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I have a 12 V DC, 80 watts fan that is powered by an 8 Amps power supply. I would like to fuse the input power entry module (Corcom) 120 VAC. It seems that most fuses out there are rated for 250 Volts.

  1. Can I use a 240 VAC fuse on a 120 VAC circuit.
  2. What current rating should the fuse be?
  3. Should I fuse the DC circuit as well?

The product is an indoor table-top small spray booth.

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    \$\begingroup\$ For the AC power, you can use a 250V fuse for any AC voltage less than 250. But don't use an AC fuse for DC power. Should you fuse the DC power? Is there a failure mode that would lead to a dangerous condition that can be prevented with a fuse? If so, then I guess you should fuse it. \$\endgroup\$ – mkeith Feb 11 '17 at 7:13
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    \$\begingroup\$ What are you trying to protect by installing fuses? \$\endgroup\$ – Andy aka Feb 11 '17 at 11:58
  • \$\begingroup\$ I have a water humidifier container above my power entry module that can spill water over the power entry plug and potentially cause a short circuit and that's why I would like to fuse the AC circuit. \$\endgroup\$ – S. Chaoui Feb 11 '17 at 21:53
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    \$\begingroup\$ A fuse might protect components, but it won't protect people. Don't put water above mains electricity. \$\endgroup\$ – Someone Somewhere May 6 '18 at 0:45
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    \$\begingroup\$ A fuse is in place to protect (i.e. prevent fire) the thinnest wiring between the fuse and power source. If you have wires to handle 50A you can use a 50A fuse but the mains distribution breaker will open at 16A (or whatever) to protect your wall wiring. \$\endgroup\$ – KalleMP Sep 26 '18 at 11:26
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Yes, you can use a 240V fuse. The voltage is not relevant in such a case. Fuses are designed to protect over-current, not over-voltage. Therefore as long as the fuse is sized appropriately for your current and as long as your fuse voltage meets or exceeds your application voltage the fuse will safely protect your system.

The current drawn from a 120V source to power an 80-Watt device will be under 1A (2/3A) but for some devices you may want to consider a margin of safety for the initial power-on spike.

In your case, it will be safe to use official 3.15A 240V fuses. It will protect the system from accidents just fine. If the initial power-on current is not very high, you can use 2A ones too.

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There's something wrong in your calculations or somewhere else.

enter image description here

if: P=80W V=12V I=8A

there's at least one parameter, that's wrong. But I'll stick to the 80W as it's the easiest and simplest.

1. And you can use a 120VAC rated fuse, no need for 230VAC. But if you don't have a 120VAC one, a 230VAC will do it too.

2. 80W on the primary side means 0,66A of current, which is a constant current, that will be drawn on the primary side, if the ac-dc converter is 100% efficient. Because we don't know either you have a linear or a switcher power supply (I'll just assume, it 's switched one), you have ~85% efficiency. Lets assume that.

Which means, your constant current flow will rise from 0.66A to 0.77A of constant current(because of losses in your power supply). And since your fan has large starting current, you have to triple it at least, so about 2.1A - 2.5A fuse would be best for this solution (on the primary, 120VAC side).

In this case, you can use a medium-blow or a slow-blow fuse. I would use slow-blow one.

3. No need for 2nd fuse, if you have one on the primary side. If your power supply is worth anything, it has also an inner thermal fuse, which will prevent destroying the PSU itself.

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    \$\begingroup\$ He never said he was using the full output rating of the power supply. Using an 8A PSU to run a 6.66A load seems perfectly reasonable to me. \$\endgroup\$ – Peter Green May 6 '18 at 0:07

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