Double check my cable calcs please?

Discussion in 'Electrics UK' started by Homer Lawtey, 17 Aug 2007.

  1. Homer Lawtey

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    I'm in the process of refitting my bathroom, and want to double check that I've sorted the right cable size for the electric shower.

    The shower unit is a 9.8 kW Mira Escape, which is a higher wattage than the old shower. The existing wiring is 6mm twin and earth fed from a 40A Type B MCB in the 30 mA RCD protected half of a split load consumer unit. It is clipped to the wall downstairs before running through the floor space in the bathroom, up a stud wall cavity and into the loft where it is connected to a double pole isolating switch, then runs down the stud wall cavity to the electric shower. Looking at table 4D2A of the IEE Wiring Regs, it seems that a 6mm T&E cable, clipped direct and/or in a non-insulated wall, 30 degree ambient temperature and not bunched with other cables, is rated for 46A. The Mira instructions recommend a 45A MCB for protection, and the showers current rating is just under 41A.

    It seems close, so I just want to double check by asking the experts on here if I've overlooked anything which could alter my cable ratings and mean I need to run a new 10mm T&E supply.

    Also, on the same subject, is it acceptable to replace the pull cord double pole isolator in the bathroom with a wall mount double pole isolator located next to the consumer unit (it's out of sight under the stairs then!). I've read that the double pol switch has to be a pull cord in the bathroom, or can be a wall mounted one in an adjacent room. Where I want to fit the switch wouldn't be an adjacent room, but I don't see why that would matter?

    Thanks in advance,

    Steve
     
  2. sparkybird

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    It looks close, but OK. You could always leave it on the 40A mcb if you were worried. I doubt you'd get it tripping unless you like very long and scalding hot showers!

    BTW the NIC recently published some guidelines on the application of diversity to water heater (eg shower) circuits, so this would defo be OK on that basis

    Hope this is helpful

    SB
     
  3. bernardgreen

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    It would only matter if the shower had to be turned off in a hurry when the person in the shower would have to run dripping wet and maybe naked through the house to find and operate a switch with wet fingers.
     
  4. GaryMo

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    BS7671 allows no diversity to be applied to instantaneous water heater circuits.
    I take it the NICEIC have over ruled BS7671? Have you got a link to this information?
     
  5. Steve

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    1. how would temperature of an electric shower make an iota of difference to the current drawn? (Think about how electric showers work)

    2. Never mind the NIC, how do you apply diversity to a fixed constant load drawn by an electric water heater? :eek:
     
  6. TicklyT

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    1. depending on the design of the shower, a cool shower may only require one out of two heating elements to be used.
    On the othe hand, a very hot shower will draw fractionally less than the maximum current unless the heater element has a negative temperature coefficient.

    2. Although not strictly diversity, it may be considered possible to make an allowance for the duty cycle by considering how the shower will typically be used. For example, after a 10 minute shower, it will be five minutes 'at rest' before the next user can get in the bathroom to switch the thing on again, and it is extremely unlikely it will be used 24/7 in a domestic situation. In fact it would probably be no more than about 30 minutes per day, which might make a small overload permissible.

    Having said that, if the BS says diversity cannot be applied, then diversity cannot be applied if the installation is to meet the BS.

    (stands back and prepares to duck the incoming.....)
     
  7. GaryMo

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    No need to duck, you're spot-on!
    It's BS7671 though ;)

    Not aimed at TicklyT, just general information.....

    Most electric showers have at least two heat settings, the highest setting obviously being full load. The dial which alters water temperature does not alter loading, it's merely altering the flow of water. The faster water flows over an element the cooler it'll exit the shower head, slow the water down and water obviously has longer to heat up.
     
  8. briwire

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    Sorry but this contravenes the regs. 6mm is only rated at 46A if it is completely clipped direct to the surface. If it passes inside walls and in lofts especially under insulation, its rating plummets to 35A. Use table 4D5A for flat cable. You should use 10mm, rated at 47A under these conditions.

    As for the switch. I think it is to ensure that the switch can be related to a nearby appliance and that it can be reached in an emergency.
     
  9. sparkybird

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    Gary Mo

    Yes, I agree that BS7671 allows no diversity to be applied to showers. The NIC guidelines were published in their mag a few months ago. Can't post a link I'm afraid.

    Briwire - the OP mentioned that none of the routes were insulated, therefore the cable would be good for more than 35A!

    SB
     
  10. Softus

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    Why even bother contemplating 6mm²? It might cost an extra £20 for 10mm², but so what, when installing an entire bathroom including shower?

    Diversity? On a shower circuit?! :eek:

    Isn't the screaming coming from your common sense so loud that it drowns out any [bizarre] thought that the NICEIC might be right?

    F.F.S. :rolleyes:
     
  11. briwire

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    It is clipped to the wall downstairs before running through the floor space in the bathroom, up a stud wall cavity and into the loft where it is connected to a double pole isolating switch, then runs down the stud wall cavity to the electric shower.
     
  12. Softus

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    The OP hasn't installed it yet. :confused:
     
  13. securespark

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    Assuming there is also no use of trunking anywhere...
     
  14. GaryMo

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    If the majority of cabling is clipped direct (method 1) but a small amount is run in trunking (method 3) at what point would you consider changing from method 1 to method 3 for cable calculations? 10%, 25%, 50% in trunking?

    I've always ignored drops from ceiling to accessory.
     
  15. Softus

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    Do you consider the effect of any trunking to be a risk of overheating?

    Do you consider the effect of any overheating to be a risk of melted insulation?

    Do you consider melted insulation to be a safety hazard?

    Do you know why you ignore it?
     

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