shower fuse rating

[topbanana]

have installed a 8.5 kW shower on 6mm cable, with a 32amp breaker.
is this ok?
cable run is 13 metres, in cavity wall and through floor without insulation.
i'm have prefered to change to 10mm but cost prevented this so i'd rather keep the 32amp breaker rather than change to a 40amp.
breakers protected by 30mA rcd.

 

[JohnD]

Hello, Neil.

I calculate 8500 watts divided by 230 volts to be 37 amps (though if it had been rated at 240 volts, which it probably was, then only 35.4). Either way, it's a bit above your MCB so likely to trip when at full heat.

Have a look at http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
if the cables are enclosed within a thermal wall, it looks to me like 10mm, but if they had been on the surface your 6 would do.

You can also look at http://www.kevinboone.com/cablecalc.cgi these are on the On-line Sources of Reference Materials: which you will find on the "For Reference" section //www.diynot.com/forums/viewtopic.php?t=7553

 

[topbanana]

thanks 4 your reply.
1. since i can't replace the cable at present would i be right in saying a lower rating mcb tripping is better than the cable overheating?
2. if and when i can replace the cable, i would only be able to replace from consumer unit to the shower switch and not to the shower. this would remain at 6mm. is this ok?

 

[Big_Spark]

TB..the current situation is the best of a bad situation really. The cable can handle higher loads than 32A, so in theory you could uprate the breaker to 40A, however this is then above the max permissable for 6mm² in this situation.

I would suggest that you leave the circuit as is, but if it keeps tripping and is an issue, call in a spark to look at the situation or if you feel confident and competent enough look for a solution yourself..

I do not advocate it, but the 6mm² could handle a 40A breaker without much danger..

 

[topbanana]

thanks for the info big.
i'll keep an eye on it.

 

[kai]

I thought that a 6.0.mm twin and earth cable could only be fed from a 40amp breaker, only if mounted on perforated cable tray throughout, and even then you will struggle in getting a low enough R1+R2 in order to fire the breaker in an earth fault situation.

 

[Adam_151]

You could argue though that the breaker is only to provide short circuit protection and that protection against overload is via the fact that the load is fixed

But if DIY Dave comes along and uprates the shower...