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The cheap 12 volt power supply I'm using has the output adjusted wide open at 15.5 volts, when I power the hotend, it drops to 15.4. when both the bed and hotend are running, it drops to 15.2.

Mac The Knife wrote:The cheap 12 volt power supply I'm using has the output adjusted wide open at 15.5 volts, when I power the hotend, it drops to 15.4. when both the bed and hotend are running, it drops to 15.2.

Wires could be adding aditional resistance?

I use an E3D hot end. It has one 40W ceramic heater cartridge. Way easier to deal with than wrapping resistors in tinfoil and praying nothing shorts out. The leads in ceramic heaters are usually insulated with double jackets, the outer made of fiberglass, so you don't have to worry about stuff shorting out unless you do something monumentally wrong. It gets the hot end up to temperature in a minute and does a fairly good job at keeping it there. If I was still using the SeeMe hot end and it was having electrical problems, I would seriously think about getting a single 40W cartridge. It has more thermal mass so it might take longer to heat up, but carefully wrapping foil around resistors to get them to fit is a bit too "chewing gum and chicken wire" for me.

Fuses are safety devices, not precision instruments. For massive current loads, usually caused by short circuits, they'll blow almost instantly. For more reasonable overloads, they take time to blow. 110% rating would likely take hours to blow. 150% rating will take a number of minutes. 200% is still several seconds. I'm generalizing here, real numbers will vary by fuse line. Ambient temperature is a factor (ratings are at 25C, colder increases rating, warmer decreases). Active cooling (fan blowing on board) may increase the rating.

So, taking Mac's scenario of two 12V 40W heaters in parallel, or 80W with a combined resistance of 1.8 Ω. Doing the math, at 15V it's 125W or 8.3 amps. 8.3 amps is 166% of 5A. If the hot end gets to temperature within a minute or so, i could see you getting away with it, barely. Certainly walking the line, however. If you're still doing it, I'd get some 10A fuses.