Electric Shower Supply

[Retired Fellow Traveller]

We own a small, one bedroom flat (built 1989).
An EICR inspection raised three C2 observations and recommendations:
(A) The shower cable, and mcb were incorrect size (9.5Kw Mira shower) - Recommend replace 6mm cable with 10mm cable (12 meters, surface run, in trunking) and replace 32 amp mcb with a 45 amp mcb.
(B) Sockets were fed via three (2.5mm) cables from a single 32 amp mcb. And, sockets were not in a "ring" - Recommend replace existing 32 amp mcb with a 16 amp mcb.

We agreed to pay £380 to rectify these observations - i.e. Replace the mcb feeding the sockets - And, to replace the shower wiring with 10 mm cable, including the installation of an RCD or an RCBO to protect the shower circuit.

He has replaced the cable, but has not installed the RCD or RCBO which was requested as part of agreed workscope, and £380 charge.

In an exchange of messages he said he didn't fit the 45 Amp (Hager) RCBO as the shower was drawing 46.5 amps, so he had fitted a 50 amp mcb.

My first question to him was:
"Why is my 9.5Kw shower drawing 46.5 amps, on a circuit that should draw a calculated current of 41.3 amps?" he said, he had got it wrong and, yes, it was drawing 41.3 amp.
I then asked where had his first figure had come from, was it measured or calculated. He said (wrongly) calculated.
So - I took a reading with a Clamp meter, set on 60 amp AC range - with shower operating at maximum flow/temperature - and it indicates a steady state current flow of 46.97 amps. The meter was clamped on to the 10mm cable, at the outlet of the 50 amp mcb - utilising a conveniently looped piece of cable that had apparently been already utilised for this function check.
When challenged, he denied ownership of a clamp meter, "they are unreliable", and is adamant that the current is the expected 41.3 amp - and that he will now install a 45 amp RCBO (Hager discontinued their 50 amp version).
I am concerned about "Safety" - life and property. Also on the potential for spurious tripping.
Appreciate comments, please.

 

[SparkyChick]

Clamp meters, just like any other test equipment, can be unreliable if they are cheap as chips and don't get regularly checked for calibration.

A purely resistive load of 10.5kW running at 240v should be drawing around 43.75A, so I would question the figure of 46.5A and thus the accuracy of the clamp meter you used. A 9.5kW load should draw around 39.6A at 240v.

What it appears he has done is measured your supply voltage or assumed the standard 230v. Power = Current x Voltage. So if you take 9500 / 230 you get 41.3A but this is not how a resistive load works.

The element inside the shower will have a power rating which will vary depending on the input voltage. This may be shown as 9.5-9.0kW along with a voltage range which may be 240-220v. The relationship between voltage, resistance and current is a linear equation known as Ohms Law. So if we assume the rating of 9.5kW is for an input voltage of 240v, then the current will be 39.6A and this allows us to calculate the resistance of the element at around 6 ohms (for clarity, a 2kW immersion heater element is around 27 ohms, so this stacks up as a valid value). If you put a voltage of 230v across that same resistance, the current will be 37.95A.

I'll also point out that a 6mm cable, when installed in perfect conditions, can handle a maximum current of 47A.

In terms of safety, the 10mm cable will be more than adequate. The circuit supplying the shower must be RCD protected, so either an MCB supplied via an RCD or an RCBO should be used to protect it. Would I have classified the shower cable as a C2... I can't say because I've not seen the cable run, but if on inspection there was no thermal damage and it was not covered by insulation I may have simply raised a C3 (improvement recommended). Do I think it was necessary work? Probably not, but then it is down to the inspector as essentially they are signing to say it's safe... if they aren't happy with it, they can classify it as they see fit.

The ring circuit.... 3 cables does not mean unsafe. It is acceptable to have the two ring conductors and a third cable connected as a spur, this is allowed. The break however does present a problem and that should be corrected, but the first step is not to replace the breaker, it is to try and find the break and look at whether it can be repaired... often it's a bad connection in the back of an accessory, sometimes it's a bad connection in a junction box somewhere (I've had one that was in a junction box, buried in the wall behind a brand new kitchen unit), but as I say, the first step should be trying to find the fault. If you can't, then splitting the ring is an option, but you should look at finding the location of the break so you can isolate that portion and then put each half of the circuit on it's own 16A breaker. If that circuit serves the kitchen, then you may find you start to get issues with nuisance tripping due to overload (running say a kettle, toaster and tumble drier for example). So in your case, space allowing, I would have sought to find the break and if that was a cable between two accessories I would have fully isolated that segment and then, space allowing I would have put each of those three cables on their own protective device.

Opinions may vary and I'm happy to be proven wrong because it would mean I'd learn something, so others may offer alternative views. In the future though, you should understand you are not obliged to have the same company who carry out the EICR carry out the remedial works and, as I advice all my clients, you should get at least three quotes.

 

[Deleted account]

^^ you won’t get a better answer than that^^ spot on.

no point in anyone else posting really!

 

[Retired Fellow Traveller]

Clamp meters, just like any other test equipment, can be unreliable if they are cheap as chips and don't get regularly checked for calibration.

A purely resistive load of 10.5kW running at 240v should be drawing around 43.75A, so I would question the figure of 46.5A and thus the accuracy of the clamp meter you used. A 9.5kW load should draw around 39.6A at 240v.

What it appears he has done is measured your supply voltage or assumed the standard 230v. Power = Current x Voltage. So if you take 9500 / 230 you get 41.3A but this is not how a resistive load works.

The element inside the shower will have a power rating which will vary depending on the input voltage. This may be shown as 9.5-9.0kW along with a voltage range which may be 240-220v. The relationship between voltage, resistance and current is a linear equation known as Ohms Law. So if we assume the rating of 9.5kW is for an input voltage of 240v, then the current will be 39.6A and this allows us to calculate the resistance of the element at around 6 ohms (for clarity, a 2kW immersion heater element is around 27 ohms, so this stacks up as a valid value). If you put a voltage of 230v across that same resistance, the current will be 37.95A.

I'll also point out that a 6mm cable, when installed in perfect conditions, can handle a maximum current of 47A.

In terms of safety, the 10mm cable will be more than adequate. The circuit supplying the shower must be RCD protected, so either an MCB supplied via an RCD or an RCBO should be used to protect it. Would I have classified the shower cable as a C2... I can't say because I've not seen the cable run, but if on inspection there was no thermal damage and it was not covered by insulation I may have simply raised a C3 (improvement recommended). Do I think it was necessary work? Probably not, but then it is down to the inspector as essentially they are signing to say it's safe... if they aren't happy with it, they can classify it as they see fit.

The ring circuit.... 3 cables does not mean unsafe. It is acceptable to have the two ring conductors and a third cable connected as a spur, this is allowed. The break however does present a problem and that should be corrected, but the first step is not to replace the breaker, it is to try and find the break and look at whether it can be repaired... often it's a bad connection in the back of an accessory, sometimes it's a bad connection in a junction box somewhere (I've had one that was in a junction box, buried in the wall behind a brand new kitchen unit), but as I say, the first step should be trying to find the fault. If you can't, then splitting the ring is an option, but you should look at finding the location of the break so you can isolate that portion and then put each half of the circuit on it's own 16A breaker. If that circuit serves the kitchen, then you may find you start to get issues with nuisance tripping due to overload (running say a kettle, toaster and tumble drier for example). So in your case, space allowing, I would have sought to find the break and if that was a cable between two accessories I would have fully isolated that segment and then, space allowing I would have put each of those three cables on their own protective device.

Opinions may vary and I'm happy to be proven wrong because it would mean I'd learn something, so others may offer alternative views. In the future though, you should understand you are not obliged to have the same company who carry out the EICR carry out the remedial works and, as I advice all my clients, you should get at least three quotes.

Thank you for your prompt, and detailed response. I guess my concern now is whether the 45 amp RCBO, protecting just the shower, will give rise to spurious tripping while using the shower

 

[Deleted account]

Thank you for your prompt, and detailed response. I guess my concern now is whether the 45 amp RCBO, protecting just the shower, will give rise to spurious tripping while using the shower

TBH it’s doubtful. It should be fine. Unless you like very long showers.

 

[SparkyChick]

Hopefully this will put your mind at ease... this may not be the exact model, but this is a capture of a portion of a Mira shower installation guide.

 

[Retired Fellow Traveller]

View attachment 95823

Hopefully this will put your mind at ease... this may not be the exact model, but this is a capture of a portion of a Mira shower installation guide.

Thanks again, Diolch yn fawr iawn

 

[philectrical]

Wouldn't 6mm be fine for that load?

 

[Mike Johnson]

Wouldn't 6mm be fine for that load?

SparkyChick covered that in her comprehensive reply above.