Mira Shower Manual / Cable Concerns

[Marmaladecoops]

Hi

Could you tell me what size cable the Mira Sport Max 10.5 KW should ideally use?

The reason I ask this is because I have just moved into a new house that has this shower installed using 10mm2 cable and a B40 MCB (40amp). Looking through the instruction guide downloaded off Mira web site I notice that the cable holder can take up to 16mm2 but does not state an exact cable size.

I also note that in the manual that shower requires 45Amp MCB, currently my consumer unit has a B40 (40Amp) MCB, connecting to 10mm2. In addition I have found out that next MCB size up is B50 (50Amp) which seems too large for the job.

I have a 100A Consumer Unit with a 30mA RCD.

Based on the Mira manual, stating that a 45Amp MCB should be used, and using the IEE Wiring Regulation as shown below, if I interpret the table correctly then this shower would use the 10KW?

Could you therefore tell me if the 10mm2 cable and the B40 MCB within my consumer unit are fine and safe for use with this shower?

Thanks in advance for your help.

MAC

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The table is on this website

http://www.diydoctor.org.uk/projects/electricshower.htm

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Ok I know the above sounds a little formal, the reason is because I have send this also to Mira without response.

Any help would really really be helpful

Cheers

MAC

 

[EddieCurrent]

Hello,
I think that we need to know a bit more info before we can advise you on this.
How long is the existing cable run ? and how it has been installed (which method,( in plaster, conduit etc) ?.


I'm sure the lads on here will be able to give you the info you need.
Cheers.

I have just read your other post about the shower, sorry.!
Sounds good to me !

A PIR is a schedule of tests that check that the characteristics of the circuits are within the minimum recommendations given in bs7671 ( the elec regs ), and advise on any 'departures' that are found when carrying out the tests. ( Periodic Inspection Report )

Hope this is of some help.

 

[Marmaladecoops]

Hi, and thanks EddieCurrent

The cable goes from the CU in the garage runs along about 1 metre and then up into a room above. The room has plasterboard walls. Therefore I assume that the cable runs in between this wall and the plasterboard. This is an external wall. From there that cable run up into the loft and then follows the perimeter of the side of the house and across to the pull switch.

I haven't measure that cable (the parts I can see) but I would say that the cable is not more than 15 metres long ((very Approx))

I will measure to double check.

Hope this helps and thank you for you help.
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11 metres +/- 1 metre

 

[EddieCurrent]

Hi,
For a 11m run , with a load of @45a , if its run in a 'thermal wall' then it should really be 16mm cable I think, but with your existing set-up the mcb is protecting the cable from overload. The breaker will not trip out straight away if overloaded for a short period of time, has it been working previous to you moving in ????

 

[DESL]

A 40A breaker will probably trip after about 10 minutes at ~44A.

I know this from experience as I have done two shower installs for customers in the past where I fitted 9.5kW on 40A MCBs

Both customers they changed the showers themselves to 10.5kW and then rang me to say breaker kept tripping.

One was an easy upgrade to a 50A MCB as the 10mm2 cable was only about 6 m run the other was go back to 9.5kW as circuit would need rewiring in 16mm2

 

[Marmaladecoops]

The shower is working fine, and so far (even though) not timed has not tripped out after 10 mins.

Is the cable of 10mm2 fine for this shower?

MAC

 

[DESL]

With the present set up you are overloading the 10mm2 supply cable the 40A breaker will protect the cable against this overload.

The main problem is the cable installation method means that it has to be de rated accordingly. Hence 16mm2 required for 10.5kW

DO NOT change the breaker for a higher rated one without upgrading cable!!

Simplest/cheapest option would be go for a 9.5kW

 

[Marmaladecoops]

Hi EddieCurrent

Yes the shower has been working fine since we moved in, currently doing timed test and has not tripped out after 20mins.

The cable looks as if the cable has be pulled up between the wall and the plasterboard which inturn is cover with plaster. This is then painted and creates theinternal wall of the bedroom (if that makes sense).

I am a production engineer with a baby and have been checking the house based on my little knowledge of electrics and knowing that new regs have come into force (Part P) based off my previous experience and wanting to ensure house is safe for us.

Cheers

MAC

 

[Marmaladecoops]

Cheers DESL

Currently up to 21 mins and no trip, so all is good?

Cable is stone cold as I would expect.

 

[Marmaladecoops]

DESL

Cheers for help,

Shower has been running for 25mintues and no trip on the 40 Amp MCB.

Checked cable for heat neat pull switch and the cable was alittle warm, I assume this is normal?

Cheers

MAC

 

[DESL]

MAC

It's the heat dissipation or lack of it where the cable runs through the plaster board wall is the main concern.

If the 10mm2 cable was surface mounted or run in trunking then it would be ok on a 50A MCB

 

[Marmaladecoops]

The cable was warm near the pull switch in the loft and it cold in the garage.

It this cause for concern ?

 

[DESL]

Your situation is far from ideal it is not good practise to run MCBs above their rating.

 

[Marmaladecoops]

DESL

The cable in the loft is in parts above the thermal loft insulation and in other areas runs along the perimeter of the house in the gap between the wall and the wooden roof support.

So as long as I don't upgrade the MCB (even to that recommended by the manufacturer to 45A) from the 40 A MCB, then the cable and shower will be fine, and it there are any problems or concerns the MCB will trip out. Then it would be time for a electrical to come in and check it out. I have got the understanding correct.

Also as mentioned in the manual 10.8Kw @ 240V AC (9.9KW @230 V AC), then if I am getting 230V AC within the house then the shower is safe and is not overloading the cable, but will the cable still become warm in normal circumstances??

Cheers and a very merry xmas to you all.

MAC

 

[DESL]

Just one more thing is it running off the RCD side of the CU?