Lighting off 30 amp cabling

Providing the addition/alteration has been protected by a RCD, this should comply to BS7671.
The issue here is the outside light and the RCD/FCU should be down fused to a suitable rating.
You can't completely condemn a whole installation just because it does not comply to the current requirements, for the sake of an alteration/addition of this kind.
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Thanks for that. I assume then that with that protected FSU I replace the internal fuse with an 5 amp one. Install 2.5 mm cable to it and 1mm cable to switch and light.
You can't completely condemn a whole installation just because it does not comply to the current requirements, for the sake of an alteration/addition of this kind.

I am not condemning the existing installation. I am saying be careful because you don't know what lurks in the existing installation. What if there is no effective earth and you place a class 1 metal light fitting on an outside wall with mains voltage sitting on it.

In practice, this is where a PIR comes in. I would be happy to fit the light but only after inspecting the existing installation to a reasonable degree. i would not want to rewire the house for the sake of a new light but I would want to ensure the earthing system is intact and functional. We do not know that.
I think as trading electricians that is what we would all do.
I hope the OP has the good sense to realise when an earth is not in existence, which would be extremely rare on a socket circuit, although not unknown.
You have every right to offer words of caution and best practice would be to test the circuit out prior to and after the alteration.
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This is where it gets tricky. A new outside light circuit is notifiable.
That means an EIC and building control notification. For a professional, registered person that means doing quite a bit of testing and most likely uncovering a non-compliance that needs to be put right before the new circuit can be signed off. Chances are there is no main protective bonding in this property. All of a sudden a simple light circuit becomes a big job, customer thinks its all overkill etc, etc.

As said, I don't do a lot of domestic work but my son does. I would not allow him (under the name of our company) to do the work as proposed. Simply because that EIC and the subsequent building control compliance doc may suggest that things are better than they actually are. I would insist on having at least the entire circuit RCD protected and any remedial work done prior to signing off.
As I understand it, if the light it fitted to the fabric of the building and cable goes directly in to the fitting. It is not deemed notifiable and we are not talking about a new circuit, as all new circuits are notifiable.
This most be fitted to the outside of the house and no exposed outdoor connection made.
The fact the circuit originates from the garage, could well give credence to your post.
There is no way that will meet wiring regulations. The requirement for an RCD is mandatory unless you can run the new circuit wiring in armoured cable or earthed metal conduit. Or keep all the new wiring at least 50mm deep in walls. Otherwise you will need an RCD in that circuit.

Let's be clear about that use of "mandatory." It is mandatory in order to comply with the current edition of BS7671, but as compliance with BS7671 itself is not compulsory, it is thus not mandatory in the legal sense, which is how many laymen would interpret the term.

More generally, you should consider looking at a consumer unit upgrade to include at least two RCDs.

Don't you think that completely redesigning the house's distribution system is a little excessive just to add one light? Maybe there are other reasons why one might want to change something else, but there seems to be a growing tendency to suggest that everything upstream of any proposed wiring change or extension needs to be brought to the current standard.

You are saying just RCD protect the new circuit from the FCU onwards. I don't like it because it is adding a new circuit to a non-compliant system but I suppose that might obey the spirit of the 17th edition

It's not adding a new circuit, it's extending an existing one, and even when complying with BS7671 for the new addition there's no requirement to add RCD protection to the existing parts of the circuit. The RCD-protected FCU for the new wiring would be fully compliant with BS7671 (subject to no other problems with the existing circuit, such as missing earths etc. already mentioned).

But and its a big but, you will be extending the circuit of a non-compliant installation. That is, your existing electrical installation does not meet latest wiring regulations.

There is no requirement for any existing installation to be brought up (or down) to current standards.

PS, How non-compliant is not known but the fact that you have wired fuses is a clue that it may have problems lurking due to age and may have been poorly DIYed over the years.

True, but equally it doesn't mean that there definitely are problems, just as a shiny new board and installation installed when the 17th edition came out doesn't guarantee that somebody hasn't been along and messed up something in the last three years. Without checking, it's impossible to say.

Chances are there is no main protective bonding in this property.

What makes you say that?
Without checking, it's impossible to say.

Exactly my point in encouraging the customer to think beyond just the extended circuit. Is the existing installation in a safe condition? Without checking, it's impossible to say.

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