Hi,

I'm installing a 10.8 kw shower and looking at other forums everyone is saying 10mm is OK however i can't see how.

10800W/230V=47A

breaker size 50A

the only way it can be a 10mm is if its clipped direct with no insulation as it can take 64A however if it is any other reference then the rating is less than 50A.

so how is 10mm enough to carry the load?

what have i missed?

Edited: 19 July 2012 at 08:40 AM by DSpark

10mm will be fine, think how long the shower will be in use for at a time, Also would you really like to try and get 2 x 16mm t/earths into a shower switch?

The other point you may want to take into account is the fact that a shower can't overload - so the cable only needs to be able to carry 47A provided the 50A MCB provides short circuit and earth fault withstand protection (and you will almost cetainly have an RCD for earth fault protection anyway).

So for reference method C and method 102, 10mm2 complies and you probably won't find any other methods in a domestic.

Equally, the ambient temp probably isn't 30C - and as dave pointed out above, how long is it on for.

It would probably take about an hour of continuous operation for 47A to warm up a 10mm2 T&E - most showers will be over by then - unless you have teenage daughters of course.

Regards

OMS

-------------------------
Failure is always an option

Thank you very much. I did have this in mind but needed it confirmed as I was always taught that the cable rating has to be higher than the breaker. the circuit is RCD protected so its all good. The only thing I was worried about is the cable over heating which you rightly mentions can not overheat and wouldn't be on for that long.

Also i have notices everyone says that the most common is method 102, people have insulation in the ceiling and as that is the worst case scenario (Lower current carrying capacity) wouldn't that be the method? even if its less than 100mm its still 45A.

Might be worth checking the small print - marketing department like big numbers - you might find that the 10.8kW rating is at 240V rather than 230V - meaning it's rated to draw just 45A. (It's a simple resistance load so the draw will be lower at lower 230V with a corresponding drop in output power.)
- Andy.

Method 100 perhaps ?

You aren't in the habit of putting the cable under the insulation are you ?

Even at a tabulated rating of 45A, that's at 30C - a few degrees cooler and you are back at 47A.

Regards

OMS

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Failure is always an option

I think you will find that the output ratings for showers are at 240V not the nominal 230V. You may wish to re-do your calculation using that voltage and you will be OK with 10mm even applying rating factors.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/

lol no but some places once ive wired the building they put insulation before they close up in the ceiling so that would be method 100... am i correct?

ohhhhh I thought we have calculate using nominal voltage ie 230V. is that not the case? do we have to calculate the current using the voltage they have worked out 10.8kW with?

meaning if 10.8kw is worked out using 240v, at nominal voltage (230) the power will be less than 10.8kW?

You should calculate at 230V - that doesn't mean you can't "interpolate" manufacturers data if they give a rating at 240V.

Regards

OMS

-------------------------
Failure is always an option

PS thanks to everyone for the prompt replies

It will - it's just a simple resistance relationship as andy pointed out above

Regards

OMS

-------------------------
Failure is always an option

so as they teach us in college that the breaker rating should always be higher than the cables current carrying capacity is not always the case, mainly where fixed items cant be overloaded. is that correct?

so college weren't entirely right however i do understand why they don't teach this as its not best practice

"so as they teach us in college that the breaker rating should always be higher than the cables current carrying capacity is not always the case, mainly where fixed items cant be overloaded. is that correct? "

Not at my college we don't!

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/

College weren't right - although I guess they have given you a simple method for every eventuality - which would have driven you to a 16mm2

There is nothing "not best practice" about it though - you just need to understand the difference between short circuit protection only and short circuit with overload protection.

Regards

OMS

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Failure is always an option

Presumably they don't teach you much about ring final circuits either then?
- Andy.

Ouch !!

OMS

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Failure is always an option

why not put a 45A fuse in? or even a 40A it wont blow unless the teenage daughters turn up!!
Not sure this was about ring mains but hey ho!!

You will often find my shower circuit cables run in lofts up on the braces, running along purlins or else a piece of timber installed for the purpose of lifting the cable up out of the insulation then dropping as near vertical as possible down to the shower unit, avoiding running cables under the insulation.

Andy

--these old timers do bang on about 230v.
i was at a split phase (or to be correct 1 phase 3 wire) tank today with a
large grain dryer installation and as i sat pondering the industry i thought
what is the voltage of the month for this 480/240 in the "wiring regs"
at the moment.
never mind--life goes on.

Regards

That will probably depend whether the dryer is running or not depending how far out on the network you were ;-)


Off topic but...I heard today that it's all about to start in central Essex with some farmers beginning to nibble at their oil seed rape and others gearing up for a start next week so wait for the calls for burning cutouts, blown fuses on pole transfomers etc as the madness of harvest and dryers running 24/7 reaches us.

Stu