Shaver socket/underfloor heating quick Q

bigstevex · 1 Feb 2010

Before i plaster the walls and hide what i've done forever can someone confirm this is ok,

From the CU is a 4.0mm cable going to a wall outside the bathroom (its from an old jacuzzi bath which is no longer there), its on the 30ma rcb side of the cu, at this wall i've connected it into a jcn box and have 2 x 1.5mm cables out, each going to their own spur on the wall and then onto their own accessory again in 1.5mm, 1 is for a 3 amp shaver socket the other is for undertile heating, also rated at 3amp. In my head this seemed perfectly fine, but in reality is it right?

I've clipped everything as straight as possible so no snakes but obviously the jcn box isn't easy to make neat! Dont want to ever have to access this again due to it going inside a wall and getting plastered.

securespark · 1 Feb 2010

What size is the MCB on that cable run?

Taylortwocities · 1 Feb 2010

If its 4mm at the CU then it should be 4mm all the way through.
If you can't avoid it then you must make a note on the CU and the MCB at the CU must be changed to a 6 or 10amp.

If you have a junction box with screw terminals it must remain accessible. That means you cannot plaster over it.

bigstevex · 2 Feb 2010

its a 16A mcb for some reason! Whats the best way to improve this? That 4mm cable is annoying to work with, its dam impossible to work with! Would 4mm into 1 spur followed by 4mm into the other spur then use 1.5mm to each accessory be more acceptable if i lower the mcb down to 10amp (can i do this myself, or is it not advised to try?)

bigstevex · 2 Feb 2010

This option removes the inaccessible jcn box too!

big-all · 2 Feb 2010

on a practical note as long as you have normal heating to warm the room as 790w max would have to be left on for 24 hours a day to have a chance to heat all but the best insulated or smallest rooms during all but the coldest days

bigstevex · 2 Feb 2010

I have a 5000 btu towel heater on the central heating too, just need the tiles too be heated for the morning/evening

ericmark · 2 Feb 2010

However much you want to write your own set of regulations sorry you can't and the regulations say

433.2 Position of devices for protection against overload
433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation.
NOTE: A reduction in current-carrying capacity may be due to a change in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions.

433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor. or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:
(i) It is protected against fault current in accordance with the requirements stated in Section 434
(ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons (see also Regulation 434.2.1).
433.3 Omission of devices for protection against overload
This regulation shall not be applied to installations situated in locations presenting a fire risk or risk of explosion or where the requirements for special installations and locations specify different conditions.

I will agree it says current carrying capacity not cross sectional area alone but the only way you can feed with 4mm cable and reduce to 1.5 is to use fuses or MCB's I would consider a grid switch would be easiest way to comply as you can have multi-fuses in the one unit. You can also have isolation switches as well. Although there is the 3 meter rule it still requires a fuse but it could be at end rather than start of run as with a spur on a ring main where the fuse in plug ensures no more than 26A can every be draw from a spur.

I also fitted underfloor electric heating to a wet room mainly to dry floor. Any hotter it would not be able to walk on with bare feet. But it does not keep room warm without the towel rail also being on and it takes 20 minutes to dry floor and so my mother still has to walk on well floor. Idea was to prevent slipping as she has one leg only.

So sorry come to conclusion underfloor heating useless in wet rooms. OK yours is bathroom but similar.

And remember the LABC will not pass installations which don't comply with BS7671:2008 and since a bathroom under Part P they need to inspect. So you need to follow regulations.

Paul_C · 2 Feb 2010

ericmark said:
However much you want to write your own set of regulations sorry you can't and the regulations say

433.2 Position of devices for protection against overload
433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation.
NOTE: A reduction in current-carrying capacity may be due to a change in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions.

Click to expand...

But that rule is clearly not applicable when the protective device at the origin of the circuit already provides adequate protection for the lowest-rated cable in the circuit. If you were running, say, a 2.5 sq. mm. circuit through an open roof space with a 20A MCB at the origin, but due to derating factors a few yards of cable down to the distribution panel needed to be 4 sq. mm., you wouldn't need to put another fuse or MCB at the junction box where you change from 4 to down 2.5 cable, since the 2.5 would already be adequately protected by the 20A MCB.

Although unconventional, there's nothing fundamentally wrong with using the existing 4 sq. mm. cable as a convenient feed and tapping down to 1.5 sq. mm to the spurs, so long as the MCB is rated to protect the smallest cable in the circuit (e.g. 10A).

And remember the LABC will not pass installations which don't comply with BS7671:2008

You know that LABC has no authority to demand that an installation complies fully with BS7671, only that it is safe, although some of them seem to think otherwise. But notwithstanding the Reg. quoted above which clearly does not apply in this situation, I'd say it would comply anyway.

If this doesn't comply due to 433.2.1, then neither do hundreds of other arrangements where a larger cable is used for part of the run due to derating factors or voltage-drop issues.

bigstevex · 2 Feb 2010

Does it matter if they are fused spurs? I thought that with it being a fused spur then thats its protection, i think i missed that bit off sorry!

I don't mind having it checked etc hence why I'm trying to do it right then ring my electrician to come and double check it for me etc!

Whats the easiest way to do this right then?

Lets start a fresh, I've got a 16A mcb at the board (30ma rcd), 4mm cable going up to the wall outside the bathroom and 2 fused spurs.

1.5mm cable going to the shaver socket and 1.5mm going to the underfloor heating controller (this can't be removed due to their routes and it would mean destroying walls. The cable has enough capacity for what its being used for!

bigstevex · 2 Feb 2010

Missed the second responce while i was typing!

So in theory i can leave the 16A mcb in because the 1.5mm would be protected by the spur fuses

So

run 4mm to one fused spur then to the next, and I can keep my 1.5mm from the switched side of each spur to the relevant device?

ban-all-sheds · 2 Feb 2010

bigstevex said:
So in theory i can leave the 16A mcb in because the 1.5mm would be protected by the spur fuses

Depending on how it's installed, 1.5mm² could be OK with nothing other than a 16A breaker.

You should know that.

bigstevex · 2 Feb 2010

should i lower the mcb anyway then to 10A to be safe? Other than that is my revised plan safe/compliant?

bongos · 2 Feb 2010

bigstevex said:
should i lower the mcb anyway then to 10A to be safe? Other than that is my revised plan safe/compliant?

ban-all-sheds said:
Depending on how it's installed, 1.5mm² could be OK with nothing other than a 16A breaker.

Taking note of how it's installed, you already have your answer.

But you must make sure that junction box is accessible.

bigstevex · 2 Feb 2010

Lol, this is getting very conflicting/confusing now....

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Shaver socket/underfloor heating quick Q

Paul_C

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