HUGE KITCHEN RE-WIRE & ISLAND (10 appliances) help!!!
- Thread starter worm
- Start date
Sponsored Links
That's not complicated.
Something of an embuggerance when it comes to running the cables, if you have a solid floor.
You've forgotten to mention tv/satellite/data/phone/alarm cabling. or what sort of ear defenders you will wear when using a downdraught fan that needs 13 amp.
Sponsored Links
Well, in general it might....
It's a self-build - surely he'll have put suitable ducting in?
No, I don't think there is any doubt - the arithmetic is clear enough. However, this was a sort-of pre-amble to my follow-up question ...
... are you not at least a little surprised (if not amazed) that 433.1.103 not only allows a ring final cable with a CCC of 20A to be protected by a 32A MCB, but it also allows that same cable with a CCC of 20A to be protected in the same circuit by a 30A 3036 fuse? If such a high degree of 'under-protection from a 3036' is tolerated in a ring final, why is a much lesser degree of 'under-protection by a 3036' not also 'tolerated' in a 4mm² radial?
Kind Regards, John
I've always thought that the 20A CCC is the minimum figure arrived at for the cable after applying all of the relevant derating factors, which meant not just installation method but includes ambient temperature, grouping, semi-enclosed rewirable factor etc.
2.5mm² T&E, Method C, BS 3036 fuse, has a CCC of 20A.
4mm² ditto is 27A.
No.
20/0.725 = 27.6A (or 27x0.725 = 19.5A) - near enough.
Isn't that WHY 20A minimum is the value required? A tweak.
Because the figures do not match.
I really don't see a mystery.
I see. I've always thought the same, in that 20A CCC is the minium after applying 'all de-rating factors' applying to the cable, but I've never regarded the "semi-enclosed rewirable factor" as being a 'cable de-rating factor'. If you look at 433.1.101, which talks about this 0.725 factor, it talks in terms of In not exceeding 0.725 times the (presumably after application of de-rating factors) CCC of the cable. If they thought the way you are suggesting they would presumably, at most, have reminded one that the de-rating factors applied to the cable's CCC had to include 'the BS 3036 factor' when appropriate.
I have therefore always taken 433.1.103 to mean that it was OK to have 2.5mm² ring cable whose CCC was 'already down to 20A (e.g. due to being Method A) protected by a 30A 3036. Indeed, I thought we had previously agreed that Table 4D5 had to be 'tweaked' to allow Method A 2.5mm² to be used in ring finals, not so that Method C 2.5mm² could be used in ring finals with a 3036?
Kind Regards, John ²
See what I've just written to BAS. That is very different from what I have always thought/'understood'. My understanding has always been that the 20A CCC minimum was nothing to do with 3036's but, rather, to facilitate use of 2.5mm² Method A in ring finals - and that, indeed, they had to 'tweak' Table 4D5 to make even that possible.
If what you and BAS have suggested were a correct interpretation, it would mean that 2.5mm² could not be used in a ring final on a 30A 3036 if there were any de-rating factors to b applied to the cable. Is that your belief?
Kind Regards, John
double post
Well. you can't get away from the 0.725 factor with 3036 fuses so I do not understand why you would have thought that.
When 3036 boards were installed singles may/would likely have been in conduit (method 3 (conduit in uninsulated wall)) for which the rating of 2.5mm² is 24A - so not quite compliant but...
(Was imperial the same?)
It is.
Were the regulations made more strict after the introduction of MCBs?
I really don't know what to think! Fortunately 3036's are a dying breed, so it's relatively unimportant, but what you and BAS are suggesting is, as I've said, totally different from what I've always thought and believed.
Since 433.1.101 talks about "0.725 times the CCC of the cable", I have always presumed that they were not intending that the "CCC" had already been reduced by a factor of 0.725 if it were protected by a 3036 (otherwise that factor would get applied twice). I have therefore always assumed that 'CCC' took into account installation method and all 'cable de-rating factors', but not any adjustment for a 3036.
That makes total sense to me. The "current-carrying-capacity" of 2.5mm² Method C is obviously always going to be 27A, no matter what the type or rating of OPD. The cable does not suddenly become able to safely carry more, or less, current just because of the type of OPD protecting it. The only difference relates to the OPD. With an MCB, the maximum permissible In to protect that cable is 27A, but with a 3036, the maximum permissible In to protect that cable is about 19.6A. However, the cable has a "CCC" (by any sensible definition) of 27A in either case.
If that interpretation were all correct, then 433.1.103 would be very clear in saying that 2.5mm² cable with a CCC (after application of all de-rating factors other than any adjustment for 3036) of 20A in a ring final is allowed to be protected by a 30A 3036, whereas we would normally say that a 30A 3036 could only protect a cable of CCC of at least 41.4A. That 'ring dispensation ratio' (41.4/20=2.07) would then be much greater than the 'ring dispensation ratio (32/20=1.6) allowed with an MCB.
I need to think more, but my interpretation seems fairly logical to me. If you say that, for a cable protected by a 3036, "CCC" already includes a 0.725 adjustment for the type of OPD, then, as above, 433.1.101 makes no sense.
Kind Regards, John
433.1.202 now.
I'm not sure I understand why you think it is applied twice.
No. With 3036s it has to be derated by 0.725
It has to be able to carry more overload when protected by 3036s.
Isn't that contradicting what you just wrote?
Yes, precisely.
MCBs have a fusing factor of 1.45 - the standard - 3036s have a fusing factor of 2 so the cable must be derated (2x0.725 = 1.45)
It doesn't matter which way round you do it.
You either have a 27A cable but must protect it with a 19.5A 3036 or
you have a 27A 3036 but must use a 37.24A cable.
If the cable has for other reasons been derated to 20A then it is too small to be used - in a ring - with a 30A 3036.
I do not understand that, either.
433.1.101 states that the In of the 3036 shall not exceed 0.725 the Iz of the cable.
Therefore a 30A 3036 must have a cable - in a radial - with Iz at least 41.38A or - in a ring - 27.6A (27A).
It does not mention other derating factors which obviously must be taken into account.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
Similar threads
