Fuse selection for RAMPS

Hi all

I'm in the middle of building a RAMPS 1.4-based machine, and have this power supply, this inlet, and this heated build plate

For whatever reason, the inlet didn't come with a fuse included as I was led to believe. This said, it is up to me to find one myself.

I have been led to believe that for this power supply I should use a slow-acting/time-delay fuse on the inlet between 5 and 6.3 amps, rated for 250v.

Is this right? I'm in the US, with 110v AC power from the grid if that helps at all.

Thank you!

That sounds very correct.

The math is like this you need 360W + something to cover heat losses. So lets say 450W (this allows as low as 75-80% efficiency). Then you get 110V (and perhaps it goes a bit lower at times - so lets say 100V).
How many amps does a 450W load use at 100V? W/V = A, so 450W / 100V = 4.5A

And thus a 5A to 6.3A fuse will allow the power supply to operate even under non-optimal conditions, but will still protect wires, connectors etc. quite well, so even wimpsy 110V wiring will not over heat in case of a short (or internal breakdown) in the power supply.

You will want as slow fuse, as capacitors inside the power supply might draw a substantially higher amperage, for a few milli seconds, when you turn the power supply on. And a slow fuse will allow this to happen.

Awesome. That's good to hear! So, would this be a suitable fuse?

The inlet does not specify what physical size fuse it uses, but it is extremely likely that it does indeed use the standard 5x20mm fuse.