# Lighting Circuit off of a 32amp Ring Main Circuit?



## Galileo

I am a Marine Electrical Engineering Contractor, and therefore not very conversant with domestic wiring regulations, i.e., 17th Edition Regs. Consequently, I'm not 'qualified' to install the wiring in a recent garage conversion, but certainly more competent than the electrician (note lower case 'e') that installed the new wiring.

The existing spur was replaced with a ring main, supplied from a 32amp C/B. Consequently, the lights go out, when the C/B trips. Can anyone tell me if it is legal to power the lighting circuit, off of there ring main circuit. 

The wiring was completed while I was away at sea, and is hidden behind the plasterboard, but there is definitely only one cable running from the Consumer Unit, to the garage conversion. I cannot afford the £400 to purchase a copy of the 17th Edition Regs; could someone please advise me?

Cheers!


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## Damien

Galileo said:


> I am a Marine Electrical Engineering Contractor, and therefore not very conversant with domestic wiring regulations, i.e., 17th Edition Regs. Consequently, I'm not 'qualified' to install the wiring in a recent garage conversion, but certainly more competent than the electrician (note lower case 'e') that installed the new wiring.
> 
> The existing spur was replaced with a ring main, supplied from a 32amp C/B. Consequently, the lights go out, when the C/B trips. Can anyone tell me if it is legal to power the lighting circuit, off of there ring main circuit.
> 
> The wiring was completed while I was away at sea, and is hidden behind the plasterboard, but there is definitely only one cable running from the Consumer Unit, to the garage conversion. I cannot afford the £400 to purchase a copy of the 17th Edition Regs; could someone please advise me?
> 
> Cheers!


The lights might be supplied by a spur in the garage but if the garage is on a ring why is there only one cable at the mains


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## Damien

Is there a spur in the garage that may be supplying the lights


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## Docara

Hi Galileo,

Little concerned both with your question and statement. Even though you state or are working in a marine environment as a sparks the use of terminology and question is strange. So there is no confusion in my response:-

Back to basics :- 

Is/was the original socket in the garage at the end of a radial ie one end of the cable was at the distribution board and the other ending in the socket. There would have been one cable connected to the faceplate. If correct it would generally be fed from a 20A MCB. - basically one breaker one circuit one socket (but sometimes there might be 2or3) but not all the ground floor (say).

or 

Was it a true spur i.e. from a socet on an existing ring a third cable was taken to supply your garage socket in question. If this was the case then when you switch a breaker (probably 32A) then the whole (say) down stairs sockets would also switch off.

Now presuming there is no easy access to the DB, the nicest and most professional way to add a socket from an existing ring would be to put a NON-SWITCHED FUSED SPUR on it and take a feed from the load side, via a 13A fuse in the spur, to the garage socket. 

Obviously, confusion/uncertaincy can creep in if you take the faceplate of and see two cables feeding it - is it a ring or is it a multi socket radial. 
For professional electricians knowing the risks the quickest (but not foolproof) way to find out is to pull the two lives out of the faceplate and see of BOTH are live - if yes then it is a ring. Faults on the ring not withstanding (broken ring) if one is live then it is a radial.
I would also,as a professional knowing the risks, pull of the DB cover off and look to see how many cables come from the breaker.

So, in your installation, god only knows what he has done and I can't advise because I'm not there but you should be able 'to get your head round it' 

If it is a radial just find the last socket and pull another cable back to the DB, via additional sockets and Fused Spurs as necessary. 

If it is a spur from a socket you've had it - You need to install a Fused Spur (13A fuse) before the (existing) first socket then feed additional sockets from the existing socket MAINTAINING the RADIAL layout. 

The theory behind it is that 2.5mm Twin+Earth is capable of carrying approx 27A. so in a ring the maximum theoretical current carrying capacity would be 54A so we install 32A protection at the DB so thatthe ciruit can never be overloaded. BUT if a true spur were installed (remember only one double socket spur per ring socket) on ONE cable the maximum load would 
be 2x13A = 26A absolute maximum. Now everytime we add a point we have to assume 13A is going to be added to the leg, so how do we protect the cabling from overload (and fire) we fuse down the assosiated leg at the origin of the spur then it doesn't matter haw many point gets installed because the 13A fuse will flow before the cable rating is overloaded.

Answering your lighting question:-

Now remembering lighting circuits are 6A normally at the DB, as long as you fuse down the new lighting circuit (install a 5A fuse in a fused spur) being fed from the ring (or radial after a FS) you have complied with the regs. If a fault on the lighting circuits occurs it wouldn't trip a 32A MCB but would melt its 1 or 1.5mm feeding cable potentially causing a fire hence the 5A fuse.

The regs book only costs £70 quid or so the info you would be looking for is buried very deep. 

Lastly, you are forgetting Building Regs Part 'P' unless you inform building control (£150+) or are a member of one of the 5 competant persons schemes your are breaking the law installing new circuits. You also should have had test sheet/results from your previuos 'electrician' 

Regards
Matt


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## Mattman

Docara knows his stuff! 

Reg books are expensive but not £400 worth, thou mine has probably saved me thousands cos I refer to it if any doubt and to show customer that I'm right. If you don't wanna pay the £70 for the full version u can get on site guide for £24 in Waterstones. 

If you only have one cable at the dis board for the garage circuit then it's a radial and should be a 20amp c/b

If you get the 17th ed on site guide (ISBN barcode 978-1-84919-287-3) and refer to Appendix H, you'll find useful info for standard circuit arrangement which you may find helpful

Hope this helps a little more


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## chris_cranch

*I have found a similar issue*

I am currently training to be a sparks mate . I noticed the conservatory lights remained on , when I had switched the downstairs lights off at the cu . so I tried the downstairs ring ' breaker switch' and the lights went off . I think I is safe to assume there is / could a spur . I am I correct ? I have notified the housing ass and they will put in a work order so I shall find out in due coarse ... However I am curious as to the ' legality' of 3 wall mounted 40+ w fittings running off a ' ring main ' 

Any serious comments would be helpful . I do have reference material :- 17th edition , osg and several more ... So even telling me where to look would be helpful


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