cooker wiring
- Thread starter barty74
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The all idea of safety is to protect the cable, there are many more factors involved in derating the cable capacity, just an example; if you run the cable across insulated area for more tha a certain length (about 400mm I think) the cable capacity will be halved, meaning if it can carry 30A it should be considered as 15A and so on. The recommendations of the On Site Guide For a 2.5 cable T&E in a radial configuration is to be protected by max. 20A MCB not higher.barty74 said:securespark said:
even though the overall load is more than 20A and the cable is capable of carrying more? dont get me wrong, i am not questioning your knowledge, but simpy understanding everything involved.
My advice to you is to stay on the safe side, use the good and free advice given in this site, and if you are still not sure call in a qualified electrician.
barty74 said:securespark said:
even though the overall load is more than 20A and the cable is capable of carrying more? dont get me wrong, i am not questioning your knowledge, but simpy understanding everything involved.
But the cable is not capable of carrying 30A, while In (nominal current of the OPD) needs to be greater than Ib (design current after applicable diversity applied), In (with factors applied) must also be less than It (tabulated current for cable using correct ref method)
Looks like you need a larger cable is the load is more than 20A
(exceptions do exist where corrected In is allowed to exceed It, but these are where overload protection is allowed to be ommited due to the fact that the nature of the load means it simply cannot cause an overload, and additional calcuations are required to prove you still have fault current protection, only one that is common is the unfused spur from a RFC and thats in the OSG as a standard circuit so no calcs required in that situation, other than that, its probably easier to install a larger cable rather than trying to justify a smaller one, unless the cost saving is huge)
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Albert said:The all idea of safety is to protect the cable, there are many more factors involved in deteriorating the cable capacity, just an example; if you run the cable across insulated area for more than a certain length (about 400mm I think) the cable capacity will be halved, meaning if it can carry 30A it should be considered as 15A and so on. The recommendations of the On Site Guide For a 2.5 cable T&E in a radial configuration is to be protected by max. 20A MCB not higher.barty74 said:securespark said:
even though the overall load is more than 20A and the cable is capable of carrying more? don't get me wrong, i am not questioning your knowledge, but simply understanding everything involved.
My advice to you is to stay on the safe side, use the good and free advice given in this site, and if you are still not sure call in a qualified electrician.
As Albert said, in layman's terms. Although the hob should only draw 20A, the breaker will not trip until 30A is being supplied. In a fault situation, your hob could start to draw more current, not a sudden failure but a gradual failure. This means that the weak point, your 2.5mm cable could start to carry more load. Remember the MCB is there to protect the wires from breaking down in a fault situation and needs to be rated for the weakest cable which in this case is the 2.5mm and therefore as mentioned several times already, the breaker needs to be changed to 20A or find a way to change the cable to 4mm.
To expand on the above:
In a domestic situaltion, the MCB or fuse normally provides protection again both fault and overload conditions
The best case current rating for 2.5 T+E is 27A if clipped direct ( Method 1, table 4D5A in the regs, or 6F in the OSG). So, if there were no other derating factors, and you were prepared to do the full design calculations, you might be able to go as high as a 25A MCB or fuse.
Without doing the full calcs, you have to use the conventional circuits which limits you to 20A as already stated, or 15A if the fuse is a rewirable (OSG table 7.1).
If the fuse you have is a rewirable (semi-enclosed to BS3036), then you already have a derating factor of 0.725 which reduces the best-case capacity of 2.5T+E to 19.6A - this is necessary because these fuses do not provide very good protection against overload conditions.
On top of this you have to add other derating factors for installation methods if not M1 and possibly ambient temperature and grouping.
Bottom line - if you are competent to do the design calcs, you can deviate from the conventional circuits. If you're not, best stick to them![/i]
In a domestic situaltion, the MCB or fuse normally provides protection again both fault and overload conditions
The best case current rating for 2.5 T+E is 27A if clipped direct ( Method 1, table 4D5A in the regs, or 6F in the OSG). So, if there were no other derating factors, and you were prepared to do the full design calculations, you might be able to go as high as a 25A MCB or fuse.
Without doing the full calcs, you have to use the conventional circuits which limits you to 20A as already stated, or 15A if the fuse is a rewirable (OSG table 7.1).
If the fuse you have is a rewirable (semi-enclosed to BS3036), then you already have a derating factor of 0.725 which reduces the best-case capacity of 2.5T+E to 19.6A - this is necessary because these fuses do not provide very good protection against overload conditions.
On top of this you have to add other derating factors for installation methods if not M1 and possibly ambient temperature and grouping.
Bottom line - if you are competent to do the design calcs, you can deviate from the conventional circuits. If you're not, best stick to them![/i]
guys, that's fantastic advice. thank you all very much.
of course safety is paramount, especially with the kids asleep upstairs! you guys are very knowledgable and i take everything you say seriously. i will take proffesional advice on this, but its good to understand some of the in's and out's.
of course safety is paramount, especially with the kids asleep upstairs! you guys are very knowledgable and i take everything you say seriously. i will take proffesional advice on this, but its good to understand some of the in's and out's.
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