8.5 Kw shower on 6mm tripping MCB

[olsonn]

Went to a job today where a lovely new bathroom has just been fitted upstairs with a new 8.5 Kw shower replacing a 7.2kw one. After a few minutes in the shower the MCB trips. The shower was fully turned up and when I asked the customer they said that they always ran it like this. On inspecting the cable size it was 6mm. The cable is run up an internal concrete wall and appears to have been plastered in and not run in conduit The run is about 12 metres. It also runs through insulation in the loft As a matter of course I would run this size shower in 10mm. It is possible to run a replacement 10 mm cable up from the CU using trunking, but the cable from the switch which is also in 6mm and about 1.5 metres long, is buried behind a lovely expensive tiled wall and doesnt want to come out. Would it be acceptable to leave this short run in 6mm or should I tell the customer that we have to remove the tiles and replace the cable.

 

[telectrix]

i suggested moving the insulation away from the cable. OK. brb give guidance on derating where necessary, due to insulation, but at the bottom of table4d5, page 282, read the note.

 

[PC Electrics]

I am also worried by the fact that it is burried directly in the wall for the majority of its run and not in conduit. I cant see anywhere in the tables where this type of installation is addressed. Perhaps somebody knows differently

Buried directly in a non insulating wall is same as clipped direct. So ccc = 47A. FFS!

 

[telectrix]

Is this for real?

If an 8.5 shower is tripping a 40A mcb, then chances are it's a faulty mcb. But check the shower current draw in case it's faulty (or it's not an 8.5, perhaps).

Nothing wrong with 6mm on 40A as long as it's clipped direct. just lift the cable out of the insulation and clip it to the woodwork. Extend it if necessary. Use a couple of lengths of 4 inch waste pipe to keep an air space around the cable where it must pass through the insulation layer. No need to spend £££s on 10mm.

my thoughts exactly.

 

[telectrix]

Just re-read this and IMO it is the cable that is the problem as in it is now under size for the 8500kW new shower due to it being run under that insulation. It's all ok when it starts off but as the cable warms up it can't dissipate the heat, resistance goes up current drawn increases etc. I'm looking at Table 4D5 which suggests 6mm2 sq, Ref Method 100# is only good for 34A if under 100mm of insulation and only 27A under more than 100mm.

I've had a beer so........daft idea maybe but how much room do have above the ceiling in the bathroom? You couldn't get in there with a 1m long SDS and drill a new hole down to the shower could you? I said it was mad! What's behind the wall? Come down the other side? Sorry, its the Amstel talking!

i always thought that increased resistance meant lower current. i agree that problems will arise if cable overheats, but 8.5kw ( manufacturers optimistic rating ) is only 35A. get the candy floss away and 6mm is good for 45A.

 

[i=p/u]

i have a 8.5 kw shower on 6mm and 30 1361 fuse, nevr no bother

 

[telectrix]

i think a lot of this 10mm nonsense for showers over 8KW (or 8.5, can't quite remember) came during the early years of the 16th whereby the IEE were worried about volt drop. this advice was amended as long as the total cable length didn't exceed 20meters. (that's 22 yards to those of us who still speak english)

 

[i=p/u]

yard brush, get with times, punk

 

[telectrix]

my yard brush is 6foot long. that's 2 yards.

 

[i=p/u]

multiply it by size of your mop and thats the length of run in meters

 

[telectrix]

meters are for measuring things like amps, volts, ohms, or, as my old man used to say, for putting shillings in to get the leccy on.

 

[i=p/u]

50p meter in my first flat in isle of man.. AND NOW I HAVE £1 FOR OIL....

 

[telectrix]

steady on, mate, you'll be joining OPEC next.

 

[i=p/u]

petroleum market.... dont sniff it anymore, last ages

 

[GLENNSPARK]

And maybe carry out basic insulation tests on the cabling?

so you didn`t carry out these tests first before posting?....so im assuming its on an RCD..........

 

[GLENNSPARK]

meters are for measuring things like amps, volts, ohms, or, as my old man used to say, for putting shillings in to get the leccy on.

Meters........sorry was thinking about lamb for a minute there Tel...................