UK 32A or 40A breaker for 7kW EV Charger

[LawVal]

I note that in answer to another EV Charger question a member suggested a 40A breaker would be preferred to a 32A because of many hours at continuous full 7kW load. I am not an electrician but understand the reasoning.
I have a 7kW charger (which generally runs full load for a continuous 5 hours during the off-peak period) and note that the electrician has connected to a 32A type B breaker in the CU and issued his NICEIC Domestic Electrical Installation Certificate accordingly. The circuit conductors are noted as 6mm csa
I have not experienced any problem with the installation but would the general recommendation be to increase the breaker to 40A?

Thank you.

 

[Julie.]

You cannot just increase the breaker size without confirming that it will operate in the correct time.

Each breaker must operate within a set time, which equates to the circuit having a particular resistance or less. Without knowing the characteristics of the circuit you won't know if the larger breaker will still operate within the required time.

If you can confirm the actual Zs (the "loop" "resistance") as measured or calculated is less than the required Zs for the particular breaker; and the cable is of sufficient size, and is installed to the appropriate method to withstand the higher current; then it could be possible.

It isn't just a simple answer unfortunately.

 

[LawVal]

You cannot just increase the breaker size without confirming that it will operate in the correct time.

Each breaker must operate within a set time, which equates to the circuit having a particular resistance or less. Without knowing the characteristics of the circuit you won't know if the larger breaker will still operate within the required time.

If you can confirm the actual Zs (the "loop" "resistance") as measured or calculated is less than the required Zs for the particular breaker; and the cable is of sufficient size, and is installed to the appropriate method to withstand the higher current; then it could be possible.

It isn't just a simple answer unfortunately.

Thanks for the very prompt response Julie.

I attach the extract from the certificate for the existing installation so I don't know if that helps.

I am not too concerned because it has been working without any problems so far, but I can understand that the charger will be called upon to operate at max 7kW load for approx 5 hours every 1 or 2 days for many years to come and therefore its circuit/breaker will be 'challenged' more than any other in the house. So, if considered prudent, I could get an electrician to change the breaker to 40A.

The existing arrangement must be compliant so perhaps it's a 'personal preference' consideration (but of course taking into account your points if I decide to uprate).

 

[Julie.]

Thanks for the very prompt response Julie.

I attach the extract from the certificate for the existing installation so I don't know if that helps.

I am not too concerned because it has been working without any problems so far, but I can understand that the charger will be called upon to operate at max 7kW load for approx 5 hours every 1 or 2 days for many years to come and therefore its circuit/breaker will be 'challenged' more than any other in the house. So, if considered prudent, I could get an electrician to change the breaker to 40A.

The existing arrangement must be compliant so perhaps it's a 'personal preference' consideration (but of course taking into account your points if I decide to uprate).


View attachment 113510

That Zs should be fine for a 40A mcb, however it still doesn't mean it would be ok to change.

A 6mm^2 cable, depending upon the type and installation method may not be capable of carrying 40A.

I have 32A on my EVCP, no need to go to 40A.

 

[LawVal]

That Zs should be fine for a 40A mcb, however it still doesn't mean it would be ok to change.

A 6mm^2 cable, depending upon the type and installation method may not be capable of carrying 40A.

I have 32A on my EVCP, no need to go to 40A.

Thanks, Julie.

I will stick with 32A.

 

[Markyd]

You work to manufacturers instructions.......If they say 32 amp then that's what you install

 

[LawVal]

You work to manufacturers instructions.......If they say 32 amp then that's what you install

Thank you Markyd.

It was the Charger manufacturer (Ohme) that organised the electrical contractor as that is their policy, they would not sell me the charger or allow me to get my local electrician to install it. So I assumed that as it was Ohme's selected installer all would indeed be OK without the need for me to check the installation (neither am I qualified).

Following your comment, I went to the Ohme website and it states "No overcurrent protection is provided in the Ohme Home Pro, separate provision is to be provided as part of the installation. We recommend a 40A Type B MCB."

I will take it up with Ohme but I guess they may just say that; 40A is "recommended" and as the 32A is working fine then all is OK. We will see.

Thanks for your input Markyd & Julie. This is my first posting and I must say I am very impressed, this forum is by far the best I have come across.

 

[littlespark]

Not related to the 32/40 amp question, but i have recently been to 2 repairs where the charge point people had arranged their own installer, and in both cases it was a loose connection where these installers had connected to an SWA that the customers own spark had put in for them.

(customer gets their own electrician to run in the power to an IP rated box.... Charge point installer simply connects to this box)

They're obviously on a price per point, and must rush through the installs without checking.

 

[Bsharp]

Has this charger been installed to an existing split load board??
I.e. one which contains 2 RCDs protected half (ish) the circuits each??

 

[Bsharp]

Also fyi the cable type installed is ‘F’ so swa (steel wire armoured) with an installation method C (clipped direct) so therefore cable capable in this situation of safely pulling 53amps. So 40 amp breaker would be fine

 

[Julie.]

Also fyi the cable type installed is ‘F’ so swa (steel wire armoured) with an installation method C (clipped direct) so therefore cable capable in this situation of safely pulling 53amps. So 40 amp breaker would be fine

I don't quite trust reports or EIs with installation methods, it's virtually impossible for the same method to be used all the way through the installation of a particular circuit.

It's usually a combination of clipped direct over most of it, a bit through duct/conduit and a bit behind insulation (thermal) etc - only the installer truly knows if the C is true in it's entirely, or just mostly true (and what the non-true bits are).

 

[LawVal]

Has this charger been installed to an existing split load board??
I.e. one which contains 2 RCDs protected half (ish) the circuits each??

Hello Bsharp thanks for the info and yes it is a newish (10 years) split board.
The circuit chart is as follows reading from the main switch on the right-hand side of CU.
100A mains switch
63A RCD 30mA
40A MCB, electric cooker
32A MCB, lounge, bedroom & hall wall sockets.
32A MCB, not used
32A MCB, not used
6A MCB, all internal lights
63A RCD 30mA
32A MCB, kitchen wall sockets + washing machine, oil-fired boiler
16A MCB, external lamp post
32A MCB, external car charging point
6A MCB, not used
6A MCB, not used.

 

[pc1966]

I don't quite trust reports or EIs with installation methods, it's virtually impossible for the same method to be used all the way through the installation of a particular circuit.

Indeed, in my own flat the RFC is on 20A because that is what it was on the original fuses. It ought to be OK for 32A but I did not put it in so have no knowledge of just how/where the cable runs.

It was probably done that way due to a high Ze originally, but I never had the chance to measure before the original cutout and MICC feed was replaced.

 

[Bsharp]

I don't quite trust reports or EIs with installation methods, it's virtually impossible for the same method to be used all the way through the installation of a particular circuit.

It's usually a combination of clipped direct over most of it, a bit through duct/conduit and a bit behind insulation (thermal) etc - only the installer truly knows if the C is true in it's entirely, or just mostly true (and what the non-true bits are).

Fair point, but you have to trust the installer at some point. My bet would be cable cleated externally from CU to EV charger in this case. But again without seeing the install you have to trust the installers reasoning.

 

[Bsharp]

Hello Bsharp thanks for the info and yes it is a newish (10 years) split board.
The circuit chart is as follows reading from the main switch on the right-hand side of CU.
100A mains switch
63A RCD 30mA
40A MCB, electric cooker
32A MCB, lounge, bedroom & hall wall sockets.
32A MCB, not used
32A MCB, not used
6A MCB, all internal lights
63A RCD 30mA
32A MCB, kitchen wall sockets + washing machine, oil-fired boiler
16A MCB, external lamp post
32A MCB, external car charging point
6A MCB, not used
6A MCB, not used.

I’d be slightly more concerned about the type of RCD present in the consumer unit.
If as you say it’s 10 years old then the RCD is more than likely going to be a type AC. Bs7671 requires a type A RCD to be installed for reasons which I won’t confuse with at this point.
Worth checking.