RCD and MCB - is this ok for my new shower?

Discussion in 'Electricians' Talk' started by Taddy, Mar 20, 2019.

  1. Taddy

    Taddy New Member

    Hi everyone,
    I want to replace my dead 7kw electric shower with a Triton T80si 7.5kW (I don't think I can go higher power as it has a 6mm cable supply). The manual says it requires a 32A MCB and also a 30mA RCD.
    I did put in the previous electrical shower and have worked with and built guitar amps so I am not really a novice but I am a little unsure about current domestic electrical systems. When I put in the previous shower we had an old style fuse box with an isolator switch for the shower - no consumer unit, MCB or RCD. Now I have all these and it looks like everything is ok but I wanted to check with you guys. Please check out the pic...
    DSC_0120 (1).JPG So the way I'm reading this is that the shower is supplied via a 32A MCB and there is a 30mA RCCB (=RCD?)for the whole bank. Is that correct? If so then is this basically all good for the shower so I don't need to change anything? I have an isolator switch already outside the bathroom so all I have to do is plumb and wire in the shower - right?

    Thanks in advance for your help...
  2. Yes,that is fine for the shower.
    Taddy likes this.
  3. Comlec

    Comlec Screwfix Select

    Yup. Just ensure all your terminations are tight.
    Taddy likes this.
  4. Taddy

    Taddy New Member

    Thanks guys - just needed to check for peace of mind. I'll make sure everything is tight.
  5. Philip Hyde

    Philip Hyde Screwfix Select

    Tight? Or to the correct Torque ? ;-)o_O:)
  6. gas monkey

    gas monkey Well-Known Member

    sounds likke a plan he who dares
  7. JP.

    JP. Screwfix Select

    Hmmmm - on the presumption that the DB portrayed is a so called 16th edn split loader - then myself would have planted an appropriate rcbo in the non protected side tbqh.
  8. Bazza-spark

    Bazza-spark Screwfix Select

    Welcome back sir!
    seneca likes this.
  9. ecoplumbing

    ecoplumbing Active Member

    Correct me if I'm wrong but is there any reason why taddy can't install a 8.5kw shower? I've fitted loads on a 6mm cable and various sparkies say this is ok with a 32a mcb
  10. robertpstubbs

    robertpstubbs Screwfix Select

    Does a 8.5kw shower only use that much power when the heat is set to 10, which I doubt anybody actually uses.

    If the MCB is 32A it would limit the current anyway.
  11. Philip Hyde

    Philip Hyde Screwfix Select

    If the shower is on High setting it will use 8.5kw to heat the water to temperature set point whether it's on 1 or 10.
    An Mcb will not limit current. Other than when it trips then it will limit it to Zero.
  12. robertpstubbs

    robertpstubbs Screwfix Select

    So does it send the water out faster if the setting is less than 10?
  13. Bazza

    Bazza Screwfix Select

    It people like you that give electricians a bad name. Done the (simple) maths?
    Ohms law. 8500watts @ 230volts = nearly 37 amps. That’s going to be perfect with your “current limiting” 32A MCB. You jump in the shower, get all nice and soapy and have leg it to the garage to reset the MCB.

    Maybe the cable route will permit a 40A MCB. It’s all down to cable calculations, remember them?
  14. Bazza

    Bazza Screwfix Select

    . In most “instant heat” showers there is no control on the electric element. The water temperature is adjusted by varying the flow of water past the electric heating element. So the electric element is a constant load.
    So the slower the water output, the hotter will be the water.
  15. robertpstubbs

    robertpstubbs Screwfix Select

    When I had an electric cooker I added up the theoretical maximum power from the 4 rings, the 2 ovens, and the grill together. It was much more than the MCB rating. I doubt the meals produced would have been edible.
  16. Comlec

    Comlec Screwfix Select

    Sorry to be a bit of a pedant but it is to expected in this forum:).
    Torque is a force and as such is a dynamic action and measured during use or application. So you can apply the correct torque to a rotating part (screw,bolt shaft etc) and some use the word 'torqued' to describe the application of the torque. The result is the fitting is secure or tight. But hey ho who apart from diligent installers, who actually owns a torque screwdrivers.
  17. Comlec

    Comlec Screwfix Select

    Your electrical installation was probably fine so maybe it was your cooking.
  18. Pollowick

    Pollowick Screwfix Select

    Nothing wrong with your calcs, just to point out that you are assuming it was defined as 8.5kW at 230v. Some showers are still defined at 240v and thus lower at 230v which for an 8.5kW would mean 33.9A - still over the limit.

    Then you get a shower defined at 230v which could be close to an MCB limit where a user has a high240v supply and the extra 4 or 5 A takes it well over.
  19. rogerk101

    rogerk101 Screwfix Select

    If we're descending own the path of pedancy ...
    Torque in NOT a force. It is a force at a particular distance, is evident from its unit of measure, which is Nm (or Newton metres).
    Torque is also not necessarily a dynamic action. I torqued my van's wheel bolts up to 180Nm using my torque wrench (dynamic action), and now the wheel bolts are tightened at a torque of 180Nm (static condition).
  20. Comlec

    Comlec Screwfix Select

    How MCBs respond to overload and why the shower wont go off suddenly
    James Billinge likes this.

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