An RCD for my shower

[Rapporman]

I am replacing a 7.5 kw shower on 6mm cable and installing a 9.8 on the same cable as the run is less that 10 metres. I intend to fit a separate 45amp RCD and MCB type 2 combination but the only ones available are 63amp RCD / 50amp MCB. My question is :- Is the any reason why I should not instal this higher rated combo unit. Any advice would be much appreciated.

 

[Steve]

So there will be only one shower there?

For a moment there i thought you were comtemplating having 2 showers on the same circuit. . .

Anyway, I suggest you buy the enclosure, RCD and MCB seperately. In fact, if this circuit starts at an MCB, why have a second one? Just put an RCD in a 2-wide box. Rating of RCD doesn't matter, as long as its trip setting is 30mA.

Also, there is no need to fit an additional RCD if there is already RCD protection at the CU for this circuit.

Please be aware of part p. //www.diynot.com/wiki/electrics:diy-electrical-work-and-the-law Read and understand that thoroughly.

6mm is rated at 46 amps clipped direct, IIRC, so you must fit maximum 45 amp circuit breaker. But we dont like seeing this, as its right on the limit. Rather you fitted 10mm² cable instead.

 

[davy_owen_88]

As crafty said, you are getting very close to the limit of 6mm² T&E when clipped direct/buried in plaster so are you absolutely sure the cable isn't subjected to any de-rating factors? Insulation, bunching, high ambient temps etc...?

Davy

 

[Adam_151]

6mm is rated at 46 amps clipped direct, IIRC, so you must fit maximum 45 amp circuit breaker. But we dont like seeing this, as its right on the limit. Rather you fitted 10mm² cable instead.

FWIW,

There is a school of thought that says that since the shower is a fixed load, its not possible for it to overload the circuit and thus the rating of the breaker can be higher (of course the design current still needs to be within the cable rating) as long as you do the calcs to prove it still provides sufficent short circuit and earth fault protection (such protection is assumed is long as In is less than Iz as long as the device has sufficent breaking capacity, etc, but if you go away from more standard things you start having to calculate stuff).

Its similar to the spur from RFC situation single bit of 2.5 fused at 32A, but the fuse in the plug limits it to 13A, except this time its a standard circuit and the calcs have been done for you

However the crux of the matter is its just not worth it for most things in houses to do the calculations, and produce design notes to go with the certs, etc, standard arrangements make things easier. I suppose if you're one of these posh people who has amtech then its probably a piece of cake to do, but for the rest of us who would have to scribble fault currents, disconnect times and cable constants on paper and scratch our heads before breaking out the calculator its more trouble than its worth!

 

[Rapporman]

Many thanks lads for your response.

Rest assured Crafty, it will only be one shower on the circut ! Re the comments from Davy Owen 88 and Adam 51, I appreciate your cautionary advice but fortunately none of the existing 6mm cable is buried and is mainly surface trunked or loose under both cellar and bathroom floor, so I considered I would just about get away with 6mm this time. Of course, had I been starting afresh, I would certainly have gone for the 10mm. Anyway, your confirmation that a 45amp MCB and 63amp RCD is ok has set my mind at rest.

Many thanks once again and by the way what a marvelous forum this is. I never expected such a quick response.

 

[securespark]

Rating of RCD doesn't matter, as long as its trip setting is 30mA.

Just to make it clear to others, it does not matter if the current rating of the RCD exceeds the value of the circuit protective device.

Don't fit two showers to one circuit, this tends to happen:

 

[Rapporman]

Point taken securespark !

 

[Adam_151]

Been thinking about that, Secure and despite having seen that type of picture far too many times, I'm not sure I'd beleive that a properly wired fuseboard and fuse would be that prone to burning up when exposed to an overload, that would kinda defeat the purpose.

I suppose there are a lot of old boards out there with loose contacts, vasaline on contacts to help it slip in easier (oh err...), poorly connected circuit and supply cables, and loosely fitted fuse elements (not to mention the ones with two of them fitted when it kept blowing!)

Never been a fan of semi-enclosed rewireables anyway, though

 

[securespark]

The trouble with 3036's is they need effectively an overload of twice the rated to blow instantaneously.

An overload of lesser quantity will stress the components.

 

[RF Lighting]

none of the existing 6mm cable is buried and is mainly surface trunked.....

Therefore it only has a current carrying capacity of 38A