How do you determine Amp rating for a fuse?

[BinaryMode]

I did a project which is two 5 volt USB powered fans that are attached to my router for cooling. I measured the Amp consumption and both fans use around 500 mA total. As a safety measure I would now like to add a fuse, but not sure what Amp rating to use. I'm not sure what determines the Amp rating other than I know I need a fuse over a 5 volt rating and probably slow blow since these are fans. Judging by the 3 Amp fuse in my mobile scanner that consumes around 500 mA at full volume, the fuse is rated at 3 Amps, so that would be 6 times the value. Would I then use a 3 Amp fuse for these fans or what?

 

[mmckenna]

You would need to consider all the parts of the circuits, not just the fan:
Current carrying capacity of the wire
Current carrying capacity of any connectors, switches, etc.
Start up current of the fans/devices.

Essentially you want the fuse to be the weakest link, so if something goes wrong, the fuse fails, not some other part of the circuit.



In other words, a 3 amp fuse on the fans is probably fine.

 

[BinaryMode]

It's just fans, no other electronics or switches. Length of wire is probably 6".

I guess current carrying capacity of the wire would be the first to consider. I should have thought that. Since the USB power wires are at least 24 gauge, I'll try a 1 Amp fuse and see how that goes.

 

[buddrousa]

Your max draw should be at start up of the fan.

 

[wtp]

bridge weight = amps
trucks = fans

so start small until they stop blowing, or start high and see the first one to blow, then go up one..science

 

[mc48]

You may not need a fuse if the fan is impedance protected.

 

[belvdr]

You may not need a fuse if the fan is impedance protected.

That protects the motor if it cannot turn, but how does that protect the circuit, for example, if a short were to occur?

 

[KevinC]

I always went 150% of the total current draw.

 

[mmckenna]

bridge weight = amps
trucks = fans
View attachment 150233
so start small until they stop blowing, or start high and see the first one to blow, then go up one..science

I love Calvin.

 

[BinaryMode]

I always went 150% of the total current draw.

Sounds prudent... Been awhile since I took classes on this stuff. Like decades...