Electricians! 8.5kw shower on 6mm cable help

zax47

Did you go to the link I put in my first post and play with those numbers? 6mm is definitely undersized for 8.5kW! Even a 7kW shower is borderline for 6mm, depending on a whole host of other de-rating factors like how long the cable is, how it is installed (run through insulation etc.), bunching (cables run together) and other de-rating factors.

It won't have helped the situation, and the heating of this cable may well be the major contributing factor to the problem in the first place (the bodge repair by BG wasn't to blame for the original failure!).

Usual case is that an old 5.5 or 6kW shower has been replaced/upgraded without a thought for the supply cable!

deano72_2

i had look at mates shower other day, 8.5kw wired in 2.5mm the plumber who wired it in said that was ok! unbelivable the cable is fine but defo needs replacing with 10mm asap.like everyone else has said,suspect you had faulty conection,nether the less it needs replacing with 10mm

Homersimpson

Hi,

I'm an electrical design engineer for domestic, industrial, commercial and healthcare buildings and have 12 years experience so hope this is of use.

An 8.5kw shower requires approximately 36A to run it. A 6mm2 cable (assuming PVC Twin and Earth 70oC) is rated at 34A (Reference method 100 table 4D5A BS7671:2008 IEE Wiring Regs 17th Edition). This does not take into account any derating factors such as grouping. Reference method 100 is for cables above a plasterboard ceiling covered by less than 100mm of insulation. If you have more insulation than this you need to reduce the values further. You can look at this table here http://www.batt.co.uk/upload/files/4d5_1236703670.pdf

Therefore a 6mm2 cable is undersized for the application and will run warm and eventually will fail. As mentioned above, 10mm2 is the way to go and the cable rating for this using the above table and installation method is 45A. I would still use only a 40A MCB maximum for this but you need to check grouping and thermal insulation etc before you can confirm this.

The reason that your 32A MCB doesn't trip is simply that any overload protective device whether it be a fuse or MCB requires a significant amount of current to flow before it trips which usually only occurs with a fault. A current slightly above a devices rating will not cause the device to trip, you need to look at the time current curves for a device to see what it actually is. I don't have my regs book to hand but I have found the table at this link on the net, not sure how accurate it is but the principle remains the same, http://www.tlc-direct.co.uk/Figures/3.18b.gif

This a for type B MCB devices and you can see that a 32A device will take 40A for in excess of 10,000 seconds. To disconnect in 5 seconds you need around 130A to flow. Therefore for the duration of your shower and beyond the device is quite happy and doesn't see a problem.

This is why its important to size the cable correctly for the load, one of the first things I was taught at college was that an overload is far more dangerous than a fault.

I hope this crash course in electrics is useful, if you need anyfurther information please let me know.

baldelectrician

Yep the BG guy is right- 10mm required

I have even had to wire showers in 16mm as there was lots and lots of insulation (completely surrounded)

vaio

It depends critically on the method of installation and particularly if insulation is involved.

The shower takes 36A,

a 4mm cable clipped to a wall with no insulation involved will carry 37A so can be used (just) up until the length gets to about 25m when volt drop becomes an issue.

Similar figures for 6 & 10mm are 47A & 64A at 50 & 75m.

Bury the cable in insulation and the current capacity drops to about 50% of the above figures.

Basil1234

one question no one has seemed to ask i take it british gas never orginally fitted the shower?

zax47

Not sure that's relevant? However, the OP says "We're covered by british gas homecare", that's why BG are involved - they did the first repair (badly) and that's why the OP is contacting them again.

vaio

mealticket wrote: »

i don't know where to buy 4mm twin and earth, but if i could, i wouldn't use it for a 6kw load, let alone 8.5k

City, Newy, even screwfix have it on the shelf, 7671 will tell you what load you can use it for depending on how it is installed

Homersimpson

vaio wrote: »

It depends critically on the method of installation and particularly if insulation is involved.

The shower takes 36A,

a 4mm cable clipped to a wall with no insulation involved will carry 37A so can be used (just) up until the length gets to about 25m when volt drop becomes an issue.

Similar figures for 6 & 10mm are 47A & 64A at 50 & 75m.

Bury the cable in insulation and the current capacity drops to about 50% of the above figures.

Very true but lets not confuse electrical theory with the practical reality of installation, if you want to talk theory PM me, I could bore for england on the subjects of Total Harmonic Distortion and non simultaneous overload!

The OP is unlikely to have a cable installed in accordance with reference method C and we have already established that a 32A MCB is not sufficently rated for the load. I personally would use a 10mm2 but I would need to know how the cable is installed (including thicknesses of insulation that the cable is under or passes through to properly calculate this).

As an interesting point, lots of people are having their lofts insulated with 10" of insulation which in many cases changes the existing cable instllation method from 100 to 101 and dramatically reduces the cables rating. On this basis a 2.5mm2 ring main with a 32A protective device would fail (based on the 0.67 rule).

Basil1234

zax47 wrote: »

Not sure that's relevant? However, the OP says "We're covered by british gas homecare", that's why BG are involved - they did the first repair (badly) and that's why the OP is contacting them again.

the relevance was if they put in the orginal cable then they could be liable apart from the fact they did an extremely poor repair boarding on dangerous really they should have just condemed it (the electrical line) and disconnected it and never repaired it in the first place!
also it would be cheaper to get in a local sparks to do the refit rather than bg. my mate is a sparks in local hospital and myself work for international air cond manufacturing company as there electrical tester point being in this case insulation would have been weaked once the first fault had occured which really should have screamed problem with draw/rating to anyone with any training.