Electrical Wiring Question

Discussion in 'Domestic White Goods Forum' started by jefflad, Feb 8, 2014.

  1. jefflad

    jefflad
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    Firstly, I'm not too sure if this is the correct forum area but here goes.

    Just had a new fire installed, it had 2, 3amp plugs coming from it... one for lights the other the fan. I've had a sparkie in to put them to a fused spur but I've a couple of questions with regard to job.

    He's wired them both into one fused spur, is this OK and finally the fused spur has a 13amp fuse in, again is this correct or should it maybe have a 3amp fuse in?

    Thanks in advance :)
     
  2. Gavtech

    Gavtech
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    The ideal thing to do is check the rating plate on the fire where the wattage used will be stated. Either for each separate feed or as a total.
    (I'm intrigued as to the nature of the fire having two separate feeds like this)

    If the total is less than 720W you can use a 3 amp fuse.
    I think it is extremely likely that lamps and a fan will fall significantly below this value so a 3 amp is probably the one to use.

    Incidentally, the 3 and 13 amp ratings are the two designated values that are made commonly available for domestic purposes... but other values are ( less commonly) available if closer protection is deemed to be necessary ( I don't think so in this case)

    Values available are 1, 2, 3, 5, 7, 10, and 13 Amps
     
  3. jefflad

    jefflad
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    Thanks Gav, I'll need to dig the manufactures stuff that came with the fire. The 2 plugs relate to a set of lights in the hearth (2 20w bulbs) and the other is the power flu. My concern is would the 13amp effectively damage either of the equipment and am I best shoving a lower fuse in... also I guess, I was looking for a nod that having both wired into one spur is fine :)

    Thanks, I really appreciate the advice on this.
     
  4. Gavtech

    Gavtech
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    The main threat is to the flexes themselves. There is little threat to the equipment in my view.

    Rating is a complex matter and the regulations have always been somewhat flexible in certain applications .
    The regulations are also always changing and I am not up to speed with them at all these days.
    But the most important thing about the regulations is that they provide informed guidance about what is safe practice, and it is more important to follow safe practice than it is to follow the letter of regulation (The regulations used to embrace this)

    Take the circuit feeding the bulbs: 2 x 20W = 40W total. Its nothing.
    But when bulbs fail , they often fail as a momentary almost short circuit. This is due to the plasma that forms when the filament breaks, which is highly conductive, much more so than the filament itself so the current demand suddenly skyrockets.

    This is why blowing bulbs sometimes will blow a fuse or trip a circuit breaker.
    The point is that the sub circuit ( in this case the flex) has to be able to safely carry the fault current until the safety device ( the fuse) operates.

    The same applies to the motor circuit. In the event of a fault (Arrested motor say ) the supply flex must be able to stand the excess current until the protection operates.

    The flexes are probably only rated at 3 amps each...although as I've implied rating is a complex and circumstantial matter - A '3 amp' cable will not fail at four... and may even take ten times that amount but it depends how long for! You want protection devices to operate long before a cable becomes a fire threat.

    So, your protection for this paired sub circuit should really be rated for what either one of them can stand i.e. probably 3 Amps, but this is only provided that the total current demand of both devices does not exceed the rating of the fuse (Equivalent to about 720 W if using a 3 amp fuse).

    I think it is very likely well within that 720W limit...but you should check to be absolutely certain.
     
  5. jefflad

    jefflad
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    Thanks for such a comprehensive reply... I've just went through all the docs and can find nothing for the fan or the lights :(

    I will give the fire company a ring tomorrow to see if they can furnish me with the info... I may just throw a 3amp fuse in and see how it goes, would that have any adverse affects?
     
  6. Gavtech

    Gavtech
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    Earlier I checked a few pdf's of power flues to see what a typical motor power requirement would be.
    Even in the specifications I found them unhelpful - simply stating a 3 amp fuse should be used.

    The one place you can be guaranteed to find the rating is on the rating plate on the unit itself - as this is mandatory. But I dare say this is not easily accessible.

    I think it is very unlikely to give you any problems whatsoever if you put in a 3 amp fuse. It would mean that the fan motor would have to take significantly over 700 watts and I would be surprised if it is a fraction of that. ( I dont know how powerful the motors are that they use in these devices but I am anticipating 100 - 200W)

    The fuse would get warm if it was near or significantly over its rating when both loads are operating.
    You could check for that. Just feel the front of the fused spur box when both items have been running for a while.
    I expect it will stay totally cool.
     
  7. jefflad

    jefflad
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    Thanks, I'll check the suppliers tomorrow and update with anything relevant.
     
  8. noiseboy72

    noiseboy72
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    As has been said, the idea is that you connect them to separate supplies, so if the lights pop the fuse, you do not lose the fan as well.

    The fuse is mainly there to protect the cable. The lamps will self-fuse - IE: they will blow, while the fan will have a thermal fuse - or will use a stall proof winding, where the motor can stall without the winding overheating.

    Unless the cables are very thin, a 13A fuse will be fine. Any sparkie would fit a suitable fuse, so I would not worry excessively.
     

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