Power in loft - Is this allowed

Discussion in 'Electricians' Talk' started by xyz123, Nov 26, 2020.

  1. xyz123

    xyz123 New Member

    Hi Good morning- Hope you all are safe and well. I need to get some power in loft so wondering if someone can help please.

    I currently have a 18W flouroscent light in loft which is connected to a 10m long plug socket. Everytime i want to use the loft, i get the cable down and plug into a socket on first floor.

    I want to add a positive input ventilation fan in loft. This has 18w fan and 400W heater and manufacturer recommends connecting with 3A FSU.

    Can I tap into the upstairs lighting circuit to get power for these two? I was thinking to take a spur to a 3A FSU and then onto a 3A fused sockets with "not to exceed 3A labels". One socket for ventilation fan and one for light. This way i dont need to install a separate switch for the loft light? Fan manufacturer manual allows installation via 3A plug and socket and doesn't require "hard wiring".

    I am open to other suggestions if i am doing something wrong. Getting socket ring extended in loft will involve too much work so i thought of this.

    Thanks for your time in reading my post.

    Edit: All upstairs lighting is LED lighting which should have reduced the load on lighting circuit if it matters.
  2. Bob Rathbone

    Bob Rathbone Screwfix Select

    Run a spur off the upstairs ring circuit, it may be 'too much work' but it is the correct method, how would you expect an electrician to do it? not the way you suggest.
  3. xyz123

    xyz123 New Member

    I would do this work myself ideally. I am am electrical engineer working in oil and gas industry and I do electrical design upto 33kV day in/day out but our projects are not in the UK so i don't know enough about UK wiring regulations and hence the question. I thought electrically there is nothing wrong with what i suggested (labels are accepted part of restrictions in other part of the world and the design (not execution) is same as adding another bathroom) but i obviously don't know about the 17th (or is it 18th) edition regs. Thank you for your post.
  4. The Happy Builder

    The Happy Builder Screwfix Select

    The regulations have a minimum requirement of 1.5 mm circuit conductors for a power circuit.

    So what do you have?
  5. Teki

    Teki Screwfix Select

    xyz123 likes this.
  6. xyz123

    xyz123 New Member

    I will check that.

    Genuine question- If lighting circuit can be 1mm2 and can have multiple old halogen bulbs and bathroom ventilation fan fitted, why cant it be used for a 400W fan with heater. I am genuinely trying to understand the technical logic behind this.
  7. unwokeable

    unwokeable Member

    Have a look at table 52.3.
    The minimum cable for lights is 1.0mm the minimum for power is 1.5mm. What you are proposing with the fan comes under power.
    xyz123 likes this.
  8. Banallsheds

    Banallsheds Well-Known Member

    There is point in an FCU on the lighting circuit. A 3 amp fuse has no discrimination against a 6 amp MCB. Don’t worry about manufacturers recommendations. FCUs don’t exist in other parts of the world.
    xyz123 likes this.
  9. terrymac

    terrymac Screwfix Select

    Don't you mean NO point ?
    xyz123 likes this.
  10. Comlec

    Comlec Screwfix Select

    Don't you mean Selectivity
    xyz123 likes this.
  11. Bob Rathbone

    Bob Rathbone Screwfix Select

    I believe that Banallsheds is pointing out that the post operation arcing time of a 3A fuse will exceed the total operating time of the 6A CB. The result will be that under fault conditions, the fuse blows and the 6A CB operates.
  12. xyz123

    xyz123 New Member

    Having looked into this I think there maybe another solution if you educated people can let me know about compliance with regs.

    I have a dedicated circuit from main fuse box upto the immersion heater on 1st floor. 16A MCB. Heater is never used by us and is infact broken so isolated currently. There is a easy cable route from hot water tank upto the loft. I understand in future immersion heater maybe used so Can I split this circuit into two and take power into loft using 1.5 or 2.5mm2 cable? Thanks.
  13. You could also try asking on this electricians forum, https://talk.electricianforum.co.uk/ UK based,very knowledgeable and helpful, quite happy to offer qualified expert advice.
  14. rogerk101

    rogerk101 Screwfix Select

    If this is to eliminate condensation in your loft then that's quite a brute force way of doing it ... not to mention being quite unfriendly to the polar bears. :)

    I used to have a really horrible condensation problem in my loft after I installed ceiling insulation between the ceiling joists. Before I installed the insulation, I guess the heat used in the house by the previous owners must have kept the loft dry because there were no signs of damp rot on the timbers.
    I did some serious investigations and discovered that there was absolutely no ventilation in the loft whatsoever, so whatever moist air that got into the loft condensed on the underside of the tar paper to the point that it dripped all over thus rendering the loft useless for any storage of stuff that could get damaged by being wet.
    I bored some 110mm holes in all the gable ends of the loft, which helped quite a lot, but not enough. I then installed some roof vent tiles on all roof faces so that there would be some through draughts from the windward to leeward sides. The helped even more. The loft is not dry enough to store stuff - even during long cold spells like the beast from the east.

    If I were to repeat the whole thing, I'd probably just install venting ridge tiles. My neighbours did that and it seems to have done the trick for them.

    Bottom line is that with a little thought and effort the loft can be kept nice and dry without resorting to more energy consumption.
  15. Teki

    Teki Screwfix Select


    Oh, what authority do you have to give people advice to ignore manufacturer's recommendations?

    Let's hope there isn't but if a fire occurred and the cause was the heater, the first thing the fire investigators would do is check if it was installed as per the manufacturer's recommendation. So unless you're prepared to underwrite any future insurance claims by insisting on a deviation to the manufacturer's recommendations, keep your advice to yourself.
  16. xyz123

    xyz123 New Member

    Thanks. While House Ventilation (PIV) is to solve condensation problems in house not in the loft.
    rogerk101 likes this.
  17. The Happy Builder

    The Happy Builder Screwfix Select

    I read the Building Research Establishment report on PIV it said there's no scientific evidence that it improves the air quality in homes, but anecdotal evidence based on people who have it all saying it did was the exact opposite.
  18. Banallsheds

    Banallsheds Well-Known Member

    Manufacturers recommendations are exactly that recommendations only. They do NOT have to be followed especially if they are wrong which is not unusual.

    Here is a typical example of incorrect manufacturers recommendations from B & Q.

    Attached Files:

  19. Bazza

    Bazza Screwfix Select

  20. The Happy Builder

    The Happy Builder Screwfix Select

    The immersion heater is 3 kW and draws around 13 amps, the circuit cable should be 1.5 or 2.5 mm, so probably rated between 15 and 20 amps.

    The most common arrangement is a 16 amp MCB, 2.5 mm twin and earth supplying a 13 amp water heater.

    So yes, there is almost certainly some spare capacity and the circuit can be rejigged.

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