At the risk of making myself look a bit of a.......

[Sparky83]

....Pratt!

I was having a disscusion with a mate of mine and radials (like you do)

and we disagreed on on feeding a 2.5mm t&e from the 32a mcb of a ring ciruit.

I thought that you could only spur off the ring from a point on the ring itself like a socket, but he said you could take it straight from the mcb itself. To me this then becomes a radial circuit sharing a mcb with a ring main?

What would happen if the ring was removed and the radial circuit left, on a 32amp breaker, that would be right, so what makes it ok when a ring shares the mcb?

I've had a look about and even on this forum but some say its ok some dont. Im happy to admit im wrong, which is how it would seem it is but i would like a good explenation as to why its ok.

Cheers
Shawn

 

[Unswitched]

So many sparks dont understand this! A Fuse doesn't have to be a the origion of a circuit to protect a cable. It can be downstream.

Fuse does 2 things:
1. protect against overload
2. Protect against fault current.
If I wired 1 x 13A s/o and fed it from a 32A or even a 40A MCB this would be fine.

It would be fine because:
1.the socket (plugtop fuse) is limiting the amount of current that can be drawn by this cable at a max of 13A continuosly. (the cable cannot be overloaded).

2.Now it has to be checked that the cable (2.5) can handle the fault current of the MCB. This is the adiabtic equation. If this passes (which it most likely would) and the 2.5mm cable can handle a high short fault current, then this is fine.

Now, if someone came along and added another socket from mine. It would be wrong. Wrong because the cable can now be overloaded because there a are more sockets to plug things in. This is why u cant spur of a spur.

This whole scenario is used in Buss bars and henley blocks. Add a 1-way CU to feed a cooker circuit. You'd use 6mm or 10mm tails, not 25mm tails. That would be ridiculous. But - 6mm or 10mm tails are protected by 100A FUSE!!!! - It's fine dont worry, as long as the adiabatic equation is satisfied.

6mm tails wont be overloaded as a 32A MCB is installed for cooker. THIS IS DOWNSTREAM of the cable.

That my friends is why you can spur from a 32A only to feed 1 point. (13A)

Dont use the argument that a twin s/o is 2 x 13A! - It's not really, they are only designed to handle 1 x 13A load total.

Hope this helps...

 

[Unswitched]

i personally would not spur a socket direct from the fuseboard on a 32a its just a bit diy, but, even a spur direct from a 32a breaker into a double socket the max it could draw is 26 amps (13 amps per socket) if T+E is rated at 27 amps at best, the cable is not going to be overloaded. but say for example mr diy comes along and spurs 5 more sockets from that spur then you have a problem!!! just break into the ring or put it on a 20a mcb just makes sense to me!

Yeah but you cant control what others do. It's not your responsibilty. If you thought like that all the time you would never do any work! ha.

If you wired a perfect ring and then I come along and spur off to feed 3sockets (all spurred off of a spur) thats my fault, not yours!

To breakinto a ring to feed one s/o under a fusebox is OTT and you end up with more connections in the circuit. Never a good thing!

 

[Amp David]

Nice post on the matter unswitched. Nice a clear and to the point.

 

[WayneL]

Just to add to what 'Unswitched' has said, the regulation allowing this scenario is:

433.3.1 (ii) ....overload protection is not necessary for a conductor, which, because of the characteristics of the load, is not likely to carry overload current.

Therefore, the regs allow you to run a 2.5mm, unfused spur from a 32A ring final or a 32A 4mm Radial, or basically a 32A MCB, - because it can't be overloaded as long as it feeds only one single or one double socket.
(Subject, of course, to installation reference method for cable current carrying capacity)

 

[Sparky83]

Thanks for the detailed reply Unswitched, i understand what your saying, just seems strange as i've always had it drummed into me that the mcb/fuse protects the cable so i always assumed i'd need a protective device that had a smaller rating than the max for the cable.

So any 2.5mm radial circuit i wire can be supplied by a 32a/40a mcb as long as there is only one socket or fcu on the end of it?

If i done a pir at a property i'd not be very impressed to see that, shows what i know i guess.

Thanks to all who tried to answer my question.

 

[keniff]

Yeah but you cant control what others do. It's not your responsibilty. If you thought like that all the time you would never do any work! ha.

If you wired a perfect ring and then I come along and spur off to feed 3sockets (all spurred off of a spur) thats my fault, not yours!

To breakinto a ring to feed one s/o under a fusebox is OTT and you end up with more connections in the circuit. Never a good thing!

i know you cant contribute to what others do once you have completed a job, i was just trying to explain why i would not do it.

 

[Colin Integrale]

So any 2.5mm radial circuit i wire can be supplied by a 32a/40a mcb as long as there is only one socket or fcu on the end of it?

A 2.5mm radial can be done off a 20A max because it's only got one way to go being a radial. Is it 24A max for a 2.5? Or maybe 27A.. Can't remember (forgive me if i'm wrong, got no book here)

A 2.5mm Ring can be put on a higher fuse because the electricity can choose which way it goes (being lazy) so shouldn't overload cables.

A spur off this in 2.5mm protected by a 13A fuse is protected fine.

A 32A Radial should always be done in 4mm

 

[Unswitched]

So any 2.5mm radial circuit i wire can be supplied by a 32a/40a mcb as long as there is only one socket or fcu on the end of it?

If i done a pir at a property i'd not be very impressed to see that, shows what i know i guess.

Well yes it can strictly speaking and would be wrong to code 1,2, or 4 on a pir. But, if there was a spare way in the fuseboard and you were wiring a new circuit to feed 1 x twin s/o, obviously you would put it on a 16a or 20A MCB, because, as I think you are trying to say, it is just standard practice. I was thinking more a long the lines of... crammed up fusebox with crazy wires everywhere, no spare ways and the only obvious place to spur off with out adding extra unneeded hidden junction boxes under the floor would be an existing 32A ring. New circuits, just put in a 20A.

As wayneL has mentioned, that seems to be the reg that this relates to. And generally, us sparks to think about the fuse being first in a circuit and protecting every change in cable size.

I think maybe domestically, we often do things as 'standard practice'. Only when you get involved with more complex circuit arrangements, like long runs, 3 phase and high loads do you start having to think differently and have to start making your own mind up as to what is right and what is wrong, as opposed to 'what is standard'.

Now I'm going to be a hypercrit (spelt that wrong)! - Last week a put a code 2 on a pir for this:
1 x smoke detector wired in 1.0mm T+E about 2 meters long fed from a 20A MCB. I'm lazy sometimes and I couldn't be bothered to think about it... But, is it strictly wrong? - Again, I still cant be bothered to think about it - so I just changed it. HA!

 

[Sparky83]

A 2.5mm Ring can be put on a higher fuse because the electricity can choose which way it goes (being lazy) so shouldn't overload cables.

I'm aware of this but as said in the first post, we are talking about spuring off the mcb of a ring instead of a mid point on the ring itself.

Thanks again Unswitched.

 

[Octopus]

Thanks for the P362 note - its clearly states that an unfused spur can be connected "to the origin of the circuit in the distribution board" which means that there would be 3 cables running into the protector (2 x 2.5 and 1 x 4.0). Thats how I read it!

Looks like I need a length of 4mm to add the double socket my customer wants.

 

[Jimmy Boy]

If you look at appendix 15 page 362 of BRB it shows various arrangements including using 1.5mm for sockets using a FCU ! Would I no, but it's there as table 6F page 134 of OSG shows the CCC of 1.5mm to prove it

 

[WayneL]

Thanks for the P362 note - its clearly states that an unfused spur can be connected "to the origin of the circuit in the distribution board" which means that there would be 3 cables running into the protector (2 x 2.5 and 1 x 4.0). Thats how I read it!

Looks like I need a length of 4mm to add the double socket my customer wants.

No, no no

An unfused, 2.5mm spur off a 'Ring Final' can feed one single or one double S/O, even from the origin (MCB) of the circuit.

You don't need to use 4mm!!

 

[Octopus]

Many thanks WayneL, I was ering on the side of caution!

 

[WayneL]

Many thanks WayneL, I was ering on the side of caution!

Of course, if you want to use 4mm it isn't a problem - it's your choice