Location: Fuse Ratings

Fuse Ratings, Fuse Deratings, and Fuse Applications

Fuses are used for many applications, in circuits in all environments. How do you choose the right fuse? We are here to help.

How are fuses rated?


When you are trying to select a fuse for a new circuit, the choices can be daunting. Typically, to size and select a fuse there is typically a list of approximately (4) things we NEED to know. So to protect any circuit, we need to know the following:
  • Voltage of the circuit_________
  • Amperage of the circuit _________
  • Type of Application (Motor or Electronics)
  • Code Compliance Requirements - USCG, NEC, etc.


  • Typically these things are extra credit and can help in the selection of a new fuse:

  • Manufacturer info if you have the old fuse
  • Rated Capacity of the Appliance per the Tag
  • We have developed quite a list here, so let's go over each independently.

    Voltage of the circuit:

    Typically the voltage of a circuit is dependent upon the power supply for the system. For instance a battery power supply will come at a specified voltage, car batteries are 12 volts typically. For a boat and RV applications, the voltage can range from 12 volts to 48 volts. Be sure to match the voltage of the fuse to the voltage of the system. A 12 volt fuse will not properly sense amperage in a 48 volt system, and will prematurely break down. In reverse, a 48 volt fuse may not sense a slight over current situation in a 12 volt system. The filament would not be sensitive enough to melt as it is designed for roughly 4x the voltage.

    Amperage of the circuit:

    The sensing, and curtailing of high amperage or current situations is the whole point. Basically a fuse stops higher current than the break point rating through a circuit.

    Type of Application for the circuit:

    Is your circuit in a car, or a home. There are fuses with supported filaments, designed for the adverse conditions associated with vehicles of all sorts. In addition, there are fuses that are designed to meet strict UL certifications for installation inside a home, office, or industrial application. Start with this section, and then move to code compliance below to dot the i and cross the t.

    Code Compliance:

    Indoors, in a power grid tied installation, versus in your car. The rules are very different. Let's just say we have inspectors in each of the worlds that this paragraph refers to. They determine whether or not your installation is in compliance. SO, to avoid scrutiny, and extra attention, know and follow their rules before you start.

    Extra Credit

    Manufacturer of the Old fuse:

    If you have the old fuse, let us know, take a picture. Send it via your phone. It makes life easy.

    Rated Capacity of the Appliance per the Tag:

    A 1000 watt refrigerator per the tag is a nice piece of information to include if we are sizing a fuse for the refrigerator.

    Another thing you may hear in conjunction with fuses will is fuse speed. The fuse's speed rating explains how long the fuse will hold up to excessive current. This can effect the derating procedure for some electrical applications. For instance a motor will always require a large initial power fluctuation, but only for a short period of time, and some fuses are designed to handle this. Some fuses are manufactured to allow this excessive flow but only for a measured length of time, although they can be more expensive. Some examples of how a fuse is speed rated include:
    F (fast)
    FF (very fast)
    Slow Blow
    Time Lag (slow)

    Now that we have the numbers, the application, and the extra credit handled, let's do the math so we can pick some fuses.
    Derating a fuse for your application



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