Electric shower replacement questions

[600rob]

My Mira electric shower recently packed up and im looking at getting it replaced with a new unit but would like a bit of advice before i buy a new unit.

The plumbing and wiring from the existing unit are all in place so im hoping that aside from any remedial work to re position the new unit and make good any exposed holes in the wall, that the replacement should be a fairly straight forward job.

The only significant difference between the old and new units is the old one is rated to 8.7kw and the new one is 9.8kw.
I believe the difference in power will have an impact on the wiring requirements so ive gathered a few pics to show what ive currently got in place:

40a circuit breaker at the CU (far left)

_20161108_215235

• 600rob
• 8 Nov 2016

DSC_0386

• 600rob
• 8 Nov 2016

at the shower end ive taken a couple of pics of the cable where it terminates in the unit

DSC_0389

• 600rob
• 9 Nov 2016

DSC_0391

• 600rob
• 9 Nov 2016

also measured the approximate outside dimension across the cable which is approximately 14mm overall


Can anyone advise if what ive got is suitable for a straight replacement with a 9.8kw unit before i go and buy something to get fitted?

Thanks

 

[ericmark]

If the shower has been fitted and wired correctly then should you select a replacement which is too big then worse case should be the trip will keep tripping.

However that is only the case if it was fitted correctly to start with.

So what you need to do is find out the size of cable used, and how it has been installed and work out if the original was correct to start with.

Many people walk into B&Q and see reels of cable marked 22A etc. If it was only that easy. Flex does have a set current rating but it has to be in free air. As soon as cable is buried then you need to consult the charts and see what the rating is when the worst case is taken into consideration.

In some cases I can look at the cable and see exactly where it runs, in other cases I have to take an educated guess. So with 16mm² cross sectional area it is unlikely the cable will be overloaded, but with a 6mm² cable there is a high chance, also not all twin and earth is the same, some is rated at 70°C and some at 90°C.

From the colours it would seem fitted after 2004 so likely there is a Part P completion or compliance certificate which will be based on an installation certificate. If you read these it may tell you what you need to know. If you don't have them there is a possibility it was not installed by an electrician or at least not one willing to put his name on the job. That would ring alarm bells to start with.

8.7kW = 38A and 9.8kW = 42.5A so likely the 40A MCB will be OK. What you need to find out is what the cable is and how it is run. So try this link it gives you the rating for PVC cable twin and earth.

 

[JohnW2]

So with 16mm² cross sectional area it is unlikely the cable will be overloaded, but with a 6mm² cable there is a high chance ....

The OP's cable appears to have a solid CPC, so presumably no bigger than 6mm². However, I would not think that there is a 'high chance' that 6mm² cable would be overloaded (although it's obvioulsy possible).

From the colours it would seem fitted after 2004 so likely there is a Part P completion or compliance certificate which will be based on an installation certificate. ...

The cable appears to be marked "2005".

8.7kW = 38A and 9.8kW = 42.5A so likely the 40A MCB will be OK. What you need to find out is what the cable is and how it is run.

The 9.8kW is very probably quoted at 240V, in which case the current at 230V would be around 39.1A (hence just OK with a 40A MCB).

Kind Regards, John

 

[600rob]

So what you need to do is find out the size of cable used, and how it has been installed and work out if the original was correct to start with.

Right, heres what ive found out:

according to my measurements, the cable is 6mm2 based on the info contained on the wiki page

i also found a part of the cable with the rating details on the outer sheath

DSC_0401

• 600rob
• 10 Nov 2016

The exact info on the cable is: 2X6mm+2.5 SQmm

in terms of how the cable is installed it goes like this:

It runs from the CU for approximately 2.8 meters as seen here

1

• 600rob
• 10 Nov 2016

it then runs up the inside of a cavity wall for about 2.4 meters into a loft......

In the loft the cable runs largely uncovered for 3.1 meters

DSC_0396

• 600rob
• 10 Nov 2016

DSC_0394

• 600rob
• 10 Nov 2016

it then goes from the ceiling mounted pull switch directly down to the shower unit through some plastic trunking for a length of about 1.5 meters.

DSC_0393

• 600rob
• 10 Nov 2016

Do you think this arrangement of cable size and routing is sufficient for me to install a 9.8kw replacement unit in place of the faulty 8.7kw unit?

 

[ericmark]

So worst case is "it then goes from the ceiling mounted pull switch directly down to the shower unit through some plastic trunking for a length of about 1.5 meters." so not really method C so as original it was just about on the mark, in real terms even with 6mm² cable because the shower is used for such a short time you would likely get away with it. However that does not make it right.

One can likely drive at 40 MPH in a 30 MPH limit without having an accident 99% of the time, the chance of things going wrong are remote. Same applies here, it is a remote chance that it will go wrong, only you can decide if worth the risk.

 

[600rob]

I guess i could just look at replacing the unit with one of the same power rating to the unit which is currently in place.
Would that be a more acceptable approach

 

[Echo the husky]

Replacing the cable with 10mm² and installing it so it is clipped properly in the loft etc. would be more acceptable, if you want to keep the new rating. If you replace with the same rating you won't be any worse off than before. I would not feel happy connecting the 9.8KW shower to your existing wiring.

Cables shouldn't really be run in the cavity as they might get damaged from the rough surfaces, or could be affected by cavity wall insulation if it is added at some point. Using the correct type of MCB is always good, your existing shower MCB does not fit your consumer unit so it has probably been butchered in some way to make it fit. A 45A MCB is available, but may be harder to find as this type of MK MCB is obsolete, they are selling on ebay for silly money. I have loads here and some RCBOs I'll have to get listing!

 

[600rob]

Replacing the cable with 10mm² and installing it so it is clipped properly in the loft etc. would be more acceptable, if you want to keep the new rating. If you replace with the same rating you won't be any worse off than before. I would not feel happy connecting the 9.8KW shower to your existing wiring.

Thanks for the advice.
I think I'll keep the existing wiring, properly clip it place where possible and replace the old unit with one that has an equivalent or lower power rating

 

[JohnW2]

.... it then goes from the ceiling mounted pull switch directly down to the shower unit through some plastic trunking for a length of about 1.5 meters.

Is that 1.5m length of conduit/'trunking' buried in a solid wall, or what?

Kind Regards, John

 

[JohnW2]

Replacing the cable with 10mm² and installing it so it is clipped properly in the loft etc. would be more acceptable, if you want to keep the new rating. If you replace with the same rating you won't be any worse off than before. I would not feel happy connecting the 9.8KW shower to your existing wiring. .... Cables shouldn't really be run in the cavity ... your existing shower MCB does not fit your consumer unit so it has probably been butchered in some way to make it fit.

I cannot disagree with the strict correctness of what you are saying, hence that the only strictly correct (and strictly reg-compliant) way of having a 9.8kW shower would be to replace the cable with appropriately-routed 10mm², but I have to say that I regard it as a very 'marginal situation' ...

... as I said before, assuming that the 9.8kW is quoted at 240V, that corresponds to about 39.1A at 230V, hence just OK on a 40A MCB. ~8.3m of the ~9.8m total cable run appears to be essentially Method C, so no problem with 6mm² cable. It's just that 1.5m cable drop in conduit that is a possible issue. That would probably be regarded as Method B, hence theoretically 38A max, but that's only fractionally under the 39.1A.

Whilst it's not the sort of advice that one can/should give to others, I would therefore personally be perfectly happy to have that arrangement in my home, particularly give the relatively short periods of usage of a shower (and the known 'safety margins' in the figures we work with). If one felt compelled to be strictly reg-compliant, and if (not sure!) one could pull 10mm² cable through that conduit, I suppose one could replace just last last 1.5mm or so with 10mm² - but I wouldn't personally want to introduce an 'unnecessary' joint into the cable just for the sake of strict regulatory compliance!

Whatever, the OP seems to have decided not to go with the 9.8kW shower, in keeping with the 'strict' advice you have given. However, your comments about the cable in a wall cavity and the MCB will still remain.

Kind Regards, John

 

[600rob]

Is that 1.5m length of conduit/'trunking' buried in a solid wall, or what?

Kind Regards, John

Hi John,

The trunking goes down through a cavity wall

 

[JohnW2]

Hi John, The trunking goes down through a cavity wall

Thanks. Is there insulation in the cavity wall?

Kind Regards, John

 

[600rob]

Thanks. Is there insulation in the cavity wall?

Kind Regards, John

Ill have to investigate and get back to you on that one.

Whatever, the OP seems to have decided not to go with the 9.8kW shower, in keeping with the 'strict' advice you have given.

Kind Regards, John

Not 100% decided yet. Just want to understand what i can do with the existing circuit.
If there were more sub 9kw units available that look half decent then it would be a no brainer. Most of them still have that cheap 80s white plastic look unfortunately

 

[JohnW2]

Ill have to investigate and get back to you on that one.

Fair enough. If there is no insulation, then that (in conduit within a cavity wall, but with no insulation) is, for some reason, one installation method which is not explicitly mentioned in the regs. However, as far as I can see it is no 'worse' (thermally speaking) than situations accepted by the regs as "Method B" ('38A max' for 6mm cable), so my previous ('personal') comments would still apply.

Not 100% decided yet. Just want to understand what i can do with the existing circuit.

Again, fair enough. You will realise from what has been said (particularly by me) that it's likely to come down to your decision as regards whether you want to be 'strictly compliant' with regulations.

Kind Regards, John

 

[Echo the husky]

I'm probably just a wuss! I know there is no reason to, but I always worry with a circuit run right at the limit the problems caused down the line. Such as when someone adds an extra layer to the loft insulation, which seems to be fashionable at the moment, the cable might start getting rather warm, a cable sized less close to the edge will be much less likely to be affected. I've certainly removed shower cables that have been thermally damaged where they pass through insulation. Cables can take quite a lot though and for the conductors to exceed 70°c would certainly take some time. Looking through the regs book I still wouldn't be comfortable doing it!