spur from a spur

[sparkwright]

Often we come acoss a socket on a ring (A) with a spur running from it to another socket (B). From time to time someone who is not in the know will add a further socket (C) from B. A spur from a spur.

To rectify it some rewiring is required, or in some instances a 13 amp fused spur unit is inserted between A and B.

However, a 13 amp fused spur unit is not a good idea when heavy appliances are to be plugged into B and C.

We cannot fit a 20 amp MCB in an enclosure between A and B.

But why can't we? 2.5 mm2 cable is rated up to around 27 amp under the right conditions, and a 20 amp MCB between A and B would certainly be an improvement.

Any thoughts?

 

[PrenticeBoyofDerry]

Or even a case for a FCU that can carry a 20A fuse.

 

[ban-all-sheds]

We cannot fit a 20 amp MCB in an enclosure between A and B.

But why can't we?

Because it's not a BS 1363 accessory and therefore may not be supplied by a ring final which uses the provisions in 433.1.5 to be deemed to be in compliance with 433.1.1.

(Note - I'm assuming that you want to comply with the Wiring Regulation - I guess if you're happy to contravene them you can do all sorts of things.)

 

[sparkwright]

There's a lot of it about though.

For example, I've seen 2.5 cable spurred from a ring circuit at the cu to a garage where it feeds a small garage board with 16 and 6 amp MCBs.

It's not perfect, but doesn't seem too bad in some ways.

 

[ban-all-sheds]

Doesn't seem too bad?

With 16 & 6A breakers, where's the overload protection for that 2.5mm² cable?

 

[sparkwright]

Doesn't seem too bad?

With 16 & 6A breakers, where's the overload protection for that 2.5mm² cable?

The same place a double socket spurred off a ring that could be supplying two 13 amp loads.

 

[ban-all-sheds]

Yes - I appear not to be able to add up any more.

 

[roberta1000]

Doesn't seem too bad?

With 16 & 6A breakers, where's the overload protection for that 2.5mm² cable?

The same place a double socket spurred off a ring that could be supplying two 13 amp loads.

And the same place as the overload protection for the double socket (many/most of which aren't rated for 26A). Nothing's perfect.

 

[JohnW2]

But why can't we? 2.5 mm2 cable is rated up to around 27 amp under the right conditions, and a 20 amp MCB between A and B would certainly be an improvement. Any thoughts?

In relation to 'good design practice', I think the main problem is that one should not really be considering a 'point loading' greater than 13A on a 32A ring final. Sure, we are allowed a double socket on an unfused spur, but I imagine those who did that 'allowing' were thinking either of the fact that most double sockets are only rated' for 13A total load or, more practically, that it would be very unusual (given good circuit design) for a sustained total load of >13A to be connected to one socket (which is why one should never install a double socket on an unfused spur in a location where it might be used for a washing machine + dryer, the only common situation in which such very high loading might be a possibility).

If you really need 20A for the 'spur', then it really should be a separate circuit - after all 20A represents over 60% of the total capacity of a 32A ring final, and to contemplate concentrating all that load at one point on the ring really flies in the face of the design concept of a ring final.

You can obviously 'cheat', just to comply with regs, by running two separate unfused spurs from virtually (or literally) the same point on the ring - but, even if that keeps the regs happy, it's really not significantly better in terms of good design practice.

If one is prepared to totally ignore the regs, there are, of course, situations in which a designer may consider it so unlikely that the total load on an unfused spur will exceed 13A that 'design common sense' might be satisfied by multiple sockets on that unfused spur - but that's not the sort of thing we talk about!

Kind Regards, John.

 

[OwainDIYer]

If one is prepared to totally ignore the regs, there are, of course, situations in which a designer may consider it so unlikely that the total load on an unfused spur will exceed 13A that 'design common sense' might be satisfied by multiple sockets on that unfused spur - but that's not the sort of thing we talk about!

If it's unlikely that the total load on an unfused spur will exceed 13A then there's no reason not to fuse it at 13A.

 

[JohnW2]

If it's unlikely that the total load on an unfused spur will exceed 13A then there's no reason not to fuse it at 13A.

Sure - other than 'convenience' (which some would call 'laziness'). Like in the scenario which started this thread. If one already has an unfused spur, supplying one socket, and then wants to add a second socket, but knows that the total load will be low, it would be easier to just 'extend the spur' than to re-do it as a fused spur.

Kind Regards, John.

 

[Adam_151]

From time to time someone who is not in the know will add a further socket (C) from B. A spur from a spur.

Thats nothing!

Came across a situation recently that appeared as if the following order of additions seemed to have happened

1) Twin socket outlet (1) spured off 4mm ring from JB (A) in 2.5mm cable
2) 2.5mm cable to above socket cut and JB'ed (B) to feed socket(2) in 2.5mm
3) Cable between JB (B) and socket (2) cut and JB (C) added to feed socket (3)
4) Two more sockets (4) (5) added by someone who thought they were looking at a 2.5mm ring... removes section of cable between JB (B) and JB(C), goes from JB(B) to first socket (4) then to second (5) back to JB (C)

Followed all that?

Oh, and one of the joint boxes had a doorbell transformer spurred off in 1mm² flex with no spur to fuse down

So what we had was spur which ended up traversing the width of the room 3 times as it picked up five sockets and the zs on the end of it was about 1.22

This wasn't in domestic, so makes you wonder if the contractors responsible were incompetant or just didn't care

 

[ericmark]

1) The double socket does allow anyone to realise it is a double socket on the end of a cable rather than if two singles were allowed one may, if not tested, think it is part of a ring.
2) I do think there is a call for a 16A fuse connection unit and to some extent a 32A fuse connection unit as to change the MCB at the consumer unit to a lower rating because the appliance requires it becomes a problem when the appliance is changed and a larger supply is required.

In the ideal world we would get out the paperwork and see how it was reduced in size to comply with manufactures requirements and be able to return it to original size. In practice even if the paperwork exists we never see it and since we don't know why the 10mm cable has a 32A MCB are unwilling to change it for a 40A MCB in case it runs through insulation.

I see no problem with a single 10mm cable supplied from a 40A MCB feeding two FCU's with 16A and 32A fuses and from that feeding hob and oven. The only thing which stops it is availability of 16A and 32A FCU's.

Again I see little problem fitting a 16A FCU feeding a "Schuko" socket except for the shutter and reversibility problem with the "Schuko" socket.

However the ring, forgetting regulations, has a problem in close to origin too many 13A sockets can produce an overload, also at the centre too many 13A sockets can produce too big a volt drop.

The system expects and is therefore calculated to have 20A drawn from centre and the remaining 12A evenly distributed along it's lenght. So when calculating volt drop one works on centre point with 26A load.

So to draw over 13A from any one point would mean the electrician would need to work out all loadings and not use the standard laid down during the war for the house rebuilding post war. With the change of cable sizes from imperial to metric and lifting of protection from 30A to 32A I think one would now be hard pressed to make a case for the ring even as used now never mind adding more spot loads.

I have run a final ring in 4mm cable with 16A MCB's twin pole all feed from a 32A MCB and with 16A BS EN 60309-2 plugs coming from the 16A MCB. It was really because the guy who started the job was a house basher and did not think that since there is no fuse in a BS EN 60309-2 plug he could not have them connected directly to a 32A MCB and easiest way to correct was to change the sockets for a version with a double pole 16A MCB. Apart from being reduced low voltage I would not want them in my house. They look wrong.