Wondering if this fuse is safe to use in this switch/plug to turn on /off a 12V DC 30A Power Supply 360W Power Supply that will power a RAMPS 1.4 board for a Prusa i3 with an external led display that contains an SD Card Reader.

I found the suggestion to use it here.

  • $\begingroup$ What's with the down vote? You be troll'n? $\endgroup$
    – leeand00
    Jan 20, 2016 at 16:00
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    $\begingroup$ It's really not a 3d printer question, it's an electrical engineering question that just happens to be about a 3d printer. $\endgroup$ Jan 21, 2016 at 4:41
  • $\begingroup$ @RyanCarlyle oh, well somebody move it to the Electrical Engineering Stackexchange then...no that's a bad idea, this site is in beta and needs the help. $\endgroup$
    – leeand00
    Jan 21, 2016 at 20:32
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    $\begingroup$ Fair point. Maybe the question could be "Is this fuse a good choice for my RAMPS Prusa i3?" or something like that :-) $\endgroup$ Jan 21, 2016 at 20:50

1 Answer 1


No, do not use this fuse. The current rating is too high to be reasonable for your printer. It will "work" in the sense that your printer will get power, but it won't provide anywhere near as much protection as a lower-rated fuse.

10A is a lot of current for mains voltage. Depending on what else you have plugged in, there is a fair chance your home's 15A breaker will trip before this fuse does, which kind of defeats the point of having it.

Even for "fast" fuses, it takes a significant amount of time for them to blow when conducting their rated current. The internal fusible link has to heat up and melt before the fuse stops conducting. The less the overload current exceeds the rating, the longer that takes. A 10A fuse conducting a 10.5A short might take 30 seconds to trip. In the meantime, your printer is melting. Lower-rated fuses will trip faster for the same short and thus provide better protection.

You need to size fuses as small as possible for the required current draw if you want to have any hope of rapidly cutting off an excessive-current event.

I would recommend a 4A fuse in the USA for this 350w power supply. (Note: the listing title says 360 but the photos show 350.) I use 4A fuses in several printers with 120v / 350w PSUs and they do not trip. But you can do the math for yourself:

350 watts / 120 volts / 80% efficiency = 3.64A

The smallest fuse you can find that is larger than this value is what you should use.

Now, we can argue over whether 80% is the right efficiency value... it could be lower. The PSU label says 6.5A input is required, but that amount of current draw implies either a <50% efficiency (which is quite poor for this kind of PSU) or would only occur for abuse/surge scenarios like starting very large motors. Such short-lived inrush events generally won't trip a fuse unless you do something dumb like lock the rotor. And none of that applies to the small microstepping-driver stepper motor systems we're working with here. This PSU should not draw more than 4A in normal 3D printer use.

Looking at this on the other end -- how much damage will 10A do versus 4A? Lots. If the short is in the 12v system, and the PSU's short protection doesn't trip in (because it's a cheap knock-off) you would roughly multiply the AC fuse current times 10 to get the DC current. And 40A is a downright scary amount of current! Depending on wire gauge, putting 40A through heatbed wiring may make it smell and smoke. Whereas putting 100A through heatbed wiring will almost certainly start a fire.

You're much safer with a 4A or even 6A fuse for this PSU than a 10A fuse.

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    $\begingroup$ Should also point out a 1-star review on the switch's Amazon entry that said it melted after an hour at 4.5A... Although it seems like a lot of different switches are sold under this listing. I actually have one of these from this exact Amazon listing in a printer right now, and the wiring to the onboard switch is very small. I would trust the main terminals to 10A but not the lighted switch part. $\endgroup$ Jan 21, 2016 at 5:09
  • $\begingroup$ So another words, with the fuse we're using the fuse to prevent power spikes coming in from the wall outlet that might send too much DC power to the heatbed and electronics and catch them on fire? So you don't think the switch is safe? $\endgroup$
    – leeand00
    Jan 21, 2016 at 20:57
  • $\begingroup$ Most often, the fuse is there to kill power in case there's a short in your mains wiring between the switch and PSU, which may be quite destructive before your home's circuit breaker trips. Or the fuse may trip if the PSU fails and starts drawing excess current. Having an oversized fuse may allow the switch to melt down without tripping any kind of protection. $\endgroup$ Jan 21, 2016 at 21:01
  • $\begingroup$ MOST of the time, shorts in the printer's wiring will be caught and stopped by the PSU's built-in over-current protection. You shouldn't really rely on that though if you're buying clones rather than name-brand PSUs (like the MeanWell that this PSU is copied from). $\endgroup$ Jan 21, 2016 at 21:04
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    $\begingroup$ If I'm not mistaken, any fuse you find rated to 125+ will be intended for AC. DC circuits over 24v are pretty rare. $\endgroup$ Jan 22, 2016 at 6:23

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