Cable widths and Socket Heights

[DJUK]

Just a quick couple of questions, I'm checking some cabling and wondering is this still correct for current regs?

32A Ringmain : 2.5mm T&E
6A Lighting : 1.0mm T&E

If the ringmain was changed to 40A breaker, is thicker cable required or is 2.5mm OK for a 40A circuit as well? These circuits haven't gone in yet, I just want to be sure they're up to current regs. Sparky will be commissioning these circuits, I just like to know what's going on.

Also, I know that building regs state all sockets should be min 450mm from floor, what about fused spurs for a particular purpose? Can these be put lower to the ground?

Ta muchly oh gurus

 

[sparkyspike]

2.5mm is normal for a 32A ring final. 1mm is normal for a 6A lighting circuit.

Not all circuits are normal; factors other than design current are taken into account, such as circuit length, installation method and cable type.

Why might you put a 40A breaker on your ring circuit????

What particular purpose do you have for your fused spur?

 

[DJUK]

Why might you put a 40A breaker on your ring circuit????

Unlikely that I would, just trying to anticipate if the sparky said "you've got a lot on here, better put in a 40A breaker", and wondering if a 40A circuit needed thicker cable...

What particular purpose do you have for your fused spur?

Well the one in question is actually a fused spur on the lighting circuit.. for a low level lighting fixture to be added later ( a hidden rope light that goes around under faux skirting to light the room at floor level ), so I was wondering if the fused spur could be placed down by the skirting within regs.

 

[ericmark]

The heights and distance from corners of the room is covered by Part M of the building regulations. The type of building does make a difference domestic is normally min of 450mm but offices etc are min of 400mm but exceptions are permitted for example you can use floor mounted sockets. On existing building often the restrictions are ignored and I must say in some places it does seem odd to cater for the disabled in areas they could not access.
So long as a bit of common sense is used most building inspectors will allow some deviation. I mounted some sockets in bedrooms low so when the bed is pushed up to the wall it will go over the sockets so reducing the risk of damage. Once this was pointed out the inspector allowed it.
As to 40A on a ring main I would not think the sockets are rated to 40A so I would not fit any breaker over 32A. The historical method of using a ring main on 32A in the main requires very little risk assessment as it has been proved over the years but once one leaves the pre-proved systems to show that the conductors of unequal length will not be overloaded near to origin etc would be a lot of work and the normal if one considers the combined load may be too high is to install another ring main.
In houses today it is quite common to find 3 ring mains. Upstairs, Downstairs and Kitchen.
With the use of computers and other IT equipment which have some earth leakage the separating of circuits so that this does not build up to a level likely to trip the RCD becomes more important. As a result the idea of 20A radial circuits is becoming more popular. However there is also the problem of when something does trip one may not notice and I know one electrician use to put freezer, Fridge and intruder alarm on same circuit so if it tripped the warming buzzer on the intruder alarm would alert one to it's failure. It was also not on RCD circuit and wired in Ali-Tube cable.
Not sure if that complies with all the regulations but seemed a good idea!

 

[securespark]

If your sparks recommends upgrading a RF circuit to 40A, sack him instantly.

 

[ColJack]

if you have too much load for one ringmain, then you install 2..

you generally install 3 in a house these days anyway.. 1 upstairs, 1 downstairs and 1 for the kitchen.

further radials are recommended for the major appliances in the kitchen such as the washing machine, dishwasher, dryer and fridge-freezer

 

[DJUK]

That's great, thanks guys.

With respects lighting circuits in ceilings, what is the SOP for cable routing and joists? Over? Under? Through? Make a notch at the bottom?

 

[sparkyspike]

Through! Don't go over, under, or notching the the joist. Drill a hole right through the centre.

The size of a ring final is based on floor area - 100msq for a single ring is the maximum. If you anticipate unusually high loads, then put them on a seperate radial, or a new ring.

 

[ColJack]

The size of a ring final is based on floor area - 100msq for a single ring is the maximum.

not true anymore,, they altered the regs..

 

[Taylortwocities]

The size of a ring final is based on floor area - 100msq for a single ring is the maximum.

not true anymore,, they altered the regs..

Oh really, where's that then?
My copy of BS7671 - Appendix 15 states:
"As a rule of thumb, a limit of 100m² has been adopted".

 

[ColJack]

corrigendum issued in july 2008 removes the words "as a rule of thumb" and replaces it with "historically", either way, they are not regs..
they are advised limits.

tut, tut, working to outdated regulations...

 

[Taylortwocities]

either way, they are not regs..

So they didn't "alter the regs" then.

Whew, that's a relief..

 

[ColJack]

ok, I'll give you that one, I neglected to put the word "book" and instead refered to the "regs book" as the "regs" as most electricians do..

 

[ericmark]

I would have thought an Appendix was not part of regs. But since the C&G2382 tests one on the Appendix as well as main part it is obviously considered by C&G as part of the regs.

Because the volt drop has increased to 5% one is now allowed up to 106 Meters of 2.5mm twin and earth in a ring main. However unless the composite beams are used using the 1/3 rule for drilling beams I still think it would be hard to wire a 100 sq meters with 106 meters of cable. If one was to consider a square house then with half the drops needing 9 meters and the other half 3 meters along ceiling and down wall plus the 26 meters ring then one would be hard pressed to get 12 sockets before the volt drop was exceeded.

So although for upper floor with composite beams one may cover 100 sq meters most houses of that size would need more than one ring.

I have debated with my son the idea of splitting side to side instead of up and down as the cable run becomes much smaller. But he feels that people expect up and down and if one goes side to side some one may end up working on a live circuit. I consider if marked left and right on the consumer unit any spark would soon work out how it was wired. With twin RCD if same done with lights it would be less likely to be plunged into complete darkness as light would likely light half that had tripped from half that has not tripped.

We had to agree to differ!

 

[ColJack]

what amperage are you basing your volt drop on?
and take into account that you have half the volt drop anyway because you are doubling up on each conductor the the socket?

so your 106m becomes 212m..

where did you get your 106 from?
13A socket outlet on a 2.5mm² radial.
11.5V permissable VD on a power cct.
mV/A/m = 18

11.5
0.018 x 13 = 49m

or did I get my maths wrong? havent used it in a bit.. might be rusty...