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Hi All,

We have just had an electrical test done and have been told that our 8.5kW shower is too powerful for the 6mm cable. We have been told it should be replaced with a 7.5kW maximum.
I was under the impression and stand to be corrected that an 8.5kw would be fine for that cable as long as the run didn't exceed 18m. It doesn't. The run is about 6m maximum.
Can anyone advise please? The shower was actually installed by an electrician who stated that it was fine.
Cheers.
Paul
 
Hi All,

We have just had an electrical test done and have been told that our 8.5kW shower is too powerful for the 6mm cable. We have been told it should be replaced with a 7.5kW maximum.
I was under the impression and stand to be corrected that an 8.5kw would be fine for that cable as long as the run didn't exceed 18m. It doesn't. The run is about 6m maximum.
Can anyone advise please? The shower was actually installed by an electrician who stated that it was fine.
Cheers.
Paul
8.5kw is approx 37A
6mm is good for between 30 and 46A depending on the installation method.

I would ask the inspector to show you his calculations and assumptions.
 
The length of the cable is fine. It's what the cable passes through on the way is the potential problem - contact with thermal insulation can reduce its current carrying capacity considerably.
Then there's the shower. 8.5kW at what voltage? Is this stated at 240 volts or 230? The declared UK mains voltage has been 230 for some time, but some shower manufacturers still state the power at 240 volts, because it looks better. Often the rated power at 230 volts is shown in much smaller print, although it's the one that matters.
 
Hi.
As James says, it depends on the installation method of the cable run. If the cable is in an insulated wall, or covered with insulation in a loft, or both, the cable size can jump to 10mm² or even 16mm². This is to allow for dissipation of heat generated, which is severely hindered when in insulating material.
 
Thank you James for your reply. I thought it was ok (I used to do my own electrics until lack of being qualified forced me to stop. I always got a qualified electrician to check over my work) and had confidence in the chap that fitted it.
I will ask.
Regards,
Paul
8.5kw is approx 37A
6mm is good for between 30 and 46A depending on the installation method.

I would ask the inspector to show you his calculations and assumptions.
 
Hi.
As James says, it depends on the installation method of the cable run. If the cable is in an insulated wall, or covered with insulation in a loft, or both, the cable size can jump to 10mm² or even 16mm². This is to allow for dissipation of heat generated, which is severely hindered when in insulating material.
Thank you Ioz. As far as I can recall, the cable runs from the consumer unit under the stairs and then up the wall to the pull switch. There would be a run of no more than 0.5m under the loft insulation before it goes down behind the wall of the shower.
We have an en-suite with a 8.5kW as well and we were told that must be changed. The cable run is probably 3m longer, but is in 10mm. However, there is a slight problem there, which has only just been ascertained. The 10mm runs to the pull switch, then the electrician ran 6mm from there to the shower for some reason. I can only imagine he ran out of cable as it wasn't what was asked or paid for!
I would imagine that shower would also be fine with an 8.5kW?

Regards,

Paul
 
Table 52.2 - cable surrounded by thermal insulation:

Length in insulation 50mm derate cable by 0.88.
Length in insulation 100mm derate cable by 0.78.
Length in insulation 200mm derate cable by 0.63.
Length in insulation 400mm derate cable by 0.51.

So a cable normally capable of carrying 47 amps clipped direct is reduced to 41 amps for 50mm of insulation.
And 36 amps for 100mm.
And 29 amps for 200mm.
And 23 amps for 400mm.

These are some of the calculations that need to be made when designing a circuit.
These are also the calculations that all too often are not made when designing a circuit.

Having said that, your shower may well be fine and not cause any problems. But there is always an associated risk when shortcuts are taken.
 
I know regulations and good design should be adhered to but there must be hundreds of thousands of 8.5kW showers running happily on 6mm cable throughout the country - some with much longer runs than the example on this thread. After all we're not looking at continuous current demand with a shower, even a teenager would struggle to be in there for 20 minutes for their monthly shower. ?
I think this is just another example of the attitude that's out there with some of these 'inspectors' now , doesn't does any of us any favours.
 
Put it this way, if it was in my house, I would be completely unconcerned.

however, on a public forum, I should point out that without inspection of the cable run it is impossible to give a definitive answer on the capability of the cable to withstand the load for any duration.

i have seen similar loads on 4mm cable and had been there for years with no problems although I would point out that 4mm is pushing your luck!!
 
The length of the cable is fine. It's what the cable passes through on the way is the potential problem - contact with thermal insulation can reduce its current carrying capacity considerably.
Then there's the shower. 8.5kW at what voltage? Is this stated at 240 volts or 230? The declared UK mains voltage has been 230 for some time, but some shower manufacturers still state the power at 240 volts, because it looks better. Often the rated power at 230 volts is shown in much smaller print, although it's the one that matters.
Thank you Brian. I'd not realised that.

What difference would the 230 or 240 volts make?

Paul
 
Put it this way, if it was in my house, I would be completely unconcerned.

however, on a public forum, I should point out that without inspection of the cable run it is impossible to give a definitive answer on the capability of the cable to withstand the load for any duration.

i have seen similar loads on 4mm cable and had been there for years with no problems although I would point out that 4mm is pushing your luck!!
We were unconcerned until the inspection! We had a complete rewire around 24 years ago, when we bought the house. Other things highlighted, due to regs changes, which are understandable are that we don't have a fireproof consumer unit and the bathroom lighting doesn't conform. We will get the things sorted out, it's just that no reason was given for the shower change. I have asked them to explain their reasoning behind it.

Paul
 
I know regulations and good design should be adhered to but there must be hundreds of thousands of 8.5kW showers running happily on 6mm cable throughout the country - some with much longer runs than the example on this thread. After all we're not looking at continuous current demand with a shower, even a teenager would struggle to be in there for 20 minutes for their monthly shower. ?
I think this is just another example of the attitude that's out there with some of these 'inspectors' now , doesn't does any of us any favours.
This is what I thought.

Paul
 
Table 52.2 - cable surrounded by thermal insulation:

Length in insulation 50mm derate cable by 0.88.
Length in insulation 100mm derate cable by 0.78.
Length in insulation 200mm derate cable by 0.63.
Length in insulation 400mm derate cable by 0.51.

So a cable normally capable of carrying 47 amps clipped direct is reduced to 41 amps for 50mm of insulation.
And 36 amps for 100mm.
And 29 amps for 200mm.
And 23 amps for 400mm.

These are some of the calculations that need to be made when designing a circuit.
These are also the calculations that all too often are not made when designing a circuit.

Having said that, your shower may well be fine and not cause any problems. But there is always an associated risk when shortcuts are taken.
Ioz, that makes it very understandable. Cheers.
Paul
 
We were unconcerned until the inspection! We had a complete rewire around 24 years ago, when we bought the house. Other things highlighted, due to regs changes, which are understandable are that we don't have a fireproof consumer unit and the bathroom lighting doesn't conform. We will get the things sorted out, it's just that no reason was given for the shower change. I have asked them to explain their reasoning behind it.

Paul
Hopefully the "fireproof" consumer unit was noted only as a C3 and not required to be changed, not as a C2? There is no requirement to replace otherwise perfectly serviceable consumer units to meet current regulations regarding non-combustible construction.

The bathroom lights issue (presumably lack of RCD protection) may well need resolving, though that is often possible without a full Consumer Unit change.

With any inspection like this, I would always ask for an explanation of their reasoning (backed up by guidance or regulations) if it is not clear from the document they have provided - and consider getting alternative quotes for remedial work.

If you can remove any identifying information from the EICR and post it here, people should be able to give you a good idea as to whether the noted issues are over officious, unreasonable, or just a matter of judgement.

I assume this is a rented property now?
 
What difference would the 230 or 240 volts make?

Paul
If it is rated as 8.5kW at 240 volt, it should be treated as 7.8kW for all calculations. Current, for design purposes is a fraction under 34A.
Did you say that you have two 8.5kW electric showers? If so, then that could be a problem for an EICR. There is no diversity allowable for an instantaneous water heater, so they have to be assumed to be continuously on for calculation purposes. 2 x 8.5kW = 17kW, which overloads a normal 15kW domestic supply without taking any other loads into consideration.
 
If it is rated as 8.5kW at 240 volt, it should be treated as 7.8kW for all calculations. Current, for design purposes is a fraction under 34A.
Did you say that you have two 8.5kW electric showers? If so, then that could be a problem for an EICR. There is no diversity allowable for an instantaneous water heater, so they have to be assumed to be continuously on for calculation purposes. 2 x 8.5kW = 17kW, which overloads a normal 15kW domestic supply without taking any other loads into consideration.
That is interesting. The overload question doesn't appear to have been raised. There is also an electric cooker and an immersion heater in the water tank should the boiler stop working.
I do know that the original electrician was an Inspector for the local electricity board at the time and he'd said that our supply wasn't enough (a 1928 built house) and arranged for it to be increased. I was at the house when a new cable was installed from the street.
 
Hello Dartlec,

Hopefully the "fireproof" consumer unit was noted only as a C3 and not required to be changed, not as a C2? There is no requirement to replace otherwise perfectly serviceable consumer units to meet current regulations regarding non-combustible construction.

The bathroom lights issue (presumably lack of RCD protection) may well need resolving, though that is often possible without a full Consumer Unit change.

With any inspection like this, I would always ask for an explanation of their reasoning (backed up by guidance or regulations) if it is not clear from the document they have provided - and consider getting alternative quotes for remedial work.

If you can remove any identifying information from the EICR and post it here, people should be able to give you a good idea as to whether the noted issues are over officious, unreasonable, or just a matter of judgement.

I assume this is a rented property now?
Thanks for the reply. Yes it is a rented property now. I'll see what I can do about the report.
Paul
 

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