10.8Kw shower on 6mm² T&E

[The Ghost]

I noticed today when wiring up switch for shower the supply is in 6mm². The load cable is 10mm².
The shower rating is 10.8Kw. So we have a load that is 47a on 6mm² the OCPD being 40a 60898 type B. Interestingly on the new installation (rewire) they had recently the particular cct. on the schedule of test results it has the supply cable as 10mm², which it is not!

I am firstly, crap at visually identifying cables and would normally measure with a micrometer and calculate the size, not having (or being able to find it!) today I thought I would quietly go away with the bit of cable(s) and do so at home. Despite the foregoing I am 90% certain of the sizes. Before you ask, I am still looking for the ......... thing!

I am thinking (as people keep saying) you can overload a circuit where it is a fixed load, and that 47a is the max 6mm² can carry, The MCB being at 40a could only be a nuisance pro tem, so I do not see an immediate problem but this is not satisfactory longer term. As well they will never have it on full load as I ran it at full load for 10 mins you can't hold your hand under it!

I hesitate to steam in as they have just had a rewire and I want to be absolutely sure of my facts before I issue this observation to the client as there will no doubt be quite a furore when the proverbial hits the fan!
Soooo...what do you think? if you bothered to read this far. And if you did you get a free guardian angel til you wear it out!

 

[Richard Burns]

You have a load of 47A(nominal), a supply cable that can take 47A (if installed suitably) you have overcurrent protection of 40A.
The cable is suitably protected by the breaker and so does not present a hazard, however the circuit is installed with the breaker designed to be overloaded which is not acceptable.

A circuit is not permitted to be be overloaded with a fixed load; it is only that if there is a fixed load that the overcurrent, as opposed to the short circuit, protection is not actually required to be in place. i.e the cable must be able to handle the fixed load but the circuit breaker does not necessarily have to be installed to protect the cable from overload, though it is a good idea if it does; especially in a domestic situation.

The easiest solution here would be to increase the breaker to 50A so that it is not overloaded.
However this does require the 6mm² supply cable to be capable of dissipating the generated heat which requires it to be in plaster or on an open surface at the minimum (method C). This would comply with the regulations but not be the best solution, just one that could be implemented easily.

 

[The Ghost]

Brilliant @Richard Burns thanks for your input. A tricky concept well explained. However now I have verified the cable sizes I will inform the client(s) the cable is not suitable for the load. Consequently either a new supply in 10mm², or a lower power shower will have to be installed. What worries me is that the house has been rewired and the EIC states it is 10mm² installed whereas it is 6mm² installed and looking at calculated figures for R1/R2 for these cables suggests strongly that the R1/R2 recorded is for 6mm². So the company installing and issuing the cert., well its going to get a bit messy somewhere down the line.

 

[Richard Burns]

The falsification of the EIC is cause for concern and does appear to have been a bit pointless.
I assume that someone installed the wrong supply cable and this was only noticed once the switch was being connected and they decided to cover up the mistake rather than correct it, which is a foolish approach.
Dropping the shower rating to 8.5kW would ensure a compliant circuit (assuming the 6mm² is suitably installed).
I wish you luck on the next steps as the customer invariably thinks it is you (who is costing them) who are at fault.

 

[westward10]

Spelling mistake or wiring "error", who knows.

 

[PEG]

It seems unlikely that the person installing the circuit,would use two different sizes of cables,so i would hazard a guess that the 6mm is original...just a guess.

 

[telectrix]

It seems unlikely that the person installing the circuit,would use two different sizes of cables,so i would hazard a guess that the 6mm is original...just a guess.

which leads me to assume that they have charged for installing new 10mm cable, but have re-used the existing 6mm, perhaps?

 

[The Ghost]

I have just advised the client of the nature of the unsatisfactory situation vis a vis the shower. She took it very well and will be speaking to the installers. It would seem they have had a raft of setbacks on there re-wire, and they appear to be quite innured to problems. I asked if they had a young person on the job, and yes they had an apprentice working on this so...some poor kid is going to get it in the neck. Poor supervision I say.

 

[The Ghost]

6mm is original...just a guess

Yes that is the case, the builder who refurbished the supply put in the cabling from the switch and lighting/fan under my supervision. He put in 10mm² as per instructions. As I say it was only when wiring up the pull switch the 6mm² came to light.