8.5 Kw shower on 6mm tripping MCB

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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.
 
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there's a problem there then. 8.5KW is 35-37A so you may have a tired MCB.or a faulty shower. i would first of all, clamp meter the cable in the CU and see what the current draw actually is. and if you clear insulation from the cable in the attic, 6mm is fine for 40A.
 
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In these cases I always say Bl**dy plumbers why because if it is a 3036 or fuse type fuseboards they are quite happy to "upgrade" the old tired 7.5Kw to a 9Kw jobbie and then muggins gets the job when he is called out to a job and has to tell them the switch is burnt out also the shower is too big for the cable then I get hassle from the customer who says you are just trying to create work and the plumber is a very nice man and I reply well yep but he aint a electrician is he.

PS have you checked for a slack connection ?
 
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1. replace MCB ( make sure it's the same brand as the CU , as, if anything goes wrong with the CU and it's got a different make of MCB fitted, the manufacturer will deny responsibility ). then clamp meter it to see if the shower is drawing excessive current. only when you have eliminated these possibilities should you think about demolishing walls.
 
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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!
 
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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
 
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clicon357 said:
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!
Click to expand...

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.
 
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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.
 
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olsonn said:
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
Click to expand...

Buried directly in a non insulating wall is same as clipped direct. So ccc = 47A. FFS!
 
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PC Electrics said:
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.
Click to expand...

my thoughts exactly.
 
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clicon357 said:
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!
Click to expand...

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.
 
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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)
 
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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.
 
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telectrix said:
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.
Click to expand...

You've confused me now Tel...

I always believed that the current drawn would stay the same, but to the detriment of the cable? Will the shower still not draw what it wants to draw, regardless of the whether or not the cable CSA is appropriate? With the consequence being that ultimately the cable would be destroyed if this was way off the mark?

Sorry if that is a stupid question :6:

Cheers
 
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8.5 Kw shower on 6mm tripping MCB
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