Calculation query for shower circuit

[cornburn]

As above really, just want to double check something.

10.5kW shower, 20m run, under floorboards in insulation, 10mm t&e.

Ib=10500/230 = 45A

As most of the run is in insulation, Ci factor is 0.5, so It must be greater than Ib/0.5

So basically I need a cable with 90A capacity under ref. method C to allow for all the insulation.

Ref method 100 (under less than 100mm of insulation) shows 10mm cable having a CCC of 45A.


Can someone please explain why the difference and what I'm missing? There must be a paragraph in the regs I've overlooked. The CCC table doesn't have any diversity applied to they so I don't get the drop.

Is it that ref method 100 assumes cable has one side touching plasterboard and one side in insulation, and calculating using figures above give the requirements for a cable completely surrounded by insulation?

So;

Clipped direct (Ref method C) - 64A
Ref method 100 - 45A (45/64 = rating factor of 0.7 from Ref method C)
Surrounded by insulation - factor of 0.5.

This means that to get the cable rating above 90A under ref method C would need a 25mm cable!!!


Cheers

 

[SirKit Breaker]

You might be looking a bit too deep mate.
Stick it on a 10mm cable with a 45a MCB and suitably RCD protected, it will be fine. I've done loads of them with no problems.

I know where you are coming from with all the maths but how long does anyone spend in the shower.

Cheers.........Howard

 

[garyinderby]

put cable in trunking or conduit. is then seperated from the insulation.

 

[bugsy]

Diversity - what diversity its a 10.5 kw shower - heating load - 100%

 

[cornburn]

Diversity - what diversity its a 10.5 kw shower - heating load - 100%

Sorry, I got caught up with mentioning diversity earlier on and that's not what I meant and have deleted as it didn't make sense. I was thinking more along the lines of like Howard said, how long the shower would be in use for (several bursts of 5 minutes through the day) and how this would be calculated.

 

[fixthemix]

http://www.tlc-direct.co.uk/Technical/index.html
Try this site loads of calculators to help

 

[SirKit Breaker]

Trying to terminate anything bigger than 10mm T&E into the pullswitch will be a barrel of laughs, 10mm is bad enough.

Ive come across 10 plus Kw showers on 4mm cable. Hidden under 2 feet of loft insulation which have never come to harm until the pullswitch burns out and i go to replace it, but have to give them the good news that yer shower needs rewiring. Usually its been like that for years as well. So i very much doubt 10mm is going to melt or whatever under anything but the most harsh of environments. Cable like any other product has a lot of redundancy built in to overcome the likes of DIYer's and Mr Electric's best efforts. Cant remember the rating of 10mm off the top of my head as i just fit it anyway on showers, but i bet it has at least 50% redundancy for overload.

 

[cornburn]

Thanks Howard. I think it is 50% as you say and I know in reality it will be fine and perfectly safe, I'm more trying to further my understanding of the differences in figures in the regs. Also if I put a job like this forward for assessment and they asked how I calculated the cable size, saying "I found it on an internet forum" or "I used an online calculator" or "because that's what most showers are" isn't going to cut it. They'll want to know how I've gone about finding it using the regs book and at the moment I'm struggling to see what the difference is between the figures above, other than the 0.5 divisor being an extremely cautious figure and covering all types of insulation in the worst case scenario.

Seems odd if that is it though as the table shows a divisor of 0.51 if 500mm of the cable run is in insulation. Surely OTT?