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

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

  1. Comlec

    Comlec Screwfix Select

    And there is no instrument that can measure that static condition. That is why once fitted wheel nuts need re-torqued after a short period to ensure they remain stable. NTS actually give you a bit of a$re-covering paper to say they have done this. So if your wheels fall off they are covered.

    The point I was making was that connections have to be tight. A term widely used and understood by the OP, who unlike me is probably not in possession of a torque screwdriver and manufacturers tables of recommended torque settings.
    Last edited: Mar 22, 2019
    Jord86 likes this.
  2. ecoplumbing

    ecoplumbing Active Member

    Ok, so putting a 8.5kw shower on with a 32 mcb- yes ohms law states it should be higher rated but what is the real chances this will trip out? Last shower I fitted was this setup, I asked a electrician on site if the mcb needed upgrading and he said no, it's still safe on a 32 and the shower wouldn't trip the mcb. So is this just commen sense or bad practice?
    seneca likes this.
  3. seneca

    seneca Screwfix Select

    Nothing wrong with that, I've put plenty of 8.5kW showers on 32 amp mcb's over the years and never had any problems at all.
    ecoplumbing likes this.
  4. Bazza

    Bazza Screwfix Select

    Look at the diversity section in the OSG. You can apply diversity to cookers etc, because they will never reach their theoretical maximum because the elements are controlled by thermostats etc.
    You cannot apply diversity to instant heat items, like showers, because they are on at full load, all the time.
  5. robertpstubbs

    robertpstubbs Screwfix Select

    Bazza - I agree that a 40A MCB could be used with 6mm2 cable - subject to the particular installation situation.

    I was just asking for clarification of how electric showers work. What you and others seem to be saying is that a 8.5kw shower will work at full power (37A at 230V) and the temperature is crudely controlled by adjusting the water flow rate?
  6. According to table 4D5 sheathed flat cable, the only time this would be allowed is if the cable is clipped direct,reference method C, and complied with table 4A2 installation method 20.
    All other methods are well below 40 amps.
  7. seneca

    seneca Screwfix Select

    Yes what you're saying is true Robert so is what Bazza said about diversity but my way of thinking is that showers are on for a relatively short time so no real danger of overheating the cable.
  8. ecoplumbing

    ecoplumbing Active Member

    Same here, in my opinion you'd have to have the shower on full for a very long time before there's a risk of it tripping out.
  9. robertpstubbs

    robertpstubbs Screwfix Select

    You obviously haven’t lived with my ex.
    seneca likes this.
  10. JP.

    JP. Screwfix Select

    Hi Bazz - how you doing m8?
  11. chippie244

    chippie244 Super Member

    Hells bells JP, where have you been and welcome back.
    seneca likes this.
  12. Bazza-spark

    Bazza-spark Screwfix Select

    Not too bad. Wife's health isn't great but we are surviving.

    How are you doing? You back on the forum?

    Best regards
  13. JP.

    JP. Screwfix Select

    Sorry to hear that about your wife Bazz..hope her health gets better m8. Not doing bad, haven't really done anything on the forum but nice to see it is chugging along nicely. Best wishes to you and your family Bazz.
    Bazza-spark likes this.
  14. JP.

    JP. Screwfix Select

    Hi Chips. Just been ducking and diving really m8..:) Have a good one Chips.
    seneca and chippie244 like this.

  15. Eh? Regs is 230v. It’s been that for a while. 8500/230 = 36.96A not 33.9A. Even at 240v = 35.42A. So not sure where you’re getting 33.9A from.

    Without knowing the length of run, whether it runs along or through insulated material, 6mm will be fine on 32A MCB and if it really is a short run. 40A MCB is acceptable.
  16. Pollowick

    Pollowick Screwfix Select

    Suggest you go back to basics and read what I said.

    A shower that is specified at 8500W at 240v - and yes they still do specify them that way will take 35.42 A and have an element resistance of 6.7765 Ohms. Put that same element on a supply running at 230v and miraculously you get 33.94A.

    So care to tell me where I have gone wrong, or admit that you are.
  17. nigel willson

    nigel willson Screwfix Select

    Yes it does use full load water temp isdjusted by altering water flow through the shower
  18. Where did I say you are wrong? All I asked was how you got to your figures as that post makes no mention of 6.7765 Ohms.

    Rather than assume a pedantic stance. Perhaps assume that most of us use a quick calculation of P/V to give us the figures we’re looking for.
  19. robertpstubbs

    robertpstubbs Screwfix Select

    My electricity is actually approx 240v and I think that is still the norm in the UK, but appliances are designed to operate within tolerances that mean that 230v appliances can operate on 240v and vice versa.
  20. Anywhere from 220v to 250v is normal operating voltage from your DNO.

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