Which MCB Type B rating for a 10.5kw shower????

Discussion in 'Electricians' Talk' started by Mag200g, Apr 9, 2004.

  1. Mag200g

    Mag200g New Member

    Please can you help, I need abit of advice before I purchase my mcb I have a crabtree starbreaker board taking type B mcbs - I am installing a 10.5 kw shower using 6mm cable over a cable run of about 8mtrs. mainly clipped to the wall and under floor boards. Can you tell me if I need to buy the 40amp crabtree type B breaker or the 50 amp type b breaker. Thanks very much for any advice you can help me with
  2. Pro-Spark

    Pro-Spark New Member

    Hi Mag

    You will need a 50A MCB type B and because of this you will have to run the cicuit in 10mm T&E cable i'm sorry to say

  3. Rabbit Rabbit

    Rabbit Rabbit New Member

    And, following on from Pro-Spark...

    If ya have NOT got an RCD with a trip sensitivity of 30mA in your Consumer Unit then fit an 50A 30mA RCBO instead of an MCB, it will be a lot cheaper to fit than an MCB and RCD. It is not a regulation requirement (I think it should be) but for the sake of your safety.

  4. Fused

    Fused New Member

    Don't forget the earthing. If something ever goes wrong and 50A's gets loose it'll sure find a flaw in any inadequate CPC/earthing . . . and the human body must remain out of any such event . . . !

    You do not rely on pipes, copper or otherwise for such an earth.

    All terminations must be carefully made and properly tightened.

    The installation must be tested.

    Do you have an RCD protecting this MCB circuit? If you do it will need to be able to handle the 50A AND the total of any other load it might serve. Do not make the assumption that only the shower will be in use and nothing else, in calculating you must allow for the shower demand plus normal power usage on the other circuits.

    You must also check your supply, main fuse, tails, consumer unit, and maximum demand are all up for this extra load. Get this tested/checked out.

    40A x 230V = 9200W, which is obviously way below the 10500W of your shower.
    Your shower is 10500W, so divided by 230V = 45.6A
    50A x 230V = 11500 . . . just safely above your needs, but not including equipment before the MCB (as mentioned above).

    Apologies if you already know all this, I'm only trying to help for your sake.
  5. Mag200g

    Mag200g New Member

    Thanks for your advice but asper usual I`ve jumped in head first and already installed the cable (6mm unfortunately.) Therefore if I was to take the 10.5kw shower back to the shop, and changed it to one which will be o.k. to install on 6mm cable - what would be the highest kilowatt shower I could install? and what rating mcb would I need to get. My consumer unit has a 30ma rcd cutout installed on it so that should be o.k. am I correct in thinking this? Thanks very much for your advice and I`m so glad I found this site otherwise I think I would have been destined for trouble.

    Thanks once again

  6. Rabbit Rabbit

    Rabbit Rabbit New Member

    Good one Fused..

    The question of fitting these showers comes up often and, I am not knocking DIY but when ya think that one stands under these things naked and soaked in water and they are hooked up to a device that limits current to 50 amps(!) its frightening. OK so the electric chair in the USA uses 2kV at some 1kA (or summit like that) but I mean to say....

    Its about time it was compulsory for them to be fitted professionally.
  7. Fused

    Fused New Member

    Thanks rabbit . . . guess you deserve a pint!

    Gasp . . . please read what I and other have said, think, and checkout other older previous topics on this forum.

    The cost of your prematurely installed 6mm cable should not dictate events. Whatever Electric shower you are installing you still need to deal with the points I mentioned. Anyway to answer some of your latest questions.

    The 30mA RCD is merely the fault current above which it will trip. There is also an Ampere figure on the device. This must be higher than the total load the RCD could carry. Viz. it could typically be 63A, 80A, 100A. What is it?

    Look on the TLC website for cable ratings, allow for HOW your cable runs, its type, its length, and load, etc. And I suggest you then work a good 10% of this carefully calculated maximum load BELOW this figure, and then choose a shower with maximum rating below this. My concern being people don't realise just how hot some pipework is under floorboards and thus the air, and thus any local cables, so in such an environment its prudent to downrate cable loads especially power-carrying cables.

    You then divide the shower power rating by 230V, which gives you the Amperes, and your MCB (and cable) must be rated above this figure . . . and all the other points I mentioned MUST be correct . . . or you'll blow your main fuse, burn-up their meter tails (which could cause a serious fire), etc, etc, and cook your goose with the local supply company!

    In short get an electrician in NOW.
  8. The Trician

    The Trician New Member

    A 8.5KW shower will take 37A.
    A 9.0kw shower will take 40A
    A 10kw shower will take 43.5A
    A 10.5KW shower will take 45.6A

    So anything above a shower rated at 9kw will need 10.00mm cable.

    Your RCD/MCB should be rated accordingly. The RCD/MCB rating in -
    AMPS - Protects the cable - not the shower.

    The 30ma rating protects YOU.

    Hope this helps

    Hope this helps.
  9. Damocles

    Damocles New Member

    Fused seems to be a bit negative here, though fair to say you jumped the gun a bit with the cable. Think he was a bit heavy on the earthing issue. Might be true that there is theoretically 50A jumping around your bathroom, but that is why the RCD was recomended to you. It is designed to cut off the supply vitually before you would feel a shock. It detects any current not coming back through the neutral wire...like escaping through you.

    RCD have just occasionally been known to go wrong, which is probably why they are not compulsory. Proper earth bonding should also be used. Although less quick to save you, it has the advantage of no moving parts to go wrong. Different posts on here have pointed out that water does not actually conduct electricity very well, so electrical risks in a shower are not quite as bad as they might seem. The main risk is grasping two bits of metal, one of which has become live while the other is earthed...zap through you! That is why metalwork should be bonded togeth with earth wire, so the wire takes the shock before you do.
  10. plugwash

    plugwash New Member

    while a narrow pipe of water has quite a high resistance (100k per meter or so) with a large body of water the resistance is much lower

    furthermore the main problem with water is wet skin skin is your bodys main defense against electric shock and when wet it conducts a whole lot better

    a 30ma rcd that trips in the proper time will protect from mains shocks in most cases but they are still only used as suplementry protection
  11. Stoday

    Stoday New Member

    There are a lot of appliances that are rated at 240V.

    The current taken by an appliance depends on its voltage rating as well as the supply voltage. Run at 230V these 240V appliances consume a lower current. Thus if the 10.5kW shower were rated at 240V its consumption at 230V would be 42A, not 45.6.

    On the other hand, the supply tolerance is higher now that the standard supply voltage is 230V (10% I think). Presumably everything is rated to accept this tolerance and that the 42A base is still OK if the supply voltage were 230V + 10% and the current then 120% greater than nominal.
  12. Stoday

    Stoday New Member

    Sorry - 20% greater than nominal.
  13. Fused

    Fused New Member

    OK Domacles. I was trying to bring this mailers concern and attention level UP, and he had not mentioned the existence of an RCD until AFTER my post and then he had obviously still jumped over many key points when he asked what he could stick through his 6mm and nothing else. Too much was being skipped over in IMO. We still don't know his type of supply, S/H capability, or CU loading. So putting a few cautionary mental trip-wires in his path I do not think was unwarranted.

    His posts did intimate he just wanted snap answers to assure him, followed by a hasty 'the rests-a-yawn' and too quick to jump in the shower, possibly with April . . . but I'd prefer everybody to be able to enjoy their showers safely with April, May and June. Like me! But not at the same time, my cubicle is too small, lol
  14. Tangoman

    Tangoman Active Member

    Don't want to throw a spanner in the works, but the manufacturer's recommended MCB on my 10.5Kw shower was 40A, fed by 10mm cable.

    Rabbit, I know exactly what you mean about pro fitted. I've got quite a lot of experience with wiring, but I still read the manufacturer's instructions cover to cover twice when fitting mine. Know some people with less experience have done the job with less care (have a neighbour for instance who plugged his 9.5kW into the ring main with a plug!!!) I think that making it compulsory is going a bit far, but certainly more attention needs to be drawn to the fact that fitting them is not for the clueless!

  15. Tangoman

    Tangoman Active Member

    That first line is more of a question actually - I've had no problems with mine on 40A so do I need to upgrade, and if so why?

  16. Damocles

    Damocles New Member

    This issue of correct mcb is a real can of worms.If its a choice of 40, 45 or 50A the difference between them is only about the same size as that due to variation in supply voltage (+-10%), or whether the shower is rated at 230V or 240V. Dont know what your nominal shower voltage was tangoman, but if it was 240 then that might account for a few of the missing amps.

    The other issue is that a mcb has only a nominal voltage. It will sustain a nominal overload for some time. Indefinitely even. It seems just possible to me that the manufacturer has deliberately specified a nominally under-current breaker to give the greatest possible current limit on what is a rather powerful-and hence potentially dangerous- supply. On the figures above a 45A mcb should certainly not trip out even if a couple of people had showers one after the other. A 40A might, though they seem to be rated to withstand a 10% overload for at least 1 hour, so it also would probably not trip. hmm.

    only, dont use an under size cable! overloaded mcbs turn themselves off. overloaded cables melt.

    You scared me fused, never mind the guy who posted. Still i've just been arguing with corgi plumbers about gas registration and safety. got me going. Understand why you justifiably might want to scare people a bit, but nice to explain the problems in a way people can join in with rather then blinding them with problems.

    I would think the most likely pinch point would be the RCD, if it were only rated 40A or something. 60A i reckon you would probably get away with unless he has lots of electrical stuff. I would like to see 80A, just to be sure. Dont really think people should be fitting 60A rcds if there is anything like a shower being used. And there is always the chance someone will fit one next year. 60/80A doesnt make much difference to the price.
  17. The Trician

    The Trician New Member

    As I previously stated - a 10.5KW-rated shower will drag 45.652A or call it 47A if it is 230V-rated.

    If the shower is rated at 240v the current will be 43.75A.

    What is 6.00mm twin and earth rated at?
    What is 10.00mm Twin and earth rated at?

    I seem to recall that 6.0mm is rated at something like 47A clipped direct?

    So a 10.5kw shower is borderline with 6.00mm, especially since I haven't taken any derating into account for the purpose of the calculation.

    I'd say that anyone running a 10.5kw 230V-rated shower will have no damp problems where the cable run is buried in plaster! If it is run under the floorboards along with central heating pipes, then you'll also have free underfloor heating too!

    It's a case of checking the rating plate/instructions for the volatge rating before you do anything else.

    Disregard the 10% tolerance on voltage - it cannot be universally applied across the country - neither can 240V come to this.

  18. Damocles

    Damocles New Member

    The spec range on the breakers is also an issue here. As far as I can make out from the spec sheet minimum overload that a particular mcb will carry for 1 hour is 15%. Maximum is 45%. I cant really believe that they are manufactured with such wide tolerence, but that's what they publish. so one 32A breaker might need 46A to trip it in an hour, another 40A might need exactly the same current. Or 58A if its on the slow edge of the spec range. So what would you do if you were a manufacturer faced with these variables? might spec a 40A for the 10.5KW shower, to err on the safe side?
  19. Stoday

    Stoday New Member

    230V is the voltage not only in this country but also throughout Europe (except for a few pockets). The UK came into line in the mid 1990's.

    So the 240V rated shower, if supplied at 230V takes 42A - I squared R see?
  20. The Trician

    The Trician New Member

    Good point, but surely the idea is not to compare how long different brands of mcb will stand the fault for before activating.

    A good and correct circuit design should enable the disconnection time to meet the regulatory requirements - otherwise what is the point?

    Many people either misunderstand, or forget, that we are protecting the cable here and NOT the appliance.

    As you have stated in other posts, a melted cable is a real possibilty with wrongly rated/mismatched protective components.


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