Need some help!

[sparks72]

Went to a job yesterday,
woman has had electric underfloor heating installed last year in her bathroom,
no certification, on a 50 amp mcb on the RCD side of the CU, installer had used the old 10mm shower cable
& jointboxed it to a 2.5 mm T&E to a switched fused spur!
Installer has dissapeared into thin air as usual (I reckon her partner did it)
but he's tight staying tight lipped!

She doesn't want a new feed installing as I advised,
anyone have any ideas of what can be done to tidy up this mess

Thanks in advance.

 

[Spudnik]

Nothing wrong with what you have described, although the 2.5 feeding the spur should really be 10mm also.

However, you could drop the MCB to a 16 or 20A

 

[sparks72]

Nothing wrong with what you have described, although the 2.5 feeding the spur should really be 10mm also.

However, you could drop the MCB to a 16 or 20A

Thanks but 10mm is way to big!
the cable going to the heating mat is 2.5mm,
I agree with reducing the MCB to 16amp but what about voltage drop?
4mm on a 16 amp mcb would have been fine,
will change the 2.5mm to 4mm or 6mm me thinks, oh well, another day another mess!

 

[Eggster71]

Install a spur on a deep surface box where the JB is. Then downsize to your 2.5. I know for a fact a 10mm can be terminated in an MK spur, but you'll have issues dressing it in a 25mm patress hence the deep box.

 

[m4tty]

Hi,

Is there a big problem with what you have described as whats the max current that will be pulled down the 10mm & 2.5 to fsu. Fused at 3A for heat mat ?? Bit rough but not a problem i dont think. Prob get shot down tho now .... hardhat on

 

[Eggster71]

Hi,

Is there a big problem with what you have described as whats the max current that will be pulled down the 10mm & 2.5 to fsu. Fused at 3A for heat mat ?? Bit rough but not a problem i dont think. Prob get shot down tho now .... hardhat on

Potentially a 50A fault current through the 2.5.

 

[sparks72]

Install a spur on a deep surface box where the JB is. Then downsize to your 2.5. I know for a fact a 10mm can be terminated in an MK spur, but you'll have issues dressing it in a 25mm patress hence the deep box.

Thanks guys,

Eggster71, I was worried about the 10mm not fitting in a sfs, haven't used MK for a while,
I'ts a plasterboard wall so getting a deep box in isn't a problem.

 

[Eggster71]

Good luck!

 

[pmac10]

Potentially a 50A fault current through the 2.5.

and??
what you want to look at is the rating and response time of your protective devices,
2.5mm cable will handle many hundreds of thousands of amperes (never mind 50), for a short period.
this is what IET 17ED Tables are for

 

[Eggster71]

Potentially a 50A fault current through the 2.5.

and??
what you want to look at is the rating and response time of your protective devices,
2.5mm cable will handle many hundreds of thousands of amperes (never mind 50), for a short period.
this is what IET 17ED Tables are for

So you'd run a 2.5 off a 50A mcb and walk away would you?

 

[pmac10]

So you'd run a 2.5 off a 50A mcb and walk away would you?

yes,my friend, work out the load and calculate accordingly, a 100Amp breaker, no doubt protects your house, is your house wired 25mm SQ copper??? doubt it.

 

[Eggster71]

yes,my friend, work out the load and calculate accordingly, a 100Amp breaker, no doubt protects your house, is your house wired 25mm SQ copper??? doubt it.

No, my house is supplied by 25mm tails which are protected by a 100A fuselink and terminated into a mainswitch rated at safe for 100A isolation. The final circuits are protected by individual MCBs each one rated below the stated maximum rating of the cables used. I did not calculate those given ratings, someone a great deal wiser did. I am not about to argue how they came to those figures as I don't know. However, I have in the past gone to jobs where a loose connection has caused an arc, this has in turn taken out the MCB. I wouldn't like to think I'd left the potential of a 50A fault on a 2.5 rated at somewhere in the region of 20A. If the wiring was in metal trunking as in an industrial set up running to a device to limit the draw then I'd have less of a problem with it. As it stands, in a domestic situation, (I'm guessing in a airing cupboard where it is quite possibly going to be buried under clothes/ towels etc) there is no way I would not take an extra half an hour and a few quid's worth of spur to make sure that the 2.5 had adequate protection!
And breathe..........
No offence dude, but I'd always prefer to cover my ---!

 

[pmac10]

good points, all correct and i agree, i,m industrial and just had a different take on the question.
by the way, power=volts*amps as im sure you know, thats how i decide on protection rating, and like you said , err on the side of caution.

 

[sparks72]

Potentially a 50A fault current through the 2.5.

and??
what you want to look at is the rating and response time of your protective devices,
2.5mm cable will handle many hundreds of thousands of amperes (never mind 50), for a short period.
this is what IET 17ED Tables are for

What i'm trying to put across is that the installation has been botched from the start,
I've been asked to sort it out, a 10mm cable to a 2.5mm that's Jointed under the floor
isn't good practice for a start, I couldn't care if it's supplied with a 185mm SWA as long as its
done correctly

BTW, got on to the underfloor heating manafucturer, they ask for a 16 amp MCB.

 

[Eggster71]

What i'm trying to put acroos is that the installation has been botched from the start,
I've been asked to sort it out, a 10mm cable to a 2.5mm that's Jointed under the floor
isn't good practice for a start, I couldn't care if it's supplied with a 185mm SWA as long as its
done correctly

BTW, got on to the underfloor heating manafucturer, they ask for a 16 amp MCB.

Happy days then, lob out the 50A sling in a 16A, chuck the 10mm and 2.5 in a 20A switch and everyone's a winner!