Advice on extending shower cable

Discussion in 'Electricians' Talk' started by Tom Bartonn, Feb 12, 2024.

  1. malbec4

    malbec4 Screwfix Select

    I can see your point. However, there are other considerations required. A guesstimate doesn't always work.

    For instance, 10500 watts @ 230 volts = 45.6 amps. Using Ref Method C, a 6mm T&E cable is big enough.

    or

    10500 watts @ 230 volts = 45.6 amps. Using Ref Method 103#, a 16mm T&E is not big enough.

    All this information comes from Appendix 4 which is informative only, not a regulation. A nice mix of engineering judgement (ie is the shower going to be continually in use) with a dash of Appendix 4 will give the correct answer.

    With this in mind, a 6mm cable will be big enough for a lot of installations concerning 10 kw or even 10.5 kw showers with a 45 amp breaker. Obviously, this will depend on the supply voltage.
    6mm is also okay, volt drop wise, for a 25+ metre cable run @ 45 amps.
     
    Coloumb likes this.
  2. arrow

    arrow Screwfix Select

    I would never put a 6mm cable on a bigger mcb than 32 amp.
     
  3. Wellwisher

    Wellwisher Well-Known Member

    Why not? A 32a MCB only required 4mm cable.
     
  4. malbec4

    malbec4 Screwfix Select

    If the cable is rated @ 46 amps, Ref Method C which includes cables being chased in and plastered over, why not? I'm genuinely interested.
     
  5. WH55

    WH55 Screwfix Select

    I did exactly this a few month ago. Bathroom fitter put in a small electric shower because it’s all we could do. I installed 10mm to the switch but then connected to existing 6mm from switch to CU. Customer said he might upgrade later when decorating different room on the path back to the CU so I future proofed it. Over specified cable isn’t always a bad thing.
     
  6. malbec4

    malbec4 Screwfix Select

    I agree. But only when realistically needed at a later date. As long as a 6mm cable is buried in plaster and/or clipped, it is fine to supply a 10.5 kw shower. Although the cable run should be less than 30 metres in length to satisfy voltage drop and max allowed Zs.

    That's not an opinion, it's just counting (thank you James O'brien -LBC presenter- for that little quote)
     
    Coloumb likes this.
  7. The Happy Builder

    The Happy Builder Screwfix Select

  8. FlyByNight

    FlyByNight Screwfix Select

    10.5kW will be te rating at 240v - thus 43.75A Showers normally have a headline power based on 240V

    Restistance of heater - 240*240/10500 = 5.4857 ohms

    At 230v 230*230/5.4857 = 9640W and 41.9 A


    But if voltage strays up to 250v current will also push up to 46A
     
    malbec4 likes this.
  9. adgjl

    adgjl Screwfix Select

    I’ve been waiting for the suggestion that a shower only needs 4mm cable for ages! What a load of “carp”!
     
    arrow, Coloumb and WH55 like this.
  10. WH55

    WH55 Screwfix Select

    4mm … Only if you want plenty of call backs from customers
     
    arrow likes this.
  11. arrow

    arrow Screwfix Select

    As you say 6mm will be big enough for some installations, however not all. Put it in some trunking and the capacity drops down. I prefer to run 10mm and I am covered.
     
    WH55 likes this.
  12. Wellwisher

    Wellwisher Well-Known Member

    Who suggested that? I said a 32a MCB only needs 4mm cable. The purpose of the MCB is to protect the cable.

    But having said that a 6kW shower would be OK on 4mm cable.
     
  13. I have shower cabling running through my garage but need to extend the cable. Can the cable be extended using junction boxes or must the whole cable be replaced with a longer length from fuse box to shower unit
     
  14. malbec4

    malbec4 Screwfix Select

    I never knew that! I thought everything was based on 230 volts.

    Thanks for the insight. Does this only apply to showers?
     
  15. WH55

    WH55 Screwfix Select

    It’s always better to replace a full length of cable if possible. That said, you can get junction boxes that are built for 60amp etc. If the join is in the garage and, therefore, visible and easy to check, a junction box may be suitable.

    EDIT - doesn’t look like our hosts sell them but I know other suppliers do.
     
    Last edited: Feb 13, 2024
  16. FlyByNight

    FlyByNight Screwfix Select

    Just had a look at the link @The Happy Builder provider in post #27 and on page 2.

    SPECIFICATIONS
    Electrical
    Nominal power Nominal power
    rating at 240V rating at 230V
    8.5kW – (40A MCB rating) 7.8kW – (40A MCB rating)
    9.5kW – (40A MCB rating) 8.7kW – (40A MCB rating)
    10.5kW – (45A MCB rating) 9.6kW – (45A MCB rating

    Don't know about other appliances, but certainly a lot of showers use the headline power based on 240v.
     
    malbec4 likes this.
  17. Wellwisher

    Wellwisher Well-Known Member

    Makes sense. That is the voltage most people get in mainland UK. Also the numbers look bigger so more attractive.
     
  18. adgjl

    adgjl Screwfix Select

    Exactly. The manufacturers instructions tell you that for your 7.5kW shower, the minimum cable size is 6mm, that a 30mA RCD is required in the circuit, and that a double pole isolator is required. The solution to extending the cable seems to be the suggested one of placing an isolating switch just outside the bathroom door.
     
  19. Wellwisher

    Wellwisher Well-Known Member

    As the regs don’t require an isolator the MI are wrong to say one is required. That’s not the only thing wrong either. Thet talk about IEE wiring regs. Now how long is it the IEE ceased to exist?

    If the MI are wrong we take account of that and don’t follow them.
     

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