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Discuss Piggy back main 12v 15A supply in the Auto Electrician Forum area at ElectrciansForums.co.uk.

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  1. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    The alarm I have has an inline blue 15A fuse. Is that ok to put a Piggy Back Fuse Holder on my main fuse (15A) constant 12v in fuse box? The Piggy Back Fuse Holder would then have 2 15A fuses, so would my main fuse then be likely to fuse(blow) if alarm had a fault as well as my alarm fuse? Logic tells me the device i am adding (alarm) should have a smaller rating fuse to the one i am Piggy Backing
     
  2. Andy-1960
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    Andy-1960 Electrician's Arms

    Location:
    Wolverhampton
    Might be best if you draw out what you are trying to do, photograph it then post it so we can understand.

    If you are trying to ask that if you put two fuses of the same size in series, which will blow first? - that is anybody’s guess.
     
    • Agree Agree x 1
  3. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    Ok. My fuse box has a fuse rated at 15A, this is where i can pick up a constant 12v supply for my alarm. The problem though is if my alarm has an inline fuse rated at 15A, is it ok to piggy back that to the main fuse in the fuse box. My concern is if alarm shorts or has a fault because the main fuse is 15A and the alarm is 15A, i could take out the vans 12v

    fuse.jpg

    Using one of these

    61YE6WCwVfL._SL1077_.jpg
     
    Last edited: Oct 9, 2018
  4. rsgaz
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    rsgaz Regular EF Member

    Location:
    UK
    It's not a good idea, no.

    Are you not fitting the alarm under the bonnet? You just need a ring terminal and an inline fuse holder, directly onto the battery positive. The fuse holder should be as close to the battery as possible.
     
  5. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    I was going to fit the alarm (brains) in the cab. Maybe just extend the cable then and go direct to battery, sounds a better way.
     
  6. rsgaz
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    rsgaz Regular EF Member

    Location:
    UK
    Yes, seeing as I expect you will be running wires for the alarm sounder, i.e. you're going through the bulkhead anyway.

    If you are determined to run from the fusebox, is your cigarette lighter permanently powered? The safe way to use those add-a-circuit taps is, say for example your cigarette socket has a 25Amp fuse, you would then plug in your new 15A fuse and change the fuse supplying the cigarette socket to a 10Amp, thus totalling the original.
     
  7. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    I looked at that, on a 20A fuse but not permanent live.
     
  8. rsgaz
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    rsgaz Regular EF Member

    Location:
    UK
    Any empty slots?
     
  9. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    But the 10A would blow before the 15A so it would never have 15A protection? I thought the main (original fuse) went in then the one needed for what you are piggybacking, ideally a lower rating so it would blow first
     
  10. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    Yes loads, not sure if anything goes to them though. Will get my thin probes out have a prod
     
  11. rsgaz
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    rsgaz Regular EF Member

    Location:
    UK
    They're not '2 in a line' like that, they are in parallel, just like all the other fuses.
     
  12. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    Oh right!
     
  13. rsgaz
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    rsgaz Regular EF Member

    Location:
    UK
    There is a right way and a wrong way round to fit those...

    addAcircuit.jpg

    If you put it in the wrong way, then yes, the current would sort of be 'back fed' through the 10A fuse first, round to the 15A and then to the new item. But the correct way round is the supply being fed to the left leg in the above diagram.
     
    • Useful Useful x 1
  14. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    Just watching this quite helpful too, I have never in all my years looked at what way a fuse goes in a car!! Now I will (6 min 20 sec in )
     
  15. PaulBerkshire
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    PaulBerkshire EF Member

    Location:
    Berkshire
    Think i should check all my fuses now :(
     
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