shower cable part 2

Discussion in 'Electricians' Talk' started by westbrom22, Nov 14, 2003.

  1. westbrom22

    westbrom22 New Member

    Having just read the thread on this forum re cable size for showers, I am ‘on the horns of a dilema’ .
    I have just removed my 9.5kw shower in order to re-tile my bathroom - - this shower has been there ever since I have lived there, and noticed its is wired using 6mm cable. I have never had any problems with this shower but now I am a little worried about putting it back without changing this to 10mm - - I’m sure your aware of the ‘hassle’ this would be.
    Am I in danger of my house going up in smoke if I leave things as they are ??
  2. George Icbar

    George Icbar New Member

    If you keep to having cold showers you will be more awake and also keep the load on the cable down.
  3. RH

    RH New Member

    The specifcations are designed to ensure that the insulation (PVC) doesn't get reach 70C as at past this temp it starts to break down.

    Given that the 'typical' shower is quite short (minutes usually) the cable doesn't have much time to warm up. However, if the same cable were connected to an identical wattage electric oven, which could be on for hours, you would have a problem.

    The regulations have to cover both, so err on the side of caution.

    I've seen quite a few 8.5KW and even 9.5KW showers connected to 6mm2 cable, and have never, as yet, seen signs of heat damage to the cable.
  4. ban-all-sheds

    ban-all-sheds New Member

  5. westbrom22

    westbrom22 New Member

    ban-all-sheds - - this indeed has been posted twice, but if you have a look at the times that they were posted, you will see they were posted at the same time - - don't ask me how it happened but can assure you it was completely accidental.
    George Icbar - was lookin for the 'easiy' way but not quite that easy, particularly this time of year - -good thought though :)
    Brian - I guess this is just the reply I was hoping for - but am still 'nervy' about it with everything thats been said - -and am still thinking about it. 'er indoors has said she wants it comnpleted before xmas but has, fortunately, forgot to tell me which Xmas ! One thing that surprises me though re your comment about cookers - was in Homebase (did I swear there ??) last week, looking at info on cables and noticed that the 6 mm stuff was suitable for showers up to 7000 w and cookers up to 13000w - - whats goin on there ??
    Thank you all for your help - -and for ban-all-sheds, believe me, if I knew a way of deleting a post I would have deleted once I realised there were 2 of the blighters there.
  6. ban-all-sheds

    ban-all-sheds New Member

    I'm sorry - I didn't notice that the time was identical to the previous one. I've never seen that happen before, but I have seen people post twice, and I lazily assumed you were one of them. Apologies for casting aspersions, and thank heavens I wasn't (I hope ;-)) too offensive in my retort...

    BRIAN - "Given that the 'typical' shower is quite short (minutes usually)" - ha! You've clearly not encountered the inattentive 15-year old dork I laughingly call a son, who never spends less than 20 minutes in the shower, and often more. I really don't want to know what he's up to in there, but if anybody knows of a shower with a timer on the heater part, but not the flow solenoid, let me know. I'd love to be able to announce to IFYOD that he can spend as long in there as he likes, but there will only be hot water for the first 10 minutes :)

    As for the Homebase advice this seems very iffy. 7kW for a shower is erring a great deal on the side of caution, as that's only 30A, and 6mm is rated at 40, so they are being very "worst case" for derating. But 13kW for a cooker? The problem with that is that although you probably wouldn't run the oven and all the rings flat out for long enough to damage the cable, you might well run them all flat out for 10-15 minutes whilst things heat up and come to the boil, which would be a current draw of 54A. What MCB are you going to use to protect your 40A cable?

    A 40A type B will typically withstand 54A for 30-odd minutes, but that might come down if heat builds up in the CU. 50A ones will never trip at 54A, but conversely they'll pass 80A for over an hour, so a fault condition might cause your cable some grief. I'm not entirely convinced that Homebase have thought this through...
  7. westbrom22

    westbrom22 New Member

    ban-all-sheds absolutely no worries mate. You have me worried now re Homebase - -gonna have to go there again and write down EXACTLY what is written on that board. I've been with this 'conundrum' (how do you spell that bar-steward ??) for some time now and up til fairly recently, the 6mm cable at Homebase stated perfectly safe for 9.5kw, the B and Q one said 9.1 kw or very near - -and now Homebase completely repackaged their stuff to say something completely different.
    As for the last paragraph, I'm sure its meant to mean something to me - - - but its so far over my head I'm gonna have to 'pass' ;)
    Thanks for your time - - now how do I link this post with the duplicate ????
  8. ban-all-sheds

    ban-all-sheds New Member

    OK - what the last paragraph is about is how fast an MCB will trip for a given overload condition. Type B MCBs (by far and away the most common in domestic installations) are required to trip in 0.1s or less at 3-5 times the full load current. But they are not "all or nothing" devices - what I mean is that there is no current level below which they will never trip and above which they will always trip - there is a current below which they won't, and there is a current at which they will trip PDQ, but those 2 values are quite widely separated, and in the "in-between" area they will trip in a shorter and shorter time as the current gets higher - the example I quoted will vary but is typical, i.e. you can put 80A through a 50A breaker and it won't trip for a long time.

    This graph shows the typical characteristics of Type B MCBs - there are 8 curves, for 6A, 10A,...50A breakers. The X-axis is current in Amps and the Y-axis is trip time in seconds. So if you want to see how long a particular MCB will pass a given current, go up from the value on the X-axis to the point where it intersects the curve of the MCB, then read across to get the time. It's not a very good quality chart - you couldn't use it in anger, and it looks as though I might have read it wrongly - on second inspection it's probably an hour-ish for 70A, 80A would cause a 50A breaker to trip in about 15-20min.

    Even so, I'm still not happy with Homebase saying that 6mm cable is OK for a 13kW cooker. Maybe I'm being alarmist, but if I had a single device that could draw 54A, even if only for 10-15 minutes I'd use 10mm.

    (BTW - the old figures of 9.5kW in Homebase and 9.1kW in B&Q are different because HB are assuming 240V, and B&Q 230V)
  9. supersparky

    supersparky New Member

    B&Q have the right idea about assuming its 230 insted of 240, the problem with assuming the whole uk is on 240 is a bad idea,at 240 an amperage is lower than it is at 230, which could make the cable overloaded.
    And just to back up b-a-s, Use 10mm for the fun

  10. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    Use 10mm...........nuff said

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