50 m2 floor area is it? I looked glanced at a 20+ year old copy I happened to have nearby.
Even so, far more options with a ring.
Even so, far more options with a ring.
ring mains in insulated walls
- Thread starter elmsoft
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It is indeed, but I'm surprised to hear that it was ever as low as 20m²
One size rarely fits all - IMO, both probably have their places.
Kind Regards, John
It USED to be 50 m2 for an A2 circuit (30 or 32 amp in 4.0 mm2), so again, a significant difference to today's 75 mm2.
Not being awkward - genuinely don't understand.
sparkwright wrote:
A 20 amp radial in 2.5 mm2 should only serve a 20 m 2 area.
What does serve mean in this context?
A 2.5mm² 20A radial with 5% VD can be ~30m long.
So, one of these running down each side of an extended Victorian terraced house.
House is 4m wide. Area 120sq.m.
What is the floor area served by each radial and what difference does it make?
A 2.5mm² 32A ring with 5% VD (at 32A) can be 80m long.
So, it could easily replace the two radials.
What has the floor area to do with it?
sparkwright wrote:
A 20 amp radial in 2.5 mm2 should only serve a 20 m 2 area.
What does serve mean in this context?
A 2.5mm² 20A radial with 5% VD can be ~30m long.
So, one of these running down each side of an extended Victorian terraced house.
House is 4m wide. Area 120sq.m.
What is the floor area served by each radial and what difference does it make?
A 2.5mm² 32A ring with 5% VD (at 32A) can be 80m long.
So, it could easily replace the two radials.
What has the floor area to do with it?
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It's a bit more than 75mm²
To use floor surface area as the determinant seems pretty naive to me - 50m² worth of bedrooms is likely to be a totally different kettle of fish from 50m² worth of kitchens, utility rooms, living rooms, workshops etc. If it's true that the guidance for A3s has (per what you've said) changed from 20m² to 50m² in a couple of decades or so, that suggests to me that it's very arbitrary, and probably not to be taken too seriously!
Kind Regards, John
Indeed - although, as I've pointed out,the OSG figure for an A3 is now 50m².
I've always taken it to mean the floor areas of rooms/places (including halls/landings etc.) in which there were sockets supplied by the circuit. I don't think it's anything to do with the cable length, which is why I keep saying that cable length/VD has to be considered as a separate (and quite possibly the limiting) issue. In other words, as far as the OSG's 'floor area guidance' is concerned (but not VD considerations) it's OK to have an A3 providing sockets in room(s) with a total floor area ≤50m², even if it's 100m from the CU, and passes through umpteen rooms and spaces (with large floor areas) on the way.
That's how I've always interpreted the guidance, anyway - but, as I've just written, I think it's daft to base anything on floor area without consideration of the nature of the rooms/spaces concerned, so I think that 'pinches of salt' may well be required!
Kind Regards, John
I gather the floor area measurement is something to with the use of electric fires, which was common when the ruling first came out. I believe it was worked out so there weren't too many electric fires on one circuit.
It is a bit of a pointless rule now.
It is a bit of a pointless rule now.
That makes some sense - although, even then, it was probably a bit silly to have a 'floor area rule' which didn't take the nature of rooms into account.
Kind Regards, John
I suppose back then (1947 perhaps? don't quote me) it was just heaters, a kettle and an iron.
True - it would be an iterative process.
3. Look for a cable with a current carrying capacity, given the Installation Method and any other applicable derating factors as per Appendix 4, which complies with Ib ≤ In ≤ Iz as per Section 433.
4. Check that voltage drop will be OK, if not then increase the cable size.
75mm²?
I know that we're using more and more insulation these days, to meet the ever more demanding requirements of Part L, and this will tend to mean cables need to be larger, but 75mm²? That's going to take some humping about, and need a complete redesign of accessories and boxes.
75 m2. I meant 75 m2.
75 m² .
I've found the right symbol at last.
I've found the right symbol at last.
Anyway, back to the plot.
Who's going to sign that? Because I really don't think you can.
And this was a genuine question:
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 2011 except for the departures, if any, detailed as follows.
Who's going to sign that? Because I really don't think you can.
And this was a genuine question:
If you do something other than what you said you'll do, or allowed them to think you'll do, then rocks and hard places will make an appearance instead of a Completion Certificate...
Indeed:75mm²?
It's a bit more than 75mm²
Kind Regards, John
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