Correct cable size?

[roundthebend]

Contentious as in you should not put more than one socket on a spur off a ring final?

If your design current is 1kW, this is approx one third of 13A. Why not take a spur off the ring final with a single 2.5 cable, put a 13A FCU in and run 2 sockets off that?

That is, rather than run a 4mm cable for the spur on its own, use 2.5mm to spur into a 13A FCU which then feeds 2 sockets from 2.5mm cable?
That would work too. I don't understand the pros/cons of each approach.

 

[securespark]

Have just looked in Green OSG.

You cannot have a single socket on an unfused spur, but you can have a twin socket or 1 item of permanently connected equipment.

Not that the OSG is regs...

 

[JohnW2]

Have just looked in Green OSG. .... You cannot have a single socket on an unfused spur, but you can have a twin socket or 1 item of permanently connected equipment.

What irrational nonsense - does it really say that? I must look at my (red) one!

Not that the OSG is regs...

Just as well

As you know, BS7671 itself is totally silent on the matter of spurs, other than to say that ring finals can be "with or without" spurs. Those who can think (rather than those who need to rely on guidance) therefore have to consider what BS7671 says, in general, about the relationships between load, cable and protective devices, whilst at the same time considering that the design has to be such that no part of the cable of the ring is 'likely to be overloaded for long periods'.

Kind Regards, John

 

[Iggifer]

Have just looked in Green OSG.

You cannot have a single socket on an unfused spur, but you can have a twin socket or 1 item of permanently connected equipment.

Not that the OSG is regs...

Do they expressly forbid a single socket? Or is it just not mentioned? HAS to be the latter

 

[roundthebend]

Have just looked in Green OSG.

You cannot have a single socket on an unfused spur, but you can have a twin socket or 1 item of permanently connected equipment.

Not that the OSG is regs...

That sounds crazy, but then I'm well aware that I'm quite naive about the complexity of the regs.
I'm an intelligent person and I'm always impressed with the technical knowledge (and application of it) that many electricians on these forums display.
I think I'd enjoy it as a career, but that's unlikely to happen in my position. Kudos to you all.

 

[roundthebend]

Going on other replies it seems I'm not the only one to think that sounded crazy. I'm sane after all!

 

[JohnW2]

That is, rather than run a 4mm cable for the spur on its own, use 2.5mm to spur into a 13A FCU which then feeds 2 sockets from 2.5mm cable? That would work too. I don't understand the pros/cons of each approach.

Both are acceptable. The only potential downside, in general, is that all the outlets beyond the FCU (i.e. 4 outlets if there are two double sockets) are obviously limited to a total of 13A. That's probably fine for you, but it might not be for everyone. The small price difference of the different sizes of cable is not likely to be a significant consideration.

Kind Regards, John

 

[securespark]

The (green) OSG says:

"A non-fused spur feeds only one twin socket-outlet or one permanently connected item of electrical equipment."

 

[Iggifer]

So it's an omission rather than an express prohibition then? Stupid IET, again

Appendix 15 does show a single socket spurred from a RFC so that trumps the OSG.

 

[JohnW2]

So it's an omission rather than an express prohibition then? Stupid IET, again ... Appendix 15 does show a single socket spurred from a RFC so that trumps the OSG.

I'm not sure that either trumps the other - both are explicitly offered only as 'guidances' - no maybe it's actually a 'No Trumps' contract

Kind Regards, John

 

[Iggifer]

No Trumps works for me!

 

[JohnW2]

The (green) OSG says: "A non-fused spur feeds only one twin socket-outlet or one permanently connected item of electrical equipment."

... and I imagine it doesn't even specify a maximum current draw for that "one permanently connected item of electrical equipment", does it? !!

Kind Regards, John

 

[EFLImpudence]

I don't wish to be rude but the OSG is for installing circuits without thinking - i.e. not having to do any working out.

That is its purpose.



I would ask the honourable gentleman to view Appendix 15B (32A Radials) where they will find more than one socket on a 4mm² 'branch' (spur) and ask them "What is the difference?".
They will also view the same unfused 2.5mm² spur limited to one (single or double) socket.
All perfectly logical.

 

[JohnW2]

I don't wish to be rude but the OSG is for installing circuits without thinking - i.e. not having to do any working out. ... That is its purpose.

Indeed ... and not just the OSG, either. As I've written a few times this evening, even Appendix 15 is (IMO) essentially for those who can't, or don't want to, think.

Kind Regards, John

 

[roundthebend]

Will 13A be enough for my TV and all the junk that goes alongside it?
Or, is this 4mm spur option going to give me more capacity?