There's nothing wrong with a 'star' radial, and no need to protect it with a 16A OPD unless the (2.5mm²) cable is deeply embedded in insulation (Method 101).
Kind Regards, John
Order Elec items for a 2 storey extn, + BS7671 imperfections
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Kind Regards, John
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Oooh!
No because of 433.1.1(iii) and the 60% but I'm not sure I remember where that is stated.
Otherwise a normal ring could have a 40A opd.
433.1.103 categorically states that the conductors shall be 2.5mm² but then goes on to refer to the minimum CCC of 20A which I do not really understand.
No, I dont think that is, in itself, the reason. 433.1.103 'over-rides' 433.1.1(iii) by saying that ring finals satisfying 433.1.103 "...are deemed to satisfy the requirements of 433.1.1" - otherwise 433.1.1(iii) would prohibit the standard 2.5mm ring final. However ...
...which I had obviously forgotten
. That clearly means that one cannot have a 1.5mm²/32A ring final by invoking 433.1.103. However, that doesn't necessarily mean that one couldn't produce a 'from first principles' justification for such a circuit (without reference to 433.1.103) - in fact, if the IET managed it for a cable with a CCC of 20A for 433.1.103, then it surely ought to be possible for anyone else to do the same for any cable with 20A CCC (Including 1.5mm² clipped direct)?Presumably to allow ring finals with Methods 100 & 102, but not with Methods 101 or 104. In other words, they did their 'justifying calculations' on the basis of a CCC of 20A, but for some reason decided to explicitly require at least 2.5mm², thereby excluding 1.5mm² clipped direct.
I'm not sure, either- are you certain that this "60%" is not a red herring?
Kind Regards, John
If he'd said 67% that would have a known origin.
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With a 30A OPD and 20A CCC, you mean? If so, one could certainly speculate upon the 'origin', although I would hope that would be the result of a reasoned electrical argument/calculation, and not an (arbitrary?) figure used as a basis for such an argument/calculation.
Kind Regards, John
How the 67% was arrived at IHNI, but that's what the rule was until Amendment No 1 : 2002 to BS 7671 : 2001
http://electrical.theiet.org/wiring-regulations/updates/bs7671-2001-amd-13628.cfm
http://electrical.theiet.org/wiring-regulations/updates/bs7671-2001-amd-13628.cfm
Yes.
It was incredibly fortuitous that when the Wiring Regulations Committee finally accepted that the issue of 2.5mm² T/E only being rated at 18.5A when enclosed in an insulating wall would not go away, that they found that the rating was wrong and that it should be 20 or 21A (with/without conduit) and that as long as they made circuit designers responsible for ensuring that under the intended conditions of use the load current in any part of the ring circuit should be unlikely to exceed for long periods the current carrying capacity of the cable, it would be OK to change from 67% to a flat 20A.
My, what a lucky coincidence that the newly rated 2.5mm² T/E would still be OK.
And of course, since then, not one single electrician has ever installed a 32A/2.5mm² ring final without carrying out due diligence to ensure that it was appropriate because under the intended conditions of use there would be no long overloads in any part of the circuit.
[/sarcasm]
And people wonder why I regard the ring final as a kludge, cobbled together with no sound engineering basis, and that were it proposed today as an alternative to radials it would be unacceptable.
The last para on p1 of this http://www.theiet.org/resources/wir.../pre14-BS7671-amd1-ring-circuits.cfm?type=pdf says it all. A pronouncement from a bunch of self-serving mealy-mouthed cowards using wet fingers in the air to do circuit design.
It was incredibly fortuitous that when the Wiring Regulations Committee finally accepted that the issue of 2.5mm² T/E only being rated at 18.5A when enclosed in an insulating wall would not go away, that they found that the rating was wrong and that it should be 20 or 21A (with/without conduit) and that as long as they made circuit designers responsible for ensuring that under the intended conditions of use the load current in any part of the ring circuit should be unlikely to exceed for long periods the current carrying capacity of the cable, it would be OK to change from 67% to a flat 20A.
My, what a lucky coincidence that the newly rated 2.5mm² T/E would still be OK.
And of course, since then, not one single electrician has ever installed a 32A/2.5mm² ring final without carrying out due diligence to ensure that it was appropriate because under the intended conditions of use there would be no long overloads in any part of the circuit.
[/sarcasm]
And people wonder why I regard the ring final as a kludge, cobbled together with no sound engineering basis, and that were it proposed today as an alternative to radials it would be unacceptable.
The last para on p1 of this http://www.theiet.org/resources/wir.../pre14-BS7671-amd1-ring-circuits.cfm?type=pdf says it all. A pronouncement from a bunch of self-serving mealy-mouthed cowards using wet fingers in the air to do circuit design.
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We did an exercise at college under the early 15th edition where we discovered it was possible to wire an immersion with 1mm2 cable within the regs. Not that you would, but you could....
Per current regs, the CCC would clearly be adequate if it were clipped direct (16A), but you'd be caught by what IIRC is a blanket requirement (i.e. without exceptions) that cables in 'power' circuits must have a minimum CSA of 1.5mm².
Kind Regards, John
Which is prescribed where?
Which says (unless they amended it last time round) 1.0mm² for power and lighting circuits.
Not that "power circuit" means anything, of course.
Not in my BGB it doesn't! - but there are lines in the margin indicating that parts of the Table have been amended. It must be about time you finished saving up for your BGB, I reckon!
Knd Regards, John
Yeah - I should get an up-to-date version.
Is there a definition of "power circuit" ?
Do lights no longer use power?
Red Table 51 explicitly says that power circuits include lighting circuits...
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