Ring main 100 square metres?

[La Poste]

I have read in the regulations that a ring circuit can cover a floor space of 100 square metres.
I am re-wiring a bungalow at the moment and the floor space is 88 square metres.
I am using 2.5mm stranded singles and 1.5 mm earth.
I am feeding the singles down the existing conduit in the walls from the loft.
Although the ring is less than 100 square metres I am wondering if the equation will be correct due to the fact I have cable drops of 3 metres to each socket.
Does the 100 square metre rule apply if I have these cable drops?
The ring supplies 7 sockets and the kitchen will be on it's own separate circuit.
There are no heavy loads being drawn from any socket but this may change in the future.
Any advice would be most welcome.
Thanks.

 

[pushrod]

Three cables to each socket three cables out of each socket.
Ring goes DB first socket three cables in conduit.
Socket 1 to 2 three cables in conduit.
Socket 3 to 4 three cables in conduit.
socket 4 to 5 three cables in conduit.
socket 5 to 6 three cables in conduit.
socket 6 to 7 three cables in conduit.
socket 7 back to DB three cables in conduit.
One big ring.

Good point.
Does a ring count as two circuits ? The six cables (singles ?) are for one ring.

As long as you are putting just 3 singles in each conduit there is no problem.

 

[La Poste]

Just a reminder if you are putting 2 circuits in the same conduit then you are derating the cable's current carrying capacity by 20%

As long as you are putting just 3 singles in each conduit there is no problem.

Got me thinking now.
reference method 3 so as you say lose 20% or maybe it's 6 multi core cables giving a correction factor of .57
One thing I am not sure about.
What is the cable size for a ring.
If I am using using 2.5 twin and earth and am building a ring circuit does that mean I take the cable size as 5 mm?
I am looking at the regs for cable size, reference method and volt drop per metre but can see no mention of a ring circuit.
It's a little confusing.
What cable size do we use when looking at volt drop per metre and reference methods for a 2.5mm ring circuit.
Thanks.

 

[pushrod]

So if you have a ring fed from the ceiling above with 2.5 twin and earth you will have one cable going into each socket and one cable going out hence two cables in each bit of wall conduit.
Does this mean that the final cicuit will have to be derated by 20% overall current rating?
Cheers.

Yes i think it does (not 100% certain as both cables are part of the same circuit) If i'm wrong no doubt someone will correct me fairly quickly

edit: oops replied while you were editing.

 

[pushrod]

Got me thinking now.
reference method 3 so as you say lose 20% or maybe it's 6 multi core cables giving a correction factor of .57 surely just 2, 3 core cables so 0.8 correction factor
One thing I am not sure about.
What is the cable size for a ring.
If I am using using 2.5 twin and earth and am building a ring circuit does that mean I take the cable size as 5 mm?
No, still 2.5mm
I am looking at the regs for cable size, reference method and volt drop per metre but can see no mention of a ring circuit.
It's a little confusing.
Have a look at chapter 7 of the on site guide table 7.1 in particular makes things clearer, but i must admit your installation method of dropping both legs of the ring in conduit from the ceiling does confuse the issue a bit
What cable size do we use when looking at volt drop per metre and reference methods for a 2.5mm ring circuit. Table 7.1 of the OSG takes into account volt drops
Thanks.

my answers in red

edit : just read thru your post #15 and must admit i don't quite follow when you talk about 6 cables

 

[La Poste]

So if I am using a cable size of 2.5mm then why fuse at 32 Amps?
There are six cables going through the wall conduit.
Sorry for the confusion.
six cables to the socket through the wall conduit from the loft.
Six cables = 3 cables (live, neutral and cpc from the DB = one leg of a ring) plus three cables (live, neutral and cpc = the other leg of the ring) all run in singles because the space is tight inside the wall conduit so six cables.
Hope this makes sense.

 

[Risteard]

For voltage drop I would divide by four (half the length and double the csa).

 

[JUD]

Yes i think it does (not 100% certain as both cables are part of the same circuit) If i'm wrong no doubt someone will correct me fairly quickly

edit: oops replied while you were editing.

Looking at Table 6C in the OSG the rating factors are for "Number of circuits or multicore cables."

I would say that if the ring is wired in singles then it would count as one circuit surely.

I'm sure someone will correct me if I'm wrong

 

[pushrod]

six cables to the socket through the wall conduit from the loft.
Six cables = 3 cables (live, neutral and cpc from the DB = one leg of a ring) plus three cables (live, neutral and cpc = the other leg of the ring) all run in singles because the space is tight inside the wall conduit so six cables.
Hope this makes sense.

You are confusing cores with cables !!! A singleT&E cable has 3 cores.
Because you are using singles it would count as two circuits in terms of grouping (table 4C1, page 268 in brb)

edit Hi JUD - i agree

 

[Deleted member 9648]

This comes under standard circuit arrangements in the OSG....It is not correct that the requirement for max floor area on a ring in domestic situations no longer applies.A ring should not serve a floor area exceeding 100sq m (Table 8a p158).
The minimum cable sizes are given in table 8a....however the actual size will be after rating factors are included,for a ring circuit the as installed current carrying capacity must not be less than 20a (8.2.3 p159)

 

[La Poste]

The one thing that confuses me is on the one had a ring can be 100 sq m ( Table 8a p 158) but on the other hand table 7.1 ( OSG page 44 ) gives maximum cable lengths.
A ring circuit covering a floor area of 100sq m could easily use up more than 106m of cable.
Anyway I've decided now, because the cable lengths are getting quite long due to me routing them around the corners of the loft I am going to split the ring in two.
Can't be too careful and never skimp on safety.
Interesting conundrum though.

 

[Risteard]

It is not correct that the requirement for max floor area on a ring in domestic situations no longer applies.

Why has the word "Historically" been introduced then?

Surely so long as disconnection times and voltage drop constraints are met then there isn't an issue. The On-Site Guide offers a means of compliance for simple (domestic and similar) installations. It is not the only means of compliance. In fact this is explicitly stated in the On-Site Guide.

The Design must be to BS 7671 (not the OSG).

 

[vambo]

Surely so long as disconnection times and voltage drop constraints are met then there isn't an issue. QUOTE]

I would agree, the OSG is a guide only.

 

[Deleted member 9648]

Hi Ristard....where is 'historically' used.?..I cant see it in my copy of the regs or the OSG...

I would agree that the 100sq m rule is a guide only....but it is clearly stated in the OSG so IMO should be adhered to.

 

[Risteard]

The Corrigendum replaced references in the Informative (i.e. does not form part of the Regulations) Appendix 15 (pp. 362-363) to "As a rule of thumb" to "Historically".

So: "The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:

[...]

"(iv) taking account of the total floor area being served. (Historically, a limit of 100 m[FONT=Times New Roman, serif]² has been adopted.)"[/FONT]

 

[Deleted member 9648]

Take your point,but in the absense of any other guidance the only figure given is still 100sq m.