Correct cable size?

[JohnW2]

Will 13A be enough for my TV and all the junk that goes alongside it?

Plenty, unless the "things alongside it" include electric heaters, welders or the like

Or, is this 4mm spur option going to give me more capacity?

Yes, it would give you more capacity, but I don't think you need it at present. It's really only worth going that route if you have any thoughts for far more 'general' use (including heaters etc.) of those sockets in the future.

Kind Regards, John

 

[roundthebend]

OK, thank you all very much.

I think I will go with 2.5mm cable spur to an FCU. Then 2.5mm cable from the FCU to feed 2 double sockets. I think we've said that was OK.
I'll run the cable just behind the plasterboard, following safe zones - horizontal from and to sockets.

 

[securespark]

The full section on socket outlets is as follows:

7.2.2 Socket-outlet circuits

The length represents the total ring cable loop length and does not include any spurs.

As a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.

The total number of fused spurs is unlimited but the number of non-fused spurs is not to exceed the total number of socket-outlets and items of stationary equipment connected directly in the circuit.

A non-fused spur feeds only one twin socket-outlet or one permanently connected item of electrical equipment. Such a spur is connected to a circuit at the terminals of socket-outlets or at junction boxes or at the origin of the circuit in the distribution board.

A fused spur is connected to the circuit through a fused connection unit, the rating of the fuse in the unit not exceeding that of the cable forming the spur and, in any event, not exceeding 13A. The number of socket-outlets which may be supplied by a fused spur is unlimited.

The circuit is assumed to have a load of 20A at the furthest point and the balance to the rating of the protective device evenly distributed. (For a 32A device this equates to a load of 26A at the furthest point.)




I think this whole piece is very clumsily written. The first sentence being stuck at the top when it would make more sense to put it after the second sentence. "rule of thumb" is wishy-washy and old fashioned. Either it is or it isn't. If it is a rule, then take the ROT bit out.

 

[EFLImpudence]

The full section on socket outlets is as follows:
7.2.2 Socket-outlet circuits
The length represents the total ring cable loop length and does not include any spurs.

Obviously, So?

As a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.

Why? No such regulation.

The total number of fused spurs is unlimited but the number of non-fused spurs is not to exceed the total number of socket-outlets and items of stationary equipment connected directly in the circuit.

No such regulation.
Some ring circuits - in bungalow roofs - are all spurs with nothing connected directly in the circuit.

A non-fused spur feeds only one twin socket-outlet or one permanently connected item of electrical equipment. Such a spur is connected to a circuit at the terminals of socket-outlets or at junction boxes or at the origin of the circuit in the distribution board.

Why, if the conductor can handle the full current of the circuit OPD?
No such regulation.

A fused spur is connected to the circuit through a fused connection unit, the rating of the fuse in the unit not exceeding that of the cable forming the spur

...and if that cable can handle the full current of the circuit OPD?

and, in any event, not exceeding 13A.

No such regulation.
Why, if an unfused spur can have a double socket to which 26A might be connected?

The number of socket-outlets which may be supplied by a fused spur is unlimited.

They got that right.
Why does the same not apply if the cable can handle the full current of the circuit OPD?

The circuit is assumed to have a load of 20A at the furthest point and the balance to the rating of the protective device evenly distributed. (For a 32A device this equates to a load of 26A at the furthest point.)

To what amperage device does the 20A relate?


Throw it away - unless you want to get scheme registered when, for some reason, ownership of this rubbish is compulsory.

I think this whole piece is very clumsily written.

You could say that.

The first sentence being stuck at the top when it would make more sense to put it after the second sentence. "rule of thumb" is wishy-washy and old fashioned. Either it is or it isn't. If it is a rule, then take the ROT bit out.

Take it all out.

 

[JohnW2]

The full section on socket outlets is as follows: ..... I think this whole piece is very clumsily written. The first sentence being stuck at the top when it would make more sense to put it after the second sentence. "rule of thumb" is wishy-washy and old fashioned. Either it is or it isn't. If it is a rule, then take the ROT bit out.

EFLI has done the line-by-line critique of all thi, and I agree with all he has said - it's not just clumsily written, but it contains several things that the authors have 'invented', which do not appear in BS7671. For example, what you mention above cannot be a 'rule' (the OSG cannot create 'rules') - and since it's talking about something which is not even hinted at in BS7671, it should not really be in the OSG at all.

Nor do I even understand the thinking (and practicality) behind that 'ROT'. As I understand what it's saying, when one gets near the middle of the ring the 'permissible' length of a spur approaches zero. The length of any spur should surely only be limited by considerations of Zs and VD. That will, indeed, mean that the permissible length will reduce as one gets closer to the mid-point of the ring but, depending on the actual situation, the maximum possible length might be very much more than that arbitrary "1/8" figure, particularly with a small ring.

Kind Regards, John