Shower help!

Discussion in 'Electricians' Talk' started by Portaloo Pete!, Apr 14, 2019.

  1. Portaloo Pete!

    Portaloo Pete! New Member

    Having an arguement with so called electrician....Fitting a 8.5kw shower in modular building. He is saying 32amp breaker and 6mm cable is ok....cable length is approx 15m and in conduit 95% of it.
    I thought 10mm cable and 40amp would be better option.
    Currently there is a 7.5kw shower there with 32 amp breaker or would best option be just replace for another 7.5kw?
    Many thanks
  2. CGN

    CGN Well-Known Member

    2 so called electricians then.
    Davebuilder and Jord86 like this.
  3. KIAB

    KIAB Well-Known Member

    A 10mm cable would future proof you ,& allow larger showers to be fitted, nice in the winter.:)
  4. unphased

    unphased Screwfix Select

    Yes, the existing setup, 32A breaker on 6mm2 cable is standard for that 7.5kW shower. If you want to go to an 8.5kW shower you would step up to a 40A breaker on 10mm2 cable.
  5. seneca

    seneca Screwfix Select

    Agreed that's how it should be although I've seen plenty of 8.5kW showers working quite happily on 6mm and 32amp breaker, although in this case the 15m run might be an issue.
    CGN likes this.
  6. peter palmer

    peter palmer Well-Known Member

    My old electric shower in my previous house was an 8.5 Kw on a 6mm cable, probably just less than 15M and that was absolutely fine for years, not sure what size MCB it was but I would think 32 or 40A would be fine with a fixed 8.5Kw load.
    KIAB, CGN and seneca like this.
  7. unphased

    unphased Screwfix Select

    It all depends if you want to follow Regs or not. I agree that an 8.5kW shower can probably run quite happily for years on a 32A MCB on 6mm2 cable. But it doesn't comply with Regs! The power formula. I=P/V proves the point.

    7500/230=32.6A. 8500/230=37A. You wouldn't comply with Regs by putting a fixed load of 37A on a 32A breaker! As for sizing the cable, that's worth a calculation too. You really need to understand the point of electrical installations before posting opinions. You either comply with Regs or you don't. There are many non-compliant cases that exist without issue. Multiple spurs off a ring being one that never seem to be a 'problem'. I am sure that is the case but nevertheless they don't comply with Regs.
    KIAB and seneca like this.
  8. peter palmer

    peter palmer Well-Known Member

    Usually only 8.5kw at 240v. I think it boils down to do you always do as you are told.
  9. CGN

    CGN Well-Known Member

    I’ve had this chat with my sparky before as I presumed a 6mm on a 32 for anything bigger than a 7.5 would be under specced.
    Like others have mentioned, although not ideal,not necessarily a major issue.
    An 8.5kw shower would have to be on full hot to come anywhere close to pulling its maximum, and the water flow rate at that setting, would be that of a mouse with cystitis and therefore unusable.
  10. peter palmer

    peter palmer Well-Known Member

    The shower will be at max power and current draw from the second its switched on no matter where the water flow is set (unless the half power button is pressed)

    It regulates the temperature of the water by allowing the cold water to flow quicker or slower over a maxed out element. The quicker the flow the less time it has to heat up.
    nigel willson and CGN like this.
  11. Bob Rathbone

    Bob Rathbone Well-Known Member

    The 8.5 kW shower will draw 35A if you use 240V and 36A if you use 230V, the load is bigger than the design current of the circuit, non compliance ooops! I agree with others on the site that 10mm with a 40A breaker would be a better option as it would comply on design current and would be more likely to be OK on VD. I assume that the cable running to the modular building feeds more than just the shower, a heater, a light, a fan? Has this been taken into account.
  12. Comlec

    Comlec Well-Known Member

    If the shower is designed to run on 230V and is rated at 8.5kW then the current drawn will be 8500/230 = 36.95A
    From this the resistance of the element can be calculated 230/36.95 = 6.22 Ohms
    From this we can calculate the current drawn at different voltages
    At 240V 240/6.22 = 38.59 Amps
    At 245V 245/6.22 = 39.38 Amps

    And 245 Volts is not uncommon in the UK
  13. peter palmer

    peter palmer Well-Known Member

    Do you not think the values are way over cautious on current limits for cables. I know it depends on install methods and insulation etc but take a piece of 2.5, generally its fine on a 20A MCB, even a 25A one possibly so it follows that 2 x 2.5's in parallel which is equivalent to 5 sqmm would be fine on a 40A MCB yet a 6mm cable can only go on a 32A in general situations. Also 4mm can be protected by a 32A MCB so whats the point in 6mm then?
  14. unphased

    unphased Screwfix Select

    Peter I share your point of view on the real ability of cables to sustain loads far greater than the design tables suggest. The reason for the over design is to ensure that installations remain safe when bodgers get involved doing things they aren't supposed to do. For example, putting higher rated showers on circuits without proper knowledge. I believe most design codes include a factor of safety of 3, in other words, whatever the known capacity of a cable is, divide the capacity by 3 to make darn sure it can cope and make that the design criteria. It would be pointless designing everything down to the bone because the least error would render the design unsafe. People make mistakes and people bodge, its a fact of life. The design codes ensure a margin of safety to account for it to a degree. They can't cope with complete morons like putting showers on 2.5mm2 cable for example.
    sparky steve likes this.

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