will be able to advise about all those issues and options.

There will be no Water/Gas Pipes in the Garden room, will an earth spike be required? Or would I use the earth from the House CU?

The house earth would quite probably be OK but there are many factors to take into consideration and electricians’ opinions about this issue vary a lot - so you would need to ascertain the view of the electrician who was designing, and was going to certify, the new circuit.

Kind Regards, John

 

[ban-all-sheds]

I‘ve run the cables, and a sparky will be connecting everything.

Has this electrician agreed to sign declarations to say that he was responsible for all of the work, and that it all complies in all aspects with the Wiring Regulations and the Building Regulations?

When you applied for Building Regulations approval, what did you tell them would be the way you'd ensure compliance with Part P? Because if you don't do what you told them you would do then rock and hard place will start to become a reality for you.

How many amps will this allow me to have at the garden room Consumer Unit after voltage drop is accounted for?

That's the wrong way around - proper circuit design involves choosing the cable to suit the load, not the other way around.

Anyway - you should ask the competent designer that question - he must know. You did use somebody who knows what they are doing to do the design, didn't you?

2) Would it be suitable / possible to able to use a RCBO in my House Consumer unit (in spare MCB socket) so that if something outside trips it won’t take out half the house? Or would it be better to have a couple of RCBO’s in the consumer unit in garden room, Or some sort of secondary fused isolator (for a totally different supply) near the main CU indoors? What would be the best practice here?

3) There will be no Water/Gas Pipes in the Garden room, will an earth spike be required? Or would I use the earth from the House CU?

Ditto to all of those - they are all things that the person who did the design should know. If he doesn't then he had no business doing the design, and you should start to ask yourself why on earth you engaged someone incompetent to do it.

I will be getting a qualified sparky in to do this, but want to order all the parts in advance.

It doesn't work like that. And why are you keen to pay too much for materials?

 

[rob29london]

Thanks for the replies! The reason I’ve asked these questions, is because I want to understand this system more than anything. Me ordering the parts in advance is a bit pointless which is fair enough.... I will be letting the Spark order the parts then. The cable has been run in accordance to Regs.

Personally, I won’t be using anything that isn’t properly signed off.

 

[JohnW2]

Thanks for the replies! The reason I’ve asked these questions, is because I want to understand this system more than anything. Me ordering the parts in advance is a bit pointless which is fair enough.... I will be letting the Spark order the parts then. The cable has been run in accordance to Regs. Personally, I won’t be using anything that isn’t properly signed off.

That all sounds good/right. I don't doubt you when you say that the SWA has been installed per regs but, if it's already been covered up, it remains to be seen whether your electrician will be happy to 'sign it off'.

Kind Regards, John

 

[ban-all-sheds]

The cable has been run in accordance to Regs.

That means that the designer (i.e. the person who calculated the size, and chose the type) of cable MUST sign the above declaration to say that he did it in accordance with the regulations.

I being the person responsible for the design of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2011 except for the departures, if any, detailed as follows:

Ditto the constructor, i.e. the person who installed it.

I being the person responsible for the construction of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the construction hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2011 except for the departures, if any, detailed as follows:


Without those declarations the cable has not been "run in accordance to Regs", as certification is a mandatory part of complying with them.

Personally, I won’t be using anything that isn’t properly signed off.

If that be true then before you installed the cable you would have had to apply for Building Regulations approval.

Did you?

 

[JohnW2]

That means that the designer (i.e. the person who calculated the size, and chose the type) of cable MUST sign the above declaration to say that he did it in accordance with the regulations.

In reality, it's only really the 'construction' (and, of course, testing) part of the work/declaration which presents potential problems. In terms of design, only the obsessively pedantic would attempt to distinguish between 'ab initio' design and retrospective approval of a design. In either case, the electrician concerned would be approving, and taking responsibility for, the design, and declaring that it was compliant with the regulations.

Kind Regards, John

 

[ban-all-sheds]

If he is the sort of electrician prepared to falsify documentation, yes.

 

[JohnW2]

If he is the sort of electrician prepared to falsify documentation, yes.

As I said, it would only be considered to be falsification by the obsessively pedantic.

Kind Regards, John

 

[ban-all-sheds]

Or those who actually possess standards of honesty, truthfulness and trustworthiness.

If you regard classifying a lie as falsification to be obsessively pedantic then you clearly have no standards of honesty, truthfulness and trustworthiness which I would expect to find in a decent person.

Perhaps you should go and join your fellow travellers on the GD forum.

 

[EFLImpudence]

In reality, it's only really the 'construction' (and, of course, testing) part of the work/declaration which presents potential problems. In terms of design, only the obsessively pedantic would attempt to distinguish between 'ab initio' design and retrospective approval of a design. In either case, the electrician concerned would be approving, and taking responsibility for, the design, and declaring that it was compliant with the regulations.

I do not really understand your distinction or, rather, when it could happen.

Are you saying an electrician - who would sign for the design - can or could construct an installation to someone else's design without thinking and/or knowing that that design were correct?
Conversely, would that electrician blindly follow a design which he would then be willing to sign for without giving it a thought?

When rightly not willing to sign for someone else's construction surely the design part is moot.

 

[JohnW2]

I do not really understand your distinction or, rather, when it could happen.

I would have thought the distinction was clear (designing from scratch oneself, or examining someone else's design and, if one is happy with it, 'approving' it).

Are you saying an electrician - who would sign for the design - can or could construct an installation to someone else's design without thinking and/or knowing that that design were correct?

Of course not, but it was BAS who chose to separate consideration of design and construction declarations. I thought it was clear that I was talking about the situation in which the electrician had fully considered, and was completely happy with, the design.

BAS is attempting (as I've said, IMO "obsessively pedantically") to suggest that if someone else does the design first, after which the electrician 'considers, checks and approves' the design, then for that electrician to sign the design declaration represents a 'falsification', since someone else thought about the design first! To my mind that is quite a ridiculous suggestion and, as (if I have time!) I'm about to write to BAS, is a view which would mean that much of the the work I have done in the past few decades has been 'falsified'!!

Kind Regards, John

 

[EFLImpudence]

Of course not, but it was BAS who chose to separate consideration of design and construction declarations. I thought it was clear that I was talking about the situation in which the electrician had fully considered, and was completely happy with, the design.

Ah, well, of course that is acceptable but then it is not signing for someone else's design at all so the premise does not arise.

That is what I was puzzled about regarding when it could happen.

I would say that in a domestic situation it is going to be standard circuits but something out of the ordinary such as a detached distant garage supply it would literally be a waste of time the householder doing it as it would all have to be checked again.

 

[JohnW2]

Or those who actually possess standards of honesty, truthfulness and trustworthiness. ... If you regard classifying a lie as falsification to be obsessively pedantic then you clearly have no standards of honesty, truthfulness and trustworthiness which I would expect to find in a decent person.

No, that is not my view, but you do not seem to understand that (except in a stupid, pedantic, sense) there is no "lie".

If I sit down and carefully consider a circuit, do all the necessary calculations, and end up concluding that it is electrically satisfactory and in compliance with regulations, are you seriously suggesting that for me to sign the design declaration would become "a lie" if someone else had previously thought about the design of the circuit? Is it really your view that only the first person to undertake the thinking and calculation etc. can describe themself as 'the designer'?

I spend much of my life putting my name and signature to reports and other formal documents which are declared as being my considered views,opinions and judgements about the matter in question. However, those documents often follow long periods of discussion, with input from many people, and often many drafts which others influence. I obviously make my own assessments of those other inputs before I end up formulating my own position - but that will often be to agree with some of what others have said. By your reasoning, I would be "lying" when I declared these things to be my view and assessment, since someone else had "thought of it first"!

Kind Regards, John