Porch light wiring

[JeffK]

Hi,

Looking for opinion / advice.

I just wired in a new porch light.

Originally intended to put in a spur off the lighting circuit (off the light in the hallway). However, when floorboards removed I could not see the lighting circuit, only the wire to the light itself.

At this point I was fairly sure that it would not be what I needed as the porch light would only work when the hallway light was turned on - I tried this anyway and confirmed that this was the case.

Luckily, there was also a mains socket circuit exposed by the floorboards which were already lifted. I wanted to take a spur off the socket but could not get to the wires without making a real mess in taking (digging) out the socket assembly from the wall and probably destroying the wooden skirting-board.

I therefore elected to cut the socket circuit and use a junction box so that I could create a spur.

Sorted... everything works!


However it now occurs to me that:

1. I have no fuse in the installation... probably not an issue? (as the main fuse box will 'trip' if there is an issue)

2. I now have a porch light that is NOT on the lighting circuit - could this cause an issue in the future if someone is expecting it to be on the lighting circuit? (they should always check and not make assumptions)

Thanks!

 

[Andy™]

1: how do you know the one in the fuse box will trip? did you measure your Zs to ensure its within limits? the CSA will most likely be smaller. will this comply and be safe to carry the fault current long enough to allow the MCB  to open? what about the nice new bit of cable that is also inadequately protected against overload? its also a reg that lamp holders are to be fused within specific ratings. easiest option would have been to use a fused spur...

2: forget about that one, not really an issue. its also very common to have a light on a socket circuit, properly wired and fed from a fused spur

as usual, just because it works doesnt mean its safe

 

[Murdoch]

However it now occurs to me that:

1. I have no fuse in the installation... probably not an issue? (as the main fuse box will 'trip' if there is an issue)

2. I now have a porch light that is NOT on the lighting circuit - could this cause an issue in the future if someone is expecting it to be on the lighting circuit? (they should always check and not make assumptions)

Thanks!

1. Oh dear , you could well have a fire under your floor boards

2. Very typical of DIYers and bodge artists

Crack on and lash up - its your own home!

 

[JeffK]

Appreciate the comments (even the negative ones! lol) 

...was kinda expecting that it was not being done in line with regs etc.

must admit I don't understand all the techy bits (I am a computer engineer for a living) - "did you measure your Zs to ensure its within limits? the CSA will most likely be smaller"

bit more info... the porch light is an LED light so won't be carrying much current.

So, it has occurred to me that I could get to the junction box... should I somehow add a fuse to the installation? maybe replace it with a socket box (under the floorboards) and make the connection a fused plug?

I may even be able to add a fuse to the outside wire just before it goes into the porch light?

What I can't do, is to start ripping out walls and re-wiring the whole house just to put in a small LED porch light.

Not an ideal situation, but will do a bit more if it will make things safer.

 

[Doc Hudson]

However it now occurs to me that:

1. I have no fuse in the installation... probably not an issue? (as the main fuse box will 'trip' if there is an issue)

Thanks!

Fuses are there to protect the cable.  Every bit of cable has a maximum amount of electrical current it can carry. Electricity always generates heat whilst passing through a circuit. More current = more heat. In the event of an overload fault the weakest link, MUST be the fuse. otherwise there is risk of the cable overheating, melting and/or catching fire. (electrical faults cause a large proportion of house fires). So every single piece of cable in every circuit must be protected by the correct rating of fuse appropriate to the cable size. Also every fuse must operate with a maximum time for safe operation. The length of the circuit and the resistance of its conductors will affect this operation time. Without any details of you cable sizes and/or your resistance readings or calculations it is impossible to say how safe or dangerous your modified arrangement is. But assuming your workmanship is similar to the same level as many of the other DIY attempts that are seen by our members here, (visit our black museum forum for some photos of substandard workmanship), then I would suspect it is probably hazardous to dangerous rather than reasonable to safe and compliant with BS7671 wiring regulations. I would be very concerned about your selection of protective devices for your modified circuit.

Doc H.

bit more info... the porch light is an LED light so won't be carrying much current.

It will get a high fault current if you get a short circuit fault in the wiring or the electronics that work the LED.  I have seen some LEDs fail in quite a catastrophic manner. Normal load current may be small, but you design for fault current, Not normal working.

Doc H.

 

[Andy™]

it being an LED light that doesnt use much current is irrelevant. id also bet that the manual (you did read it, didnt you?) says something alone the lines of it must be on a circuit of 6a max of similar.

if there is a fault, then there is the potential for hundreds, sometimes thousands, of amps to be flowing through a cable. unless the cable is adequately sized and matched to the OCPD there there is a potential for the cable to get hot enough to start a fire

unless its a maintenance free junctions box or another suitble joint that requires no access, then the junction box must always be accessible...

but what the hell, its your house. just make sure you have some working smoke detectors and the number for your insurance handy....

 

[Doc Hudson]

A GU10 LED less than 12months old, electronics failed, blew the lighting circuit fuse, unable to reset until lamp removed. (The actual LED part was still functional if you tested it with a 12v battery).

So be assured LED's can take a lot of current down the supply cable if they fail. As Andy quite rightly says, with DIY work you can choose to do whatever you want, its your risk, your home, your family etc..

But any competent electrician will have a legal duty of care to ensure their work is designed not just to operate, but to Fail-Safe.

Doc H. 

 

[JeffK]

OK... think I get the message...  :facepalm:

So... time for the 'constructive' bit... I will be able to get access to the junction box without taking all of the floorboards up again so... 

what is the best way to get a fuse into the installation... a socket under the floor boards with a fused plug?

If there is another better way then please let me know

 

[Doc Hudson]

If you are using the socket circuit to supply the lamp, I would have thought a suitably positioned and labeled fused spur with a 3A fuse fitted, would probably be suitable. But hard to say precisely as I cannot see the exact layout.

Doc H. 

 

[ProDave]

Okay, thinking outside the box.

You don't want to mess up the walls or the decoration. A plug and socket under the floor is a really bad idea (nobody mentioned junctions and accessability yet?)

The only solution I can see that would be safe and not mess up the decor much, would be a switched fused connection unit, set in a plasterboard back box in the ceiling below.

 

[Doc Hudson]

(nobody mentioned junctions and accessability yet?)

See Andy's posts.

Additionally for general information JeffK, Although some may think a fuse is there to make a circuit work, it is actually not needed during normal operation. In fact in an ideal world where everything worked correctly no circuit would need any fuses at all. Similar to 99.9% of car journeys never need an airbag or seat belt. and some people may go the whole of their driving life and never ever need a seatbelt or air bag to operate as it is designed to operate. But in the event of an accident (fault) you are less likely to come out seriously or fatally injured if they are there and functioning. Fuses serve a similar purpose in electrical circuits. 99.9% of the time never needed, but when they are needed they must be correctly selected and installed to provide the protection. We have of course made no reference to RCD's and electric shock protection yet?

Doc H.

 

[Rob_the_rich]

.....So... time for the 'constructive' bit...

Not quite, please post some pictures. your junction box, your porch light, your manufacturer's instructions, your cable route, your fuseboard (including main head) and anything else you or we can think of.

They paint a thousand words apparently.

 

[JeffK]

ok thanks for the comments guys... what I am hearing is that I have to get a fuse into the mix.

My biggest concern is safety (along with not wanting to mess up the decor)  .

A junction box may not be suitable as it would be located beneath the floorboards and therefore not accessible.

A socket box under the floorboards is a bad idea.

So, a "switched fused connection unit, set in a plasterboard back box" is the way to go.

(I did a quick google and they look like the things that are installed with an electrical cooker)

Thanks for the advice... looks like I have another job this weekend.

 

[JeffK]

Rob... just saw your post... can't get all the info right now as everything is put back.

Will be able to access everything at the weekend (gotta move furniture to get access to lift the carpets so I get at the floorboards and junction box which I put in).

 

[Doc Hudson]

Spur example: http://www.screwfix.com/p/mk-13a-dp-switched-fused-connection-unit-with-flex-outlet-white/13479

Back box example: http://www.screwfix.com/p/appleby-1-gang-35mm-dry-lining-box/44827

Don't forget, put a 3A (or 5A) fuse into the spur and take out the 13A it will come with.

3A Fuses:  http://www.screwfix.com/p/3a-fuse-pack-of-10/94488?_requestid=521485

Other brands are available, but some of the cheaper stuff is absolute rubbish!

Doc H.

 

[phil d]

As someone else said, just because it works, doesn't mean it's either correct, or safe, to educate you a little better and give you an idea of how things work think about this scenario and then you'll have a better idea of why we go on so much about the importance of testing.

Your socket circuit will probably be connected to a 32 amp type B MCB, this has a maximum earth fault loop impedance of 1.1 ohms. Now along with measuring the earth loop impedance we get various other measurements to ensure the circuit is safe, too high an earth loop and not enough current will flow to cause the breaker to trip.

Imagine if the loop value was so high that under fault conditions only 20 amps would flow through the fault, this obviously would be nowhere near enough to trip a 32 amp breaker, however it would be more than enough to cause the insulation to melt and cause a very nasty fire!

you say that you have fitted a joint box into the socket circuit to feed you light, I hope it is in 2.5 twin and earth, not 1 or 1.5 that is normally used for lighting circuits (a lot of people do this) you should come from the joint box in the same size cable as the circuit you are connecting to, in this case I would suspect 2.5 mm, take that into the fused spur, then and only then, after it has been fused down to either 5A or 3A may you reduce the cable size to either 1mm or 1.5 mm. Another point to remember is that a lightswitch is generally rated a maximum of 10 amps, so even if you had wired everything in 2.5 cable you still have an issue with not only the fitting, but also the switch!

Please take heed of the advice being offered by people on here, in a "worst case scenario" situation, not only could you find your house insurance not paying out, but it could result in the death of a loved one.

As someone who has worked in the fire service please remember "fire kills in minutes, smoke kills in seconds", Trying to save a few quid now could cost a lot more later.

 

[Rob.]

You should come from the joint box in the same size cable as the circuit you are connecting to, in this case I would suspect 2.5 mm, take that into the fused spur, then and only then, after it has been fused down to either 5A or 3A may you reduce the cable size to either 1mm or 1.5 mm.

For the purpose of what the OP is doing this is ok advice. 

Technically it's not correct. You can spur of an 'X' sized cable supply with a smaller CSA conductor. There are requirements it must meet.

 

[Doc Hudson]

Your socket circuit will probably be connected to a 32 amp type B MCB, this has a maximum earth fault loop impedance of 1.1 ohms.

The 'maximum' for a 32A type B, is actually 1.37ohms. I think you are referring to the rule-of-thumb 80% value?

Doc H.

 

[phil d]

The 'maximum' for a 32A type B, is actually 1.37ohms. I think you are referring to the rule-of-thumb 80% value?

Doc H.

correct, the values in the OSG now are the 80% values,lol

For the purpose of what the OP is doing this is ok advice. 

Technically it's not correct. You can spur of an 'X' sized cable supply with a smaller CSA conductor. There are requirements it must meet.

Which is why I went down the route I did, I didn't want to throw more complications into it for a diy'er.lol

 

[Andy™]

isnt it great having all these different books, 7671, OSG etc, all about the same thing, yet they dont all contain the same values