Order Elec items for a 2 storey extn, + BS7671 imperfections

[ricci1212]

[Title edited by site admin after the subject of the discussions changed.]
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Hi,
We are building an two storey extension.
Just preparing a order list for electrical installation.
We will order the goods and wire the cable in the dry wall a Napit Electrican will connect the extension electrics to the house mains supply.
Just preparing my order list.

We have in the ground floor
20 Spot lights
Tv Video computer

Lights and socket for the garden


First floor
20 spot
Tv

There will be five circuits driven by separate MCBs ( Miniature Circuit Breakers).

6 Amp: Lighting in the extension.
16Amp: 13 Amp ring in the extension
16Amp: 13 Amp feed to the garden


Is this ok?

Regards Ricci1212

 

[Gaz the Sparky]

What are you actually asking ?

You should speak to the electrician who will sign off your work

16a for a ring circuit is too low for the circuit do you know how to design the circuit properly

 

[ban-all-sheds]

What are you actually asking ?

He's asking people here to fill in the yawning chasms in his knowledge which should not be there if he wants to do electrical design work.

You should speak to the electrician who will sign off your work

Absolutely.

Ricci - you must ask the question of your electrician - nobody but she can do the design work and the installation of the cables etc, because you'll be needing her to sign this:

FOR DESIGN, CONSTRUCTION, INSPECTION & TESTING
I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to ............. (date) except for the departures, if any, detailed as follows:

How can you reasonably expect her to sign to say she did things if she didn't actually do them?

do you know how to design the circuit properly

No, he doesn't have a clue.

 

[riveralt]

We have in the ground floor
20 Spot lights
First floor
20 spot


There will be five circuits driven by separate MCBs ( Miniature Circuit Breakers).

6 Amp: Lighting in the extension.
16Amp: 13 Amp ring in the extension
16Amp: 13 Amp feed to the garden

Is this ok?

Nuff said!

 

[ban-all-sheds]

Hi,
We are building an two storey extension.

When you applied for Building Regulations approval, what did you say would be the way you would ensure that the electrical work would comply with Part P?

Because if you said, or allowed to be said on your behalf that they would be done by an electrician, or tacitly accepted an assumption or default position of your LABC, and then you do something different you could easily find yourself in a world of pain when they refuse to issue you with a completion certificate.

And yes, that does happen - it's not unheard of.

Again - if you need your electrician to certify that the electrical work she did complied with the Building Regulations (a legal requirement), you need her to have actually done it.

20 Spot lights.
.
.
20 spot

Are you having that many because the rooms are enormous, or because you need that many to get around the fact that spotlights are useless at lighting up rooms?

 

[OwainDIYer]

20 Spot lights
20 spot

6 Amp: Lighting in the extension.

How are you getting 40 spot lights past Building Control under Part L energy efficiency? at 20W each that's 1.6kW of lighting?

How are you getting 40 spot lights on one 6 amp circuit when a 6 amp circuit can supply about 12 lights maximum (using the 100W per point or actual rating if greater principle)?

How are you getting 40 spot lights on one circuit to comply with avoiding danger by splitting circuits so a fault on one circuit does not cause a risk?

You said 5 circuits but listed 3 - what are the other 2?

 

[JohnW2]

How are you getting 40 spot lights on one circuit to comply with avoiding danger by splitting circuits so a fault on one circuit does not cause a risk?

I totally agree with all your other points/questions but, as for the above one ... the OP is only talking about the supply to the new extension, which, when built, will presumably only be a relatively small part of the whole house. Given that the house will presumably have other lighting (and other) circuits, I would imagine that the requirement for 'division into circuits' (in the installation as a whole) would be satisfied, without any further 'division' within the extension, wouldn't it?

Kind Regards, John

 

[pjcomp]

16A for a ring is only "too low" if you put too much through it. Given his list of appliances I doubt he'll get anywhere near it. In any case in the event of overload it's erring on the side of safety - it will trip too soon, not too late.

If he's after a shopping list I'd want to suggest the right sizes of cable and enough of it, and I'd want to know whether to get knock-out boxes and screw them to the wall or if there'll be enough airspace to fit dry-lining boxes. Dimmer switches? Need deeper back boxes. So much to buy, so little information.

PJ

 

[EFLImpudence]

There is absolutely NO point having a ring with a 16A opd.

It can be debated whether it is actually allowed.

 

[JohnW2]

16A for a ring is only "too low" if you put too much through it. Given his list of appliances I doubt he'll get anywhere near it. In any case in the event of overload it's erring on the side of safety - it will trip too soon, not too late.

Yes, but there are far more issues than that. The only point of a ring final circuit is to enable one to use cable smaller than that which would be required for a radial circuit fed from an OPD of the size in question - and, even then, the regs only 'dispensate' such a ring if the cable has a current carrying capacity of at least 20A and the circuit is protected by a 30A or 32A OPD.

With a 16A OPD, assuming the cables were 'clipped direct', one could, electrically speaking, have a 1.5mm² (or, theoretically, probably even 1.0mm²) radial circuit - although some will cite the fact that Appendix 15 ('informative') of the regs only mentions 2.5mm² and 4mm² radial circuits. There is, in fact, no cable available which is so small that there would be a benefit from having a ring circuit, even if the regs did allow this with a 16A OPD (which they don't).

Whether or not 16A is enough for the OP's requirements is one question, but it's really the 'ring' proposal that should be being questionend.

The OP's level of understanding of these matters suggests strongly that he really needs the advice, at this stage, of the electrician who he hopes will certify the work.

Edit: I see that EFLI beat me to mentioning these points, albeit in a somewhat less verbose/detailed fashion than myself

Kind Regards, John

 

[Gaz the Sparky]

16A for a ring is only "too low" if you put too much through it. Given his list of appliances I doubt he'll get anywhere near it. In any case in the event of overload it's erring on the side of safety - it will trip too soon, not too late.

When I was saying too low I was just meaning that this doesn't conform to convention and if too many appliances are plugged in it will over load the MCB causing it to trip. I know technically its safe due to it being less than the maximum safe rating of the RFC.

I have once put sockets on a 16a mcb when carrying out a consumer unit change but this was due to the sockets being wired unconventionaly so I had to cover the 1x 2.5 cable

 

[JohnW2]

When I was saying too low I was just meaning that this doesn't conform to convention and if too many appliances are plugged in it will over load the MCB causing it to trip. I know technically its safe due to it being less than the maximum safe rating of the RFC. ... I have once put sockets on a 16a mcb when carrying out a consumer unit change but this was due to the sockets being wired unconventionaly so I had to cover the 1x 2.5 cable

It's up to the designer to decide on likley loading. 20A 2.5mm² radials are very common and there's rarely going to be so 'bad' an installation method (Method 101?) that requires one to go down to 16A with 2.5mm².

Kind Regards, John

 

[EFLImpudence]

(or, theoretically, probably even 1.0mm²) radial circuit

The minimum for power circuits is 1.5mm².


More than that -

Hypothetically, had you a ring wired in 1.5mm² then the CCC (method C) of the two conductors would be 40A.
Taking the maximum opd for a ring to be 60% of 54A CCC (from 2 x 2.5mm²) then 40 x 60% = 24 means you would have to fit a 20A mcb.

As this is the same as can be achieved with a 20A radial with 1.5mm², it is, as I said, absolutely pointless.

 

[Gaz the Sparky]

When I was saying too low I was just meaning that this doesn't conform to convention and if too many appliances are plugged in it will over load the MCB causing it to trip. I know technically its safe due to it being less than the maximum safe rating of the RFC. ... I have once put sockets on a 16a mcb when carrying out a consumer unit change but this was due to the sockets being wired unconventionaly so I had to cover the 1x 2.5 cable

It's up to the designer to decide on likley loading. 20A 2.5mm² radials are very common and there's rarely going to be so 'bad' an installation method (Method 101?) that requires one to go down to 16A with 2.5mm².

Kind Regards, John

Yes I know there are lots of 20a radials. The one I'm refering to was wired as a spider with 1 central junction box so I felt that it wasn't the best to be installing it at 20amps so limited it to 16a

 

[JohnW2]

(or, theoretically, probably even 1.0mm²) radial circuit

The minimum for power circuits is 1.5mm².

I know. That's why I said 'electrically speaking' and 'theoretically'. As you say, regs-wise, 1.5mm² is the smallest permitted.

More than that - Hypothetically, had you a ring wired in 1.5mm² then the CCC (method C) of the two conductors would be 40A. Taking the maximum opd for a ring to be 60% of 54A CCC (from 2 x 2.5mm²) then 40 x 60% = 24 means you would have to fit a 20A mcb.

Actually (and I've never previously thought of this!), if the cable were 'clipped direct' (CCC=20A, per 4D5), would it not actually qualify under 433.1.103 for a ring final with a 32A OPD?

As this is the same as can be achieved with a 20A radial with 1.5mm², it is, as I said, absolutely pointless.

.. and as I said, too - although what I've just written above about a 1.5mm² 32A ring final may change that!

Kind Regards, John