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I am reviewing the electrical specs for a few different devices, and I am just confused why some devices have a fuse rating that allows for a higher power input than what is listed as the "max power".

For example, the specs of one device states that it has a max power consumption of 200VA and has a 4 amp fuse. But with that type of fuse, technically it could allow for a max power of 4ampx120V = 480VA before the fuse blows. Why not just have a 2 amp fuse or lower to be closer to the max power consumption? Is this just bad design?

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3 Answers 3

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Running 2A through a 2A fuse will significantly shorten its life. Also, many "2A" devices will have inrush currents or spikes that exceed 2A for short periods.

Fuses are a safety device, not a current limiter. They're really only intended to blow if the device short-circuits, to prevent excessive current in the device itself from starting a fire or electrocuting the user if the case becomes live.

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  • \$\begingroup\$ Fuses prevent wires and traces from burning. That is about the best they can do. \$\endgroup\$
    – user105652
    Commented Aug 24, 2020 at 17:34
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A fuse is used (in the vast majority of cases) to protect upstream cabling infrastructure from over currents and prevent fires. It is not used to prevent a device (downstream) taking too much current except when that downstream current is excessive enough to endanger the upstream cabling infrastructure.

If the downstream device is consuming excessive power it is likely that it is damaged. A fuse will not stop a downstream device from becoming damaged (except in the rare case of zener barriers used in intrinsically safe equipment).

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  • \$\begingroup\$ The upstream cabling infrastructure should be protected by another fuse, even further upstream, no? \$\endgroup\$ Commented Aug 24, 2020 at 17:57
  • \$\begingroup\$ Not necessarily. \$\endgroup\$
    – Andy aka
    Commented Aug 24, 2020 at 17:59
  • \$\begingroup\$ I disagree, fuses protect downstream cables and traces not upstream. You are correct that fuses wont protect device itself. \$\endgroup\$
    – Rokta
    Commented Aug 25, 2020 at 6:50
  • \$\begingroup\$ Of course fuses protect upstream cabling - you can't rely on a 40 amp fuse on a ring main to protect the cabling on a spur or the cabling from the socket to the equipment where the fuse is housed. \$\endgroup\$
    – Andy aka
    Commented Aug 25, 2020 at 8:40
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A fuse is intended to protect the wiring, not the device.

There are many possible reasons not to use a fuse of the lowest possible capacity as dictated by the apparent demand of the device.

  • The device may take an inrush current, and a larger fuse may be cheaper than a time delay fuse
  • Running a fuse close to its rating can shorten its life
  • The manufacturer may only have a few fuse sizes approved within their organisation, and it saves them doing a load of documentation to add a new fuse size if one of their standard fuses will protect the wiring adequately.
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