To be more precise, I'm interested in the amount of "created" heat during normal operation from a diazed 16 ampere fuse.

How hot will the fuse be after delivered 16A in seven hours with the voltage of 230VAC, in a nominal environment of 30C.

  • 1
    \$\begingroup\$ Depends on how good you cool it and what its resistance is. \$\endgroup\$
    – PlasmaHH
    Commented Mar 7, 2016 at 10:10
  • \$\begingroup\$ Normal working conditions and 25C ambient temperature. \$\endgroup\$
    – Christian
    Commented Mar 8, 2016 at 10:01

2 Answers 2


To solve that you need to calculate the power dissipated in the fuse (P = IR) and then the total thermal resistance between the fuse and the ambient. This would be then similar to a heatsink calculation and there are many examples of this on the web.

You are unlikely to find thermal resistance of a fuseholder published so your best bet would be to form an experiment:

  • Set up a fusebox and place the fuse in question into the holder.

  • Run an appropriate current through the fuse into a test load. This can be a low voltage setup such as car battery and lamps. Using low voltage would allow you to install a temperature sensor beside the fuse.

  • Run the test until the temperature stabilises.

  • Calculate the thermal resistance using formula:

    $$ R_{Th} = \frac {\Delta T}{P} $$

where \$R_{Th}\$ is the thermal resistance of your fuse and holder, \$ \Delta T \$ is the difference in temperature between your fuse and ambient and P is the power dissipated in the fuse. The result will be in K/W (kelvin per watt).

You can then use this figure to predict temperature rise for any other current through that fuse.


The circuit voltage is irrelevant, only the 15A current matters. Why?

After 7 hours, it will have reached steady state. The thermal conductivity from the fuse to ambient will be all-important, and it's not only not well controlled, it's usually unspecified anywhere. It's easier to measure the fuse temperature, than it is to try to work it out.

  • \$\begingroup\$ I can see in standards, that fuse carrier have a 4W (DII) power loss and shall be able withstand 185C in x days. Just want to know the nominal working heat temperature actually at max rated power at x ambient. 15A was just an example. This to get a sens of how hot the environment can be. \$\endgroup\$
    – Christian
    Commented Mar 7, 2016 at 11:13
  • \$\begingroup\$ The fuse carrier and fuse box will have a maximum environmental temperature specification. The temperature of the fuse is irrelevant to you, the user, if you are using it in a correctly mounted fuse holder in a fuse box. \$\endgroup\$
    – Neil_UK
    Commented Mar 7, 2016 at 11:55
  • \$\begingroup\$ Neil, I'm asking cause it matter to me, what the heat could be. I'm not asking this question out of an user perspective :) \$\endgroup\$
    – Christian
    Commented Mar 7, 2016 at 13:01

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