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Any Rules of Thumb for Fuse Sizes

Discussion in 'Power Supplies' started by BundyRoy, May 23, 2014.

  1. BundyRoy

    BundyRoy Senior Elf

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    Is there a rule of thumb for the safety factor when putting in an inline fuse. My largest current load is 5.4 Amps on any one length of strip. I have 4 lengths of strip being powered by the one PSU. Each strip is coming off a separate output on the controller. The strips are powered downstream of the controller not through the controller. Do I put an inline fuse into each line that is:

    a) just above the calculated current load for each strip or
    b) a percentage below the rated current load for the power wire
    c) just below some calculated maximum current load for the strip
    d) some other criteria.

    I'm just about to order my fuses, so wanted to double check.

    Thanks
     
  2. ecbailey

    ecbailey Full Time Elf

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    I think people generally put in ones that are below or at the maximum current for the part of the circuit that can handle the least current. But, as done in some controllers that can handle high current, there is no harm in putting a lower one in if you don't expect to get anywhere near the maximum. Just remember to change it out if you load more onto the fuse.
     
  3. ԆцряєсϮ

    ԆцряєсϮ Senior Elf

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    The purpose of the fuse is to protect the cable from over current.
    If the fuse size is greater than the current carrying capacity of your cable, and a fault occurs, that cable will draw a greater current than it can handle, heat up and most likely either melt or catch fire.
    If the cable you are using is rated at 10 amps then a 10 Amp fuse is the largest you should use. More likely a 7.5 would be best.
     
  4. OP
    OP
    BundyRoy

    BundyRoy Senior Elf

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    Thanks for the replies. Wouldn't the wire in the strip lights be the wire in the circuit with the lowest current carrying capacity. I've got no idea what wires are in the strip lights but it doesn't seem to have any big wires in it. What sort of amps can you safely put through strip lights.
     
  5. OP
    OP
    BundyRoy

    BundyRoy Senior Elf

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    Another question about fuses. I have two cables going from a PSU to two arches. If I put then both through the one fuse does it still work. IE if I get a short in one of the cables or lights will it blow the fuse or does having the other cable there somehow effect it. I realise that if it blows the fuse then both lights go out, but was just wondering what effect two circuits on the one fuse has. I also guess it double the amps through the fuse. Any advice will be greatly appreciated. Thanks. The fuse is on the positive, so it goes PSU to fuse and then joins the +ves of the two circuits to the downstream end of the fuse.
     
  6. AAH

    AAH I love blinky lights :) Community Project Designer

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    Ideally the fuse would be rated such that it protects the thinnest wire on the circuit. If you tee off from a point after the fuse it's probably overkill to go with wire twice the diameter than what you need just so you can have the 10A fuse protecting the 2 5A cable runs for instance. Having a fuse in the circuit is the most important thing and generally a fault condition will result in an overload well above the typical current.
     
  7. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    The fuses primary role is not to actually protect the device that is connected to it but to in fact protect the wiring, so the fuse must always be rated below the correct derated current load of the cable to protect it from causing a fire if a short occurs as then the cable will be drawing more current than it is rated for.

    What most people dont know is that cables current ratings are derated when installed in different situations due to the ability to remove heat. A single cable in open air will have a higher current rating than a bundle of cables in a conduit and thus a different fuse/CB rating to protect it.

    So the fuse must be rated to protect the smallest cable on that circuit.
     
  8. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Thanks Alan and Eddy. What I have is my arches are powered by the same PSU as the pixlite 16. I have power running from the pixlite 16 to the arches through the 14/.20 cable (10m long). I then have another 14/0.20 cable going straight from the PSU to each of the arches (one cable to each) again both are 10m long. I figure the pixlite has a fuse so I have not done anything there. Both direct injection cables come out of the one fuse at the PSU. I started with a 7.5A fuse as this was the smallest I had at the time. I then looked up the rating for 14/0.20 cable in the wiki and it said 4.5A. So I then went looking for a 4A fuse. Couldn't seem to find one locally so I put in a 5A fuse to replace the 7.5A fuse.

    I have 96 pixels of 12V ws2811 3 led/pixel 10 pixels/m strip on each arch. By my calculations this should require 5-6A. I figure this is shared over the two cables, so 2.5-3A on each give or take.

    I do have some 1.5mm2 and 2.5mm2 cable I could use for the power injection if you think it is warranted. I have only run this a couple of times so far. I know the 14/0.20 cable is not that good for power but it was all I had at the time after changing the position of my arches at the last minute. My mini directors were very keen to get the show going on Sunday whilst we had Gran staying. I also have extra fuses, just been lazy so far.
     
  9. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    With the 14/020 cable all i did was double up the cores for power injection because you only need to use 2 cores, so may as well double the capacity and use all 4 cores of the cable.

    A 5 amp fuse will be good enough
     

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