Hi, quick “theoretical” question here (don’t you just love these!?):

Say for an extension or garage conversion done a long time ago, someone simply spurred off the house ring main to put in a couple of sockets in the new room, and for reasons best known to himself added a new lighting circuit (with correctly-sized cable) and powered this by simply connecting the cable messily and dangerously to the newly-spurred ring main which fed the new sockets instead of properly extending the existing lighting circuit.

To make it safer (although technically not by the book I know), would it sound reasonable to install a fused connection unit (FCU) on the ring main and run the lighting from that, as well as changing the sockets from being a spur to being part of the existing house ring main?

If so, would you wire the FCU like a socket, in that it has 2.5mm T&E cables coming to AND from it (plus the solitary lighting cable)? So in essence it would look a little like a spur, and this “spur” would be the start of this mini lighting circuit (say 2 or 3 lights) just like the consumer unit would be for a normal lighting circuit.

Also, just to make certain, what size fuse for the FCU would be required in this hypothetical lighting circuit?

Would it make the scenario any better at all if there was an option to put this ring main and lighting circuit (via the FCU as mentioned above) for the extension / garage conversion on a completely separate MCB circuit instead of connecting it all to the existing ring main? I figure this way at least it’s all separate / isolated and has less chance of overloading the existing ring main.

I would appreciate a sympathetic answer to this theoretical scenario please! I know I’ve asked a lot of questions, and I know it’s tempting to just tell me it would be better to do it all again and all properly (which I would do if this theoretical scenario was my house), but sometimes people just want things sorted ASAP and without walls and ceilings coming down.

Many thanks in advance.

Nothing desperately unusual about that

And what do you mean by "correctly-sized" cable? 1.5mm²? 1.0mm²? 2.5mm²?

So in what way was the connection messy and dangerous? You don't tell us.

If you're going to all that work, why not just connect the lights in the extension into the main lighting circuit?

But then you'd be adding yet another FCU and ring circuits aren't really intended to have many FCUs on them.

What size of cable has your lighting circuit been run in? Remember that the fuse protecting the lighting cable must be lower than the current-carrying capacity of the lightest cable used downstream of it.

Your existing FCU is presumably fused at 13 amps. If your lighting wiring is 1.5mm it can comfortably carry that so it could be connected into wiring to the sockets in the extension. Not exactly good practice, though, and in any case, if the existing sockets in the extension were used for heating, the circuit is liable to be overloaded.

If you're able to bring the ring main into the extension, then you could keep your costs to a minimum and used a FCU to take the supply for the lights in the extension from that and use a more conventional 5amp fuse for the lighting in the extension -- and that would be appropriate if it's been wired in 1.00mm².

If access to the lighting circuits in the main building is relatively feasible, then best practice would be to connect into that.

Would it make the scenario any better at all if there was an option to put this ring main and lighting circuit (via the FCU as mentioned above) for the extension / garage conversion on a completely separate MCB circuit instead of connecting it all to the existing ring main? I figure this way at least it’s all separate / isolated and has less chance of overloading the existing ring main.

I would appreciate a sympathetic answer to this theoretical scenario please! I know I’ve asked a lot of questions, and I know it’s tempting to just tell me it would be better to do it all again and all properly (which I would do if this theoretical scenario was my house), but sometimes people just want things sorted ASAP and without walls and ceilings coming down.

Many thanks in advance.[/quote]

It would meet the regs if you installed a 13A FCU somewhere between the ring main and the first socket and then used a 5A FCU to power the lights.

Most has been answered A1. As to fuse size for lighting there are two limits. The regulations limit it to 16A see 559.6.1.6 but also many ceiling roses are only rated at 5/6A so 559.4.1 says must be to manufacturer's instructions. So in most cases the maximum size of fuse to supply lights is 5A.

The problem with any FCU is you can stick in any fuse up to 13A. Many lamps when they blow can draw a huge current due to ionisation as they blow. They should have an internal fuse as a BA22d lamp holder is rated at about just 2A. However often the internal fuse does not exist and when the lamp blows it can weld the contacts to the bulb holder so you have to change whole holder rather than just the bulb when it blows. Using a switched FCU as a light switch is a common way to fuse down but you must be sure it only has a maximum of 5A fuse put in it.

It is not ideal but often we are confronted with a bodge. And it's not unknown for electricians to swap the standard 32A MCB for a 20A MCB to make a bodge safe. However one has to be able to test and even to hire the test set is around £75. To buy around £750.

So you need to do a risk assessment. Is your family's safety worth it. I knew a farmer who always used his hat to turn on the lights as switch was live. He never forgot so for him quite safe. But not for me. I did have a few Anglo Saxon words to say! And this is the point may be safe for you but what about your grand children? That's if your children live to produce them of course! My father-in-law still does not have an earth leakage trip yet it was him who pressured me into fitting one in 1991. But he has no children in his house.

So think about it. Is it worth the risk?

Thanks for all the replies!

Let me clear a few points up:
1) when I meant correctly sized cable, I meant 2.5mm for the ring main and either 1mm or 1.5mm for the lighting.
2) I wasn't there when the connections were taken apart, but a plastic bag buried in plaster was mentioned and a fused connection unit wasn't mentioned i so i guess it was just spurred without a fuse onto the ring.
3) I don't have the option of extending the house lighting circuit to the extension unfortunately.
4) so currently there are no FCU on the circuit.

"It would meet the regs if you installed a 13A FCU somewhere between the ring main and the first socket and then used a 5A FCU to power the lights."
I must admit, I don't get this. Why install two FCUs, one at 13a and one at 5a?

"So you need to do a risk assessment. Is your family's safety worth it."
Not my family. And surely anything is better than what is currently happening (the lights are temporarily wired into a fused plug!).

5a for a lighting circuit. Many thanks.

Further help would be greatly appreciated.

As to meeting BS7671:2008 requirements if below 16A it will comply, unless the manufacturer states otherwise. So likely it will need to be 5A.

It is normal in for example a shed or garage to supply the lights from a switched FCU with a 5A fuse so removing the need to install a consumer unit in the shed or garage. So rather than having a FCU and a light switch one combines the two together.

It is however hard to recommend a route without being on site. Very easy to miss some thing. So I am wary at saying this is best method in case there is something missed.

I think this post is running away from me! All the help has confused me.

Did we get a consensus that it's not the worst thing in the world to have a small lighting circuit wired in to the ring main, and the best way to achieve this would be via a FCU with a 5amp fuse?

Thanks.

I would say that there is nothing wrong with having a 5A FCU supplying the lights. It is not ideal but it it is legal.

As for my "It would meet the regs if you installed a 13A FCU somewhere between the ring main and the first socket and then used a 5A FCU to power the lights." comment and your question "I must admit, I don't get this. Why install two FCUs, one at 13a and one at 5a?

The answer is because a unfused spur from a ring circuit should only power one single or one double socket, however a 13A fused spur is allowed to power several sockets ie multiple spurs and several double sockets.

Sorry for the delay in replying but I have been very busy at work

Thanks for the reply, i do appreciate your help.

I must not be getting this though (or we're talking at cross purposes). Bear with me once more please...


I think i'm on the cusp of understanding what you mean about the FCU's. Are you working on the assumption that the power to the new room will be spur from the old ring main, hence the need for a 13A FCU at this point and then a 5a FCU for the lights off this?

If so, I'm working to another scenario: either just extending the ring main from the rest of the house into the new room, or possibly creating a separate ring main for just this room. This would surely negate the need for a 13a FCU anywhere? If i need one of the above, the question would then be purely: how to correctly take power off an existing ring main for a light circuit.

I have probably mis-led you with my rambling text.

If this is all correct(!), is it just a 5a FCU somewhere in the ring main? And is it wired in with 2 2.5mm cables and the one 1.5mm cable?

If I'm still wrong, just tell me to go jump off the pier.

Thanks.

Yes


Well if you are extending the ring then you will only need the 5A fused spur for the lighting

Woo hoo.

Thanks for the confirmation, and the patience!

Just to extend everyone's favourite post with another stupid question...



To do what I want to do (spur from a ring main for a couple of lights), would I need a 13amp FCU and put in a 5amp fuse? Thought I'd ask before I go to screwfix and find you can't buy a 5amp FCU and so plump for a 45amp one or something stupid.

I assume 13amp because of the ring main, and the 5amp for the lights.

Oh, and is there any advantage in making the FCU part of the ring main or spurring it from a socket? I would have plumped with making it part of the ring main myself...


That's my last question on this thread . Many thanks in advance.

Ring main via spur to fcu (which is rated maximum 13A) with a 5 Amp fuse is correct...
Sure that's your last question??

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