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20 amp mcb for 3kw immersion with 1.5mm cable

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  • 24-05-2018 7:50am
    #1
    denismc
    Registered Users Posts: 1,399 ✭✭✭


    We moved into a house 18 months ago and haven't really used the immersion that much until now.

    So I was checking the circuit and it looks like the immersion element is a dual 2/2.85 with a pre-installed cable.

    The cable from the immersion to the isolator seems to be 1.5mm but this is protected by a 20 amp MCB from what I understand is the maximum load for the cable.
    Should this MCB not be lower like 16amp? Should I be worried?


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    #2
    Tuco88
    Registered Users Posts: 1,052 ✭✭✭Tuco88


    It should be a 2.5mm^2 Heat Resistant flex. Good spot...


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    #3
    denismc
    Registered Users Posts: 1,399 ✭✭✭denismc


    Tuco88 wrote: »
    It should be a 2.5mm^2 Heat Resistant flex. Good spot...

    From what I can tell the immersion comes with 1.5m of prewired cable at 1.5mm. This unit is over 18 years old so would have been installed under older regs.
    I may just get an electrician out to check it to be sure.


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    Dual element (2 and 3 kw) immersion elements are normally supplied with heat resistant 4 core 1.5 mm sq. flex

    My understanding is:
    The current carrying capacity of a cable depends on a number of factors, conductor size is just one of them. As the cable run is short, the insulation is heat resistant coupled with the fact that the element has a high temperature cut out stat this cable choice is considered acceptable (between the immersion switch and the element).


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    #5
    Risteard81
    Registered Users Posts: 2,567 ✭✭✭Risteard81


    2011 wrote: »
    Dual element (2 and 3 kw) immersion elements are normally supplied with heat resistant 4 core 1.5 mm sq. flex

    My understanding is:
    The current carrying capacity of a cable depends on a number of factors, conductor size is just one of them. As the cable run is short, the insulation is heat resistant coupled with the fact that the element has a high temperature cut out stat this cable choice is considered acceptable (between the immersion switch and the element).
    It should also be remembered that the immersion heater is a fixed load and therefore overload is not possible, therefore the circuit protection need only provide fault protection and not overload protection. On the face of it (bearing in mind I am not familiar with the installation, installation methods or components used) I see no issue with 1.5mm^2 heat resistant flex, and indeed I would suggest that this is not uncommon with immersions.


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    #6
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    Risteard81 wrote: »
    It should also be remembered that the immersion heater is a fixed load and therefore overload is not possible, therefore the circuit protection need only provide fault protection and not overload protection.

    What do you mean by the bit in bold?


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    #7
    Risteard81
    Registered Users Posts: 2,567 ✭✭✭Risteard81


    2011 wrote: »
    Risteard81 wrote: »
    It should also be remembered that the immersion heater is a fixed load and therefore overload is not possible, therefore the circuit protection need only provide fault protection and not overload protection.

    What do you mean by the bit in bold?
    Like an electric shower, the immersion is a fixed load. The element means that it can only draw what current it will draw. Overload isn't possible. Fault protection is required (short circuit and earth fault protection). Therefore if the protective devices provide fault protection and the flex is suitable for the load then there is no issue.


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    #8
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    Risteard81 wrote: »
    Like an electric shower, the immersion is a fixed load. The element means that it can only draw what current it will draw. Overload isn't possible. Fault protection is required (short circuit and earth fault protection). Therefore if the protective devices provide fault protection and the flex is suitable for the load then there is no issue.

    Ok, I know what you mean now. Definitely reduced risk of an overland alright.
    I was thinking I have seen situations where cables have been undersized for fixed loads (around 1.2 kA of heat tracing when it was designed for 1.2 kA). I have also seen immersion element stats fail casting the water in the cylinder to boil and cables to fail.


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    #9
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Risteard81 wrote: »
    Like an electric shower, the immersion is a fixed load. The element means that it can only draw what current it will draw. Overload isn't possible. Fault protection is required (short circuit and earth fault protection). Therefore if the protective devices provide fault protection and the flex is suitable for the load then there is no issue.

    There is a slight difference, as only one part of the immersion element should be on at a time. In theory, possible overload. Reality, unlikely.


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    #10
    denismc
    Registered Users Posts: 1,399 ✭✭✭denismc


    Thanks for the replies , no harm to check these things from time to time.


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