Spur off shower cable???

[ceolmar]

Is it possible (and/or safe and/or allowed) to take the power from a 10mm shower supply cable to provide power to a combined shower extractor fan/LV downlight? If so, how is it done?

 

[BigJon]

You would need to terminate a 10mm loop feed in to a 3 amp FCU and take fan/light from that in 1.5 or 1mm T+E, But to be honest its not good practice and a bit rough.
Can you not use the existing lighting wiring or get a loop feed from somewhere close by?

 

[plugwash]

is this shower circuit still in use for a shower and if so is there enough spare capacity.

once you've determined you have the capacity then if the circuit is still in use for a shower use a bigass junction box (e.g. http://www.tlc-direct.co.uk/Products/AAJB60.html) to take the spur. Otherwise just disconnect it from the shower pull cord.

While not strictly nessacery its best practice to take the spur in the same size as the rest of the shower circuit.

then take the cable to a FCU (use a good brand like MK on the deepest box you can and if at all possible arrange it so the 10mm can slide straight in and out of the box as the front is fitted/removed) fit an appropriate rating of fuse and your done.

 

[securespark]

While not strictly nessacery its best practice to take the spur in the same size as the rest of the shower circuit.

It is absolutely necessary to match the cable size to the device rating.

 

[plugwash]

pretty sure overcurrent protection can be downstream as long as short circuit protection is upstream.

i'm also pretty sure that modern breakers can provide adequate short circuit protection for short runs of cables with a far lower current rating than the breaker.

 

[Adam_151]

pretty sure overcurrent protection can be downstream as long as short circuit protection is upstream.

i'm also pretty sure that modern breakers can provide adequate short circuit protection for short runs of cables with a far lower current rating than the breaker.

473-01-04 Devices for protection against overload need not be provided:

(ii) for a conductor which, because of the characteristics of the load or supply, is not likely to carry overload current

You then have to size your conductors and cpds in accordance with 434-03-03 (the adiabatic), as 434-03-02 no longer applies

one day when I have time I'll write a document on it and how to apply it differernt situations (fuses, breakers, etc)

(its something that seemed interesting to me at one point... so I researched it)

 

[plugwash]

473-01-04 Devices for protection against overload need not be provided:

(ii) for a conductor which, because of the characteristics of the load or supply, is not likely to carry overload current

that would i presume cover a conductor with a fuse downstream

You then have to size your conductors and cpds in accordance with 434-03-03 (the adiabatic), as 434-03-02 no longer applies

the saving grace with modern breakers is that once you hit the fast trip point they trip very fast (standard says 0.1 sec iirc but some manufacturers claim much better) so thermal effects should be minimised.

 

[Adam_151]

When you get to faster than 0.1 seconds you should refer to the I²t value published by the device maker (says 434-03-03), with a fuse this will be a constant (you need a set amount of energy to vapourise a fuse, once you get to the point that its fast enough that very little excapes its a constant value whatever the PFC [until you get higher than the fuse can safely break!])

However with a breaker you have a graph like this (this is for MK sentry B curve domestic breakers)

http://www.twobeds.com/upload/userfiles/adam_151/mkbreakerisquaret.GIF

(The standard says IIRC that this information has to be make available and I extracted the above from a MK sentry technical PDF, I have similar infor for hager and wylex, and for bussman service fuses)

as there is a physical limitation on the speed the contacts will open at so higher PFC = higher I²t for breakers

The max a cable can stand is S²K² where S is size in mm² and K is a constant depending on the cable (its 115 for T&E IIRC)

 

[plugwash]

i guess 434-03-02 is a rule of thumb? care to spell out exactly what it says i don't have the regs handy.

you also have to consider that the cutout fuse will have a maximum letthrough too.

 

[Adam_151]

i guess 434-03-02 is a rule of thumb? care to spell out exactly what it says i don't have the regs handy.

you also have to consider that the cutout fuse will have a maximum letthrough too.

It basically says that where a circuit protective device meets section 433 (that is, it protects a cable from overload) and it is capable of breaking the fault current at the point where it is installed, it can be assumed that the conductors are also protected against fault current

the assumption should be checked for conductors in parralel and some non current limiting types of breaker

(not the exact wording, can scan it if the exat wording is important to you later tho )