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  1. #11

    Join Date
    Mar 2001
    Posts
    1,021
    Location
    Arkansas
    Tractor
    TN70D, 4wd, 16x16 trans

    Default Re: Sizing an inline fuse

    Dan,

    Your absolutely correct. I should have made that more clear.

    My general policy is to oversize the wire a bit since it cost little to do so for all but the longest of runs. Then the load only needs to be considered for the fuse size. Particularly for 10-20 feet of small gauge wire like you would need for this application.

    #8 and larger is a different story since this stuff gets pricey.

    Fred

  2. #12
    Veteran Member chim's Avatar
    Join Date
    Apr 2002
    Posts
    1,727
    Location
    Lancaster County, PA
    Tractor
    Kubota L3200, Ford 1210

    Default Re: Sizing an inline fuse

    There's another facet to the sizing of "protection" that is often misunderstood. A fuse or breaker sized to protect a load rarely protects the load in the sense that it prevents the motor / light / heating element from going bad. On occasion, a motor may be protected in an overload event, but the fuse or breaker most often blows or trips after the load already took a crap. It is common to have 15, 20 or larger amp protection on circuits that normally see a fraction of that current.

    For protection of motors, the common method is to either have built-in thermal protection or a motor starter with same. Fuses and breakers are there primarily for short circuit (wire) protection. There are not enough sizes of fuses available for proper protection of loads. Thermal elements for a motor starters, especially for smaller motors, have ranges like 3.56A to 3.95A, 3.96A to 4.23A, etc. They don't jump in 1,5 or 10A increments.

    For lighting and power circuits, it is not uncommon to have wiring connected to the circuit that would be considered undersized for the circuit's fuse or breaker. Two examples are a fluroescent light - they have 18ga wires for the line side of a ballast, but are connected to 15A circuits in a residence (larger circuits in commercial applications). Another is the circuit your computer is plugged into. At home, you have 15 and 20 amp circuits, depending mostly on the room you're in. The electrician has no idea what you will come along and plug in. There are requirements for circuits in different living spaces, but they do not address protection of devices you come along and connect. If you plug a radio or clock or table lamp into a 20 amp circuit in your kitchen, the fuse or breaker has no way of knowing it's not the coffee pot. The load you plugged in may need less than an amp to operate properly, have some kind of internal problem, and draw 12 amps and smoke. Meanwhile, the breaker assumes all is well because that current well below its max................chim

  3. #13
    ddl
    ddl is offline

    Join Date
    Apr 2000
    Posts
    349
    Location
    Peculiar, MO
    Tractor
    B2400 Kubota

    Default Re: Sizing an inline fuse

    My comments deal only with motors and industrial wiring. House wiring is something else and I would never attempt to tell any one that I knew much about that. Too many assumptions for me. Just give me a motor to control!!!

    Dan L

  4. #14
    Platinum Member
    Join Date
    Apr 2000
    Posts
    762
    Location
    Greater Springfield area, Massachusetts
    Tractor
    Kubota B2910, also Honda HT3813 with mower and front blade.

    Default Re: Sizing an inline fuse

    Here's yet another bit of fuel to add to the confusion fire.

    A fuse does not "blow" instantaneously at its rated value.

    For instance, a 10 amp fuse with 10 amps of current flowing through it might last for several minutes, maybe even an hour before it fails. As the current overload increases, the "fail-time" decreases. A 20 amp current draw might cause the fuse to blow in a second. A 40 amp overload might blow the fuse in 0.1 second.

    I've attached a generic pic showing the fail-time vs. % overload for three different types of fuses. The green line might represent a "slo-blo" type where the red line would represent a "fast-blo" type.

    Therefore, if the fuse is being used to protect the device, a lot of information is needed as to what current is "excessive" and then a fuse with the correct "fail-time" and current rating needs to be selected.

    Is everyone confused now?

    ~Rick
    Attached Images Attached Images

  5. #15
    New Member
    Join Date
    Sep 2005
    Posts
    0

    Default Re: Sizing an inline fuse

    I've changed in the past few years to using more in-line circuit breakers than fuses for primary power supply. Unless you have a dead short in the line, most blown fuses are caused by a surge of voltage pulled at initial activation of the light/motor/etc.
    I use either auto or manual reset breakers as I don't have to go find a fuse. If the breaker continues to cycle, then I know I've got a problem.
    Just a suggestion.
    I usually order my stuff from either Terminal Supply or <A target="_blank" HREF=http://www.delcity.net>http://www.delcity.net</A>

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