Extending ring main and lighting in to new extension

[JohnW2]

Shouldn't you be using 37A - CCC of 4mm².

I should have - I obviously has "32" stuck in my mind, hence the error. However, as you go on to say, that error doesnt aler my conclusion ...

That still doesn't result in 63A, though.

... since, as you've obviously worked out, even 37 x (32/20) is, at 59.2, still less than 63. Now corrected - thanks for noticing!

I thought the criterion was 60% of the OPD rating.
32 x 0.6 = 19.2A
63 x 0.6 = 37.8A - albeit just over but I was just making the point.

Give or take some slight 'approximations' in your method (which resulted in 19.2A, rather than 20A), the things we are saying are mathematically equivalent.

The 'ratio' of 32/30 (OPD/CCC), which is obviously using the precise 'reg figures' is simply the reciprocal of saying that the CCC has to be 62.5% of the OPD rating (whereas your "60%" only gave approximately the reg's min CCC figure).

If one applies that 62.5% to a 63A OPD, one gets 39.375A for the min CCC.

There is also the fact that 2.5mm² rings could have a 40A MCB - 40 x 0.6 = 24A

Indeed, and it still would if one used 0.625 (giving 25A) - but the reg is obviously clear in being specific to a 30/32A OPD, not specifying CCC as a percentage of any OPD's rating..

Kind Regards, John

 

[JohnW2]

Who says only one format is permitted, the regs list only one but that doesn't mean others are forbidden.

I think it is 'forbidden'. 433.1.1 requires that a (Method C) 4mm² cable be protected by an OPD with an In no greater than 37A, hence it would be non-compliant with a 63A (or even 40A or 45A) OPD. 433.1.204 makes an exception/'dispensation' for ring finals, specifically for 30/32A OPDs, allowing a cable with a CCC of as low as 20A to be protected by a 30A/32A OPD (in a ring final circuit). Since the reg is specific to a 30/32A OPD, that 'dispensation' (to 'violate' 433.1.1) would not apply in the case of any other size OPD.

Kind Regards, John

 

[JohnW2]

My understanding is that the standard 32A ring with cable that has a current rating of at least 20 amps and a size of at least 2.5mm² (1.5mm²) is "deemed to" comply with the requirements for overcurrent protection.

Indeed - provided that it is protected by a 30A/32A OPD, which is explicitly required for the 'dispensation' in 433.1.204 (to 'violate 433.1.1) to apply to a ring final circuit.

I would assume that for any other ring you would have to justify the incomplete overcurrent protection some other way.

I don't think it could be "justified in any other way". If the OPD were anything other than a 30A/32A one, any situation in which cable CCC was less than the In of the OPD would be in violation of 433.1.1 - and nothing in BS7671 allows that rule to be broken, other than in the case of a 30A/32A ring final.

Kind Regards, John