Regs regarding Joining Smaller Cable with Bigger

The previous householders have re-used a 6mm^2(??) cable, (that I presume was once used to feed an electric shower), to feed a power shower pump and the (gas) heating system. (ie. low current draw compared with the electric shower which I presume it used to feed). The cable is protected by a 30A fuse in the consumer unit.
In order to make the wiring reach the power shower pump and the heating system in the airing cupboard, they have extended the 6mm^2 with around 3metres of 2.5mm^2 using a junction box in the loft.
The current situation is clearly unsatisfactory as the 2.5mm^2 is effectively (incorrectly) protected with a 30A protector. I understand that the regs say that each reduction in cable size should be appropriately protected, but I want to know if it is acceptable just to place smaller fuse at the start of the whole circuit in the CU. (ie. 16A). [Working here on the principle that it just becomes a radial circuit serving less than 50m^2 of floor area]. The alternative, if not, will be for me to replace the 3m of 2.5mm with 6mm. Not a major issue, but I'm not sure about getting it into the FCU's, (which incidentally have also been omitted by the previous 'installers')
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Depending on the load that is being drawn from the cable it would be safer to drop the fuse rating in the consumer unit to cover it. The 30amp fuse will protect the 6mm cable, but it is much to heavy to protect the thinner gauge cables.
But do check what load is being drawn first so you get the correct fuse rating and stop any nuisance tripping.
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On 14 Jan 2004 09:47:18 -0800, mike snipped-for-privacy@peppertree-broadcast.co.uk (Mike Hall) wrote:

Either way would be fine, the easiest in your case would be to replace the 30A fuse with a 16A. For future reference you can get 6mm into fused spurs, 1 is easy enough, two are tricky, but do-able with a little effort.
SJW A.C.S. Ltd.
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As a single strand of 2.5mmsq cable is rated at 29A @ 35C, there isn't much of a problem with cable ratings. I'd change the CU fuse to 16A or so, but fit a local in line fuse @ 5A to protect against pump failure. Regards Capitol
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On Wed, 14 Jan 2004 20:20:45 -0000, "Capitol"

Surely bad advice? Protecting a 29A rated cable with a 30A rated fuse has to be bad karma. The 29A is max isn't it? And the 30A fuse could take substantially more before it blows?
I'm not up to speed on this topic, but I'd certainly question it if I came across it.
PoP
Sending email to my published email address isn't guaranteed to reach me.
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Quite right. I'd note it on a periodic test as a failure. As I have mentioned in another post tonight there is an answer which states rules and regulations willy nilly without knowing what they mean, or totally disregarding others directly related to the odd one or two you happen to know. The regulations, BS7671, is not a diy guide with a few pages of tips, it is a comprehenshive book of guidelines, which, when coupled with years of interpreting and training can be used safely. There is the other type of answer which is based on experience, years of training and a little of been there done that. I think the first couple of answers covered it, no need for any more randomly dangerous rubbish to be thrown in as well.
SJW A.C.S. Ltd.
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I think I should reply to my admirers.
1) Go out and read the 16th edition regulations IN DETAIL before committing finger to keyboard. Together with the background clauses in the IEE discussions.
2) Just to give you a clue, 2.5sqmm cable in free air WITH CONSTANT LOADING can be adequately protected by a 30A fuse in a radial circuit.
3) The IEE regulations are a code of practice and not a Bible. The IEE specifically states that the code is a guide.
4) Changing wire gauge is not bad practice. Sorry, but that is the real world.
5) This application is an intermittent loading. Thus cable ratings change.
6) Both of the admirers I noticed had failed to look at the application in sufficient detail to spot the really weak points in the installation.
a) If solid cable is used to connect to a vibrating pump( shower pumps normally are on anti-vibration mountings), it shears! The pump should be connected with stranded wire. b) The fusing problem is not in the CU but locally. The pump should be protected with a local 5A fuse.
7) If you ask what would I have done:
a2) Left the fuse @30A b2) Split the 6mm feed in the loft into a 13A socket( for local convenience) and a fused 13A spur using the local 2.5mm2 cable, both mounted in a surface box. c2) Terminate the 2.5mm2 adjacent to the pump in a 13A socket. d2) Fit a 13A plug flexible lead to the pump fused @5A.
The inherent problem in the setup was the ability of the source to over supply energy to the pump under failure conditions. A 15/16A CU fuse helps in this respect but is not ideal, hence the preference for a local fuse @ 5A
Hope this explains the reasoning behind the statements.
Regards Capitol
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No, it is fine, provided there is ONE fused connection unit at the end (or ONE single or double socket). It is no different from an unfused spur run in 2.5mm on a 30A radial/ring circuit. Overload protection in the 2.5mm spur is from the FCU/plug top fuse. Short circuit and earth fault protection for the cable is provided by having sufficiently low earth loop impedence. As the majority of the circuit is in 6mm and there is only 3m of 2.5mm, this is unlikely to be a problem.
The fuse/MCB rating will cover any cable that has that rating. The cable is designed to stand substantial overload for much longer than the protective device.
However, there is no need for this circuit to be 32A. If it only runs the central heating and a shower pump, I'd replace the MCB with a 16A or even lower. I see no reason to have higher MCB ratings than necessary.
Another alternative is to replace the junction box with a 13A FCU, if it is accessible for changing fuses and checking terminals.
Christian.
P.S. being paranoid, I tend to run shower pumps off an RCD, even when plumbed in plastic.
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But water is a poor conductor of electricity - especially over the distance a pump would normally be from the shower. Wetting *skin* and other materials increases their conductivity, but that's a different argument.
--
*Men are from Earth, women are from Earth. Deal with it.

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Indeed. Paranoia is an irrational fear, not a sensible one!
Christian.
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I will have two FCU's, (one for shower pump and one for CH), hence I believe the 16A fuse at the CU will sort me out.

Thats the next plan when I replace the CU over the next few weeks (/months). At the moment I'm just laying in the groundwork.
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