Replacing 9kw shower with another 9kw shower!

[thomasp]

Hi all,

With my Mira Supreme 9kw shower finally giving up on me I reluctantly forked out for a Mira Advance ATL 9kw shower as it was supposedly the best 'fit' in terms of pipework.

Anyway, I had a plumber visit today and aside from the pipework not being as easy as first thought he also mentioned as an afterthought that the 32A MCB in the fuse box should actually be a 40A. So the shower wasn't fitted and he walked away with a £30 callout

Now, as far as I understand it, if my cabling is 6mm then I'm really stuck but if it's 10mm then I might get away with just replacing the 32A MCB for a 40A MCB?

So, my 2 questions are:

* How can I tell which size of cabling I have?

* Is it really possible just to change the 32A MCB unit for a 40A MCB if my cabling is 10mm?

I should also maybe mention that the consumer unit is fitted with a 30mA RCD.

Hope that makes sense!

Cheers.

 

[skenk]

Hi all,


So, my 2 questions are:

* How can I tell which size of cabling I have?

* Is it really possible just to change the 32A MCB unit for a 40A MCB if my cabling is 10mm?

I should also maybe mention that the consumer unit is fitted with a 30mA RCD.

Hope that makes sense!

Cheers.

*Approximate overall dimensions of t&e are in the wiki: //www.diynot.com/wiki/electrics:cable_types:flatpvccables

*Yes but the circuit should be tested at the same time for safety (i'd wager that it's a 6mm cable anyway), could be notifiable as well?

If you weren't getting tripping with the old shower then you could try just replacing the shower and leave the 32a mcb. It's wrong design and bad practice but at least the cable would be adequately protected from overload by the 32a mcb.

 

[davy_owen_88]

As linked to above, the approximate outer dimensions of twin and earth cables are listed in the wiki, the problem is that insulation thickness varies between manufacturers so if the size of your cable comes somewhere between 6mm² and 10mm² you will either need to assume it is 6mm² or measure the actual conductor dimension with a caliper.

If you do have 10mm² it is also not a simple case of upgrading the MCB. You must confirm that the cable is capable of carrying that current which will involve inspecting the cable along its entire length and applying any derating factors applicable.

 

[Cremeegg]

As a general rule - but no more - if you can see the earth wire - for 6mm its single core; for 10mm its stranded.

 

[thomasp]

Many thanks for the quick replies.

I take it that the best place to look for cable size would be where it connects to the isolation switch?

Cheers.

 

[securespark]

That or the shower unit itself. Or the inside of the CU.

 

[thomasp]

Oh well, it's 6mm cable as predicted.

So, I suppose the obvious questions are these:

* is it possible to upgrade ONLY the 6mm cable to the shower or will it have to be the entire house?

* how expensive can I expect this to be?

Cheers!

 

[Sailbadthesinner]

The problem with shower circuits is they only get used for a few minutes at a time so overloads like using 9KW (37.5A)from a 32A MCB often go unnoticed. 40A from a 32A MCB can take over an hour to trip!

You need to check the route the cable takes from CU to Isolator to Shower. If and only if the entire run counts as 'clipped direct' ie it is mounted on wall and ceiling surfaces or embedded directly in plaster so that it is not restricting the heat flow by any loft insulation nor by being enclosed in conduit nor running near hot water pipes nor through warm airing cupboards, then you can uprate the MCB to 40A using the existing 6mm2 cable.

If you are lucky enough to be able to see the entire length of the cable then it should be easy to replace it with 10mm2 anyway . If you can't see it you probably can't be certain how it's installed

If you change either the cable or the MCB it will be notifiable under Part P

Or you could take a bath!

Sailbadthesinner.

 

[thomasp]

You need to check the route the cable takes from CU to Isolator to Shower.

Hmm, well I can see the 6mm cable leaving the bottom of the CU to presumably go upstairs within the plasterboard wall cavity.

I can see 6mm emerge in the loft and go down to the Isolator switch and 6mm return from the Isolator switch back into the loft to then go to the shower.

I just can't see the part between CU and Isolator but it is a fairly short run, about the length of a normal staircase give or take.

Thanks again

p.s. is there any rule of thumb you can apply if you know that the house was built within the last 20 years and that the walls are plasterboard? Would that help in working out how the cable is attached?

p.p.s. here's a picture of the MCBs already in situ. Anyone know if it's still possible to get a LN5940s Type 2 of this particular variety?