Spur from a spur? Theoretical reason for it being allowed...

[DNS1]

Hi chaps...

Been asked to add a light in an under-stairs cupboard.

A spur from the downstairs socket circuit runs through that cupboard to a double 13A outlet in the kitchen. I'd like to tap into that for the light, but under the regs I'd have to fuse it down first.

The thing is, the 2.5mm T+E used is rated for 27A (in this case, method C). Worse case scenario is that both those sockets are used at 13A, so that's 26A being drawn.

The light which is going to be fitted is a very low energy LED fitting, so uses next to nothing. If the light is on and the sockets are used, the maximum that could be drawn through the spur is only ever so slightly over 26A, so the cable is still not overloaded.

Would anyone NOT fuse the spur in this instance (as it's clearly not a risk), or should it be done anyway as per the regs (and have the inconvenience of the blown 13A fuse if the toaster and kettle are used together)

 

[SYKRAPS]

Fuse it down first mate as per regs, that is presuming that the socket circuit in question is protected by a 32a mcb. If it is protected by a 16a or a 20a then tap off the cable IMO.

 

[Octopus]

Insert FCU in cupboard before socket then you can add as many accessories as you want!

i.e. incupboard FCU fused to 13A, cable 2nd FCU fused to 3A for light, cable to 2G socket in kitchen

 

[Richard Burns]

A 13A fuse will take 20 amps for ever (if actually using a kettle and toaster max 3 mins), 25A for 10 sec, so put in a 13A FCU to sockets (or insert a 1 way board with 20A breaker) and have 3A FCU to light before the 13A FCU.

 

[Darkwood]

You are talking about 2 methods here, one that contravene's the regs and one in compliance and with all due respect both methods would cost the same to do, if as titled its theory you are after then consider you use the socket for many yrs as stipulated, no issue! then you move house and next owner has very different uses for the extended spur......

With this thinking i could easily wire my ring main in my house in 1.5 with a 20amp front end as i never plug any heavy loads in (kitchen ring not included) but the regs exist to cover worse case senerio's not personal habits.

 

[SPARTYKUS]

darkwood I like Murdochs idea, just FCU it prior.ANd sleep easy?

 

[GLENNSPARK]

god...its a light in a cupboard for christs sake....and were all talkin as if its goin to take out the cable...lol....
fusing the kitchen socket down is one thing but....
depends what they use it for...(toaster kettle etc)
don`t forget that a PVC 70* flat twin 2.5/1.5 will carry upto 27.5 A...

 

[Octopus]

god...its a light in a cupboard for christs sake....and were all talkin as if its goin to take out the cable...lol....
fusing the kitchen socket down is one thing but....
depends what they use it for...(toaster kettle etc)
don`t forget that a PVC 70* flat twin 2.5/1.5 will carry upto 27.5 A...

Whilst I completely agree with what you are saying the OP has the "space" to do it by the BGB so I'd say anything less would be worthy of a diy'er

 

[brman]

Hi chaps...

Been asked to add a light in an under-stairs cupboard.

A spur from the downstairs socket circuit runs through that cupboard to a double 13A outlet in the kitchen. I'd like to tap into that for the light, but under the regs I'd have to fuse it down first.

The thing is, the 2.5mm T+E used is rated for 27A (in this case, method C). Worse case scenario is that both those sockets are used at 13A, so that's 26A being drawn.

The light which is going to be fitted is a very low energy LED fitting, so uses next to nothing. If the light is on and the sockets are used, the maximum that could be drawn through the spur is only ever so slightly over 26A, so the cable is still not overloaded.

Would anyone NOT fuse the spur in this instance (as it's clearly not a risk), or should it be done anyway as per the regs (and have the inconvenience of the blown 13A fuse if the toaster and kettle are used together)

Can you clarify this before I go off in the wrong direction....

Do you mean:
Ring ->spur -> double socket -> 3A FCU -> light
or the above without an FCU?

Assuming with the FCU then I don't see a problem. The regs say only 1 device on a spur as a "standard circuit" to save you doing the design calcs. If you can do the calcs and show that the cable and accessories are protected properly and the design load is less than the fusing then I can't see what is the problem is?
imho of course

 

[Richard Burns]

Can you clarify this before I go off in the wrong direction....

Do you mean:
Ring ->spur -> double socket -> 3A FCU -> light
or the above without an FCU?

Assuming with the FCU then I don't see a problem. The regs say only 1 device on a spur as a "standard circuit" to save you doing the design calcs. If you can do the calcs and show that the cable and accessories are protected properly and the design load is less than the fusing then I can't see what is the problem is?
imho of course

If you were to do Ring ->spur -> double socket -> 1A FCU -> light
then you would be OK, double socket 26A max and light spur 1A max total 27A sorted.

 

[brman]

If you were to do Ring ->spur -> double socket -> 1A FCU -> light
then you would be OK, double socket 26A max and light spur 1A max total 27A sorted.

agreed it would be better but I don't own any 1A 1362 fuses

it is an arguable point but, depending on the light fitting used you could argue that it will never take more than an amp in normal use. Even if the low energy bulb is replaced with a 100W incandescent.
So there is no chance of overloading the 2.5mm2 cable supply the spur (in normal use). So you are then only worried about fault conditions where the 3A fuse is protecting the downstream cabling/accessories.

Given the choice I would fuse the spur but if that has a real chance of causing nuisance fuse blowing then I'd do it this way.

 

[GLENNSPARK]

agreed it would be better but I don't own any 1A 1362 fuses

it is an arguable point but, depending on the light fitting used you could argue that it will never take more than an amp in normal use. Even if the low energy bulb is replaced with a 100W incandescent.
So there is no chance of overloading the 2.5mm2 cable supply the spur (in normal use). So you are then only worried about fault conditions where the 3A fuse is protecting the downstream cabling/accessories.

Given the choice I would fuse the spur but if that has a real chance of causing nuisance fuse blowing then I'd do it this way.

never take more than an amp...even if an incandescent lamp is used......
so lets see thats 100W....tell you what no....lets go for worst case here....so that`l be 150W/230V=.....WOW..0.65 of an amp.....gonna overload the cable is it?....

 

[DNS1]

Cheers for the help guys,

If I decide not to fuse the spur and work on the fact that the calculations show the cable is adequate for the 26A + the light, should I just note it on the Minor works certificate and be done with it?

Realistically if I fuse down the whole spur, the chances of it blowing are pretty much nil, but it just seems a bit pointless if the cable is always going to be fine.

Being an understairs cupboard it's not like anyone will be changing the simple pendant for a fancy 5x100w bulb chandelier or anything!

 

[Octopus]

Not sure I agree with your decision.

You had the opportunity to fit a FCU, out of the way, in the cupboard so IMHO you should have done this.

 

[Taylortwocities]

^^^^^What He Said^^^^^^^^

 

[Guest55]

Cheers for the help guys,

If I decide not to fuse the spur and work on the fact that the calculations show the cable is adequate for the 26A + the light, should I just note it on the Minor works certificate and be done with it? I wouldnt be recording on a certificate work that i know damn well doesnt comply , you're just leaving yourself open for a kicking if it all goes pete tong.

Realistically if I fuse down the whole spur, the chances of it blowing are pretty much nil, but it just seems a bit pointless if the cable is always going to be fine. it doesnt matter if the cable is 95mm - standard circuit arrangements limit 1 unfused connection point for each spur.

Being an understairs cupboard it's not like anyone will be changing the simple pendant for a fancy 5x100w bulb chandelier or anything! very true , but we all know you're trying to justify a design decision that just isnt right.

what are the chances of this kitchen socket using more than 13a after being fuse down ?
what is being plugged into this socket now ?
if very little then just fit the extra fcu and stop dithering ;-)

 

[Shropshire]

I always got told that a spur of a spur is ok if it doesnt exceed 27 amps

 

[Guest55]

I always got told that a spur of a spur is ok if it doesnt exceed 27 amps

Im afraid you were mis-imformed.

 

[Rampage]

Have I read this wrong or are you saying that the total for a double socket-outlet is 26A and not 13A?

- - - Updated - - -

Have I read this wrong or are you saying that the total for a double socket-outlet is 26A and not 13A?

 

[Geordielad]

I just can't understand why a FCU is being tried to be left out. Surely as per the regs (although I'm qualified spark - I am still learning and happy to receive advice) a FCU is required??!
But on a side note surely you would want to switch the light off?? The method you plan to use would mean your light would be on 24/7!!
I would much prefer to fit a FCU as you'd get protection, discrimination and also a means to switch on and off.

 

[Taylortwocities]

I always got told that a spur of a spur is ok if it doesnt exceed 27 amps

You were told wrong then.

 

[DNS1]

The method you plan to use would mean your light would be on 24/7!!

How on earth do you work that out?!

Why on earth would I fit an unswitched light fitting?!

I was just curious about the regs regarding spurs as I was planning on adding a tiny load to a spur, which could only ever draw 26A or less and which was capable of taking 27.5A.

It seems that on this occasion the regulations are too restrictive... The 2.5mm T+E cable is capable of carrying the worst case scenario load of the double sockets, plus the maximum that could be draw by my addition of a light fitting. I just don't get why the regs insist on fusing in this case, when it would restrict the cable to less than half its capacity.

I know the regs are there to stop idiots chaining multiple double sockets from a single spur, I just don't see why there isn't something which allow the sparks to do the calculations and make the decision for themselves!

 

[DNS1]

Thinking about it, ANY spur on a normal (I mean, 30-32 Amp protected) ring final isn't really very safe according to the regulations...

A single run of 2.5mm T+E can take 27.5 amps maximum... Seeing as most RFCs are protected by a 32A mcb or 30A fuse, it means in the event of a wiring fault the spur could be overloaded with no trip!

A 2.5mm radial on a 32A mcb wouldn't comply with the regs, so why does a 2.5mm spur from a ring final? Is this the only time when an mcb does NOT have to protect the cable in a worst case scenario? If so, why?!

 

[spark 68]

Forward fusing DNS,

Each plug has only a max capacity of 13A (the fuse in the plug)

 

[somersetsparks]

Thinking about it, ANY spur on a normal (I mean, 30-32 Amp protected) ring final isn't really very safe according to the regulations...

A single run of 2.5mm T+E can take 27.5 amps maximum... Seeing as most RFCs are protected by a 32A mcb or 30A fuse, it means in the event of a wiring fault the spur could be overloaded with no trip!

A 2.5mm radial on a 32A mcb wouldn't comply with the regs, so why does a 2.5mm spur from a ring final? Is this the only time when an mcb does NOT have to protect the cable in a worst case scenario? If so, why?!

Hence why only one spur is allowed, therefore meaning even a double socket with 2 kettles in could pull 26A! The socket would give uip first!

 

[somersetsparks]

On a few domestic test I have come accross a few spur on spurs for 2 tiny load i.e. boiler and condense pumps and turned a blind eye. However I wouldn't do it myself

 

[DNS1]

Forward fusing DNS,

Each plug has only a max capacity of 13A (the fuse in the plug)

Isn't the MCB expected to trip before the cable is overloaded in the event of a wiring fault before the fuse?

 

[spark 68]

The difference between a spur and a radial on 2.5mm is, a spur = 1 double socket (max 26A), a radial many double sockets @ potentially 26A each.

The fuse in the plug/s protects the cable for the spur, which as SS said above, is why you are only allowed the one.

 

[somersetsparks]

With regard to spurs,
I seem to remeber someone on here putting it that -
Your Short circuit protection is provided by the 32A MCB
Your overcurrent protection is protected by the bs1362 fuse.

So is wiring a 2.5mm radial to 1 Socket protected by a 32A MCB O.K. as well :cool3:

 

[spark 68]

That was it SS, I cannot think properly this time of night lol