Lighting Circuit From fused spur on Radial Mains Circuit?

21 Jun 2005
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United Kingdom
I added a 20a mcb to a 2 way consumer unit (other mcb is shower) for a new radial mains circuit to go up to the loft. Can I run a fused spur off of the new radial circuit in the loft for six downlighters?

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OOI - what cable is supplying the mini-CU, and where does it come from?
Hi thanks for the reply.

The cables supplying the mini CU are about the same size as the cables feeding the main CU. They come from a junction box (splitter box). This is from what I can tell as I didn't install this part.
I fitted this new mini CU by upgrading an old standard single fuse-way, that was being used for the shower.

kujina said:
I fitted this new mini CU by upgrading an old standard single fuse-way, that was being used for the shower.
Did you fit one with an RCD?
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So its a definite yes to my original question? I would probably have 6x 50 watt.
Yup. Use an FCU with a 3A fuse in it to take the lighting supply off the 20A loft circuit.

You are aware of the legal requirements of Part P regarding ths work, I assume?
DIY Electrical work and the law

On January 1st 2005 legislation came into effect which brought electrical work in dwellings under the Building Regulations, and made it a controlled service. This amendment to the Building Regulations, known as "Part P", imposes safety requirements, and also classifies electrical installation work into two basic categories, notifiable and non-notifiable.

Essentially the distinction is between major work, or work in what are deemed high-risk areas such as kitchens, bathrooms and gardens, and minor work such as replacing switches or adding sockets to existing circuits. (NB to those with a knowledge of the IEE Wiring Regulations, the term "minor work" is used here in its generic sense rather than the meaning defined in the Wiring Regs.)

A very important and fundamental point to note is that DIY electrical work has not been outlawed. It has been brought within the remit of the Building Regulations and cannot be carried out as freely as it was before, and in many cases cannot be carried out without involving your local council, but you may still DIY.

Major work is classed as notifiable, i.e. it must be notified to your Local Authority's Building Control department (LABC). How this happens depends on who does it.

If the work is carried out by an electrician who is registered with one of the organisations who administer self-certification schemes, (sometimes referred to as "Competent Person" schemes) they carry out the work and report the details to their scheme organiser, who then notify the appropriate LABC that the work has taken place, and that it has been certified by the person who carried it out as being in compliance with the Building Regulations. You may be familiar with this method of operation if you have ever had windows replaced by a FENSA member.

If the work is carried out by someone who is not registered with one of the schemes, be they an electrician or another type of tradesman (e.g. kitchen fitter) who has chosen not to register, or a DIYer, then it must be notified to LABC in advance in the same way that any building work which requires their involvement is notified in advance. And just like when you are building an extension, or converting a loft, etc, there is a fee payable to LABC to cover their activities related to checking compliance with the Building Regulations.

The full text of the legislation can be found here: and it is worth making the effort to read it, and understand it, because there is a lot of misinformation on the Internet, some of it put out by organisations with a vested interest in pretending that DIY work is illegal, some of it by LABCs who have either misunderstood the legislation, or who are also attempting to mislead the public in order to reduce the amount of work notified to them by non-self-certifying people, some of it, surprisingly, by the ODPM themselves (The Office of the Deputy Prime Minister were responsible for the legislation), and much of it on electrical advice and discussion fora like this one.

At the time of writing (July 2005), the following work was classed as not needing notification to LABC:

[code:1]1. Work consisting of -

(a) replacing any socket-outlet, control switch or ceiling

(b) replacing a damaged cable for a single circuit only;

(c) re-fixing or replacing enclosures of existing
installation components, where the circuit
protective measures are unaffected;

(d) providing mechanical protection to an existing
fixed installation, where the circuit protective
measures and current carrying capacity of
conductors are unaffected by the increased thermal

2. Work which -

(a) is not in a kitchen, or a special location,

(b) does not involve work on a special installation, and

(c) consists of -

(i) adding light fittings and switches to an
existing circuit;

(ii) adding socket outlets and fused spurs to an
existing ring or radial circuit; or

(iii) installing or upgrading main or supplementary
equipotential bonding.

3. Work on -

(a) telephone wiring or extra-low voltage wiring for the
purposes of communications, information technology,
signalling, control and similar purposes, where the
wiring is not in a special location;

(b) equipment associated with the wiring referred to in
sub-paragraph (a).
The terms "kitchen", "special installation" and "special location" are defined in the legislation.

Like any law, things can change, and you are urged to ensure that you make yourself familiar with the law as it stands now, not necessarily as it was when this document was written.

The starting point on the ODPM website for information related to Building Regulations is and a list of the relevant legislation can be found at .

It is in the nature of the Internet for sites to change, and it may well be that today, when you are reading this, the links above no longer work, and you will have to search for the pages you want.

Useful Information

On the ODPM website there are a number of "Approved Documents"
( ). These are not definitions of the law, nor do they tell you what you must, or must not, do. Instead they give guidance on ways in which the law can be satisfied. They do point out that you are not obliged to adopt any solution contained in them if you prefer to meet the requirements in another way, but that said they are useful as they contain common sense advice and often there is no good reason not to adopt the solutions they contain.

Approved Document P can be found here: .

Technical Requirements

The technical requirements laid down by Part P are remarkably simple. They are:

  1. Reasonable provision shall be made in the design, installation, inspection and testing of electrical installations in order to protect persons from fire or injury.
  2. Sufficient information shall be provided so that persons wishing to operate, maintain or alter an electrical installation can do so with reasonable safety.
It is worth noting however that they apply to all work, not just notifiable work, no matter who does it. So a DIYer adding a socket or a fused spur to a ring final circuit must work to the same technical standards as a registered electrician doing a complete rewire.

The biggest practical issue that arises is testing. To carry out testing of electrical circuits requires expertise and equipment, neither of which are likely to possessed by the average DIYer. So although Part P allows a DIYer to replace an entire circuit cable if it is damaged, without notification, it is arguable that he could not be sure that he had done it properly unless he carried out a series of tests on it, something he would probably be unable to do.

The IEE Wiring Regulations, aka BS 7671

Surprisingly, perhaps, Part P does not alter the status of the Wiring Regulations. They are still non-statutory, and there is still no legal requirement to adhere to them in domestic installations. This is a source of great discomfort to many electricians, and some of them will jump through hoops to try and prove that they are mandatory, but the fact remains that they are not.

HOWEVER, adherence to them is a very good way of ensuring that you meet the technical requirements of Part P, and you would need a very good reason, and a very good understanding of what you were doing, to decide to not adhere to them.
kujina said:
Ban-all-sheds thanks for your time on this thread.

kujina, you are aware that post took ban-all-sheds all of about 30 seconds to do, arent you? No disrespect, ban, it just looks like kujina thought you had just spent 3 hours replying to his/her post!! :LOL:
I'll have to reuse that text an awful lot of times to get the average time-to-create down to 30s.....
Here is the lighting design I came up with for the lighting, is it ok?

The top two JBs only need be 3-terminal ones.

If the lights and switch are going in the bathroom, are they suitable for the zones in which they will be?
Thanks for the response, your right about the top two JB's, this lighting will be for the loft.

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