What gauge wire is recommended for the hotend heater??? I want to wire up a hotend with just a single wire that 4 insulated wires in it (just for asthetics) but I don’t want to be irresponsible with current.
First, you need to tell us some specs. Voltage of your system, Wattage of the heater at his desired rate, length from Board to heater…
12v 40 watt heater and around .5m between the hotend and board
I never considered that one day I would actually find a use for ohms law. voltage × amps = watts. So less voltage needs more amps to reach the required wattage. So 40w heater at 12v would use 3.3amps. That’s why people say 24v is less likely to burn your house down as 40w at 24v need 1.6amps now the guage of the wire should be rated but unfortunately I have no idea what the thickness for a given conductor needs to be.
@Michael_Scholtz that’s what I’m wondering. I love ohms law. Use it all the time haha but once I get a number I really don’t know what to do…
For your heater, it’s AT LEAST 0.18mm^2, that is AWG24. The formula is a little bit more advanced, than just U=R*I 
I’ve always found a way to go higher voltages, lower amperages. High amps = high heat. Whereas a smaller gauge at 110v won’t get as hot. I’ve used 22 gauge on 110v with a solid state relay. Bit more dangerous though - I once accidentally had the thermistor drop off of my bed and the heater was rated for 600w; it burnt my blue tape to a crisp and ruined the heater.
I’m severely hoping that everyone jumps to 24v systems. I’ve been using them instead of 12v and I’m liking the extra overhead it gives, while bringing down the cost of certain items. A 12v 6A PSU costs the same as a 24v 6A PSU, but the 24v is a lot more power.
@Rene_Jurack 14AWG seems way too large, did you mean 24?
Remember, for flex service, you want to use the smallest wiring gauge that will safely carry the current so fatigue is minimized.
@Ryan_Carlyle You are right! Thanks for watching. I checked the numbers again and found, that I used a false comparison-table. Still, 0.18mm^2 is correct, what equals to AWG24. I will edit my post above.
14AWG is too large, as @Ryan_Carlyle says. Based on the .18mm^2 figure, @Rene_Jurack probably did mean 24AWG.
Also, don’t forget that AWG is ass-backward, so larger numbers mean smaller wire, so 26AWG (a higher number) will be too small.
It would also be a good idea to use silicone-insulated high-flex wire such as https://www.adafruit.com/products/1437. This is beneficial both for mechanical reasons and because this wire, with its high-temperature insulation (which should even survive accidentally resting against your heater block, which normal wire wouldn’t), is rated for more amperage for the same conductor size. The 24AWG wire I linked to is actually rated for up to 8A per conductor at room temperature (temperature should also be considered, of course, and wire rated for the required current may not be sufficient for some 3D printing applications, especially if a heated chamber is used).
@Nathan_Walkner , that’s what I do; except I also prefer 24v with the stepper motors as well. Can’t run those on 110v
At one point I used a 110v Heater Cartridge too but it was so powerful that the PID autotune in marlin couldn’t figure it out, and I have no clue how to go about properly tuning a PID loop.
@Nathan_Walkner Running AC to your hot end is probably a bad idea. It’s a fairly common practice for platforms heaters, but those are all (AFAIK) continuously insulated silicone heaters, whereas the hot end gets a lot hotter and usually has several points where a short circuit is much more likely. These platforms also tend to move less, particularly in the large printer designs that are likely to use AC platform heaters (small printers can get away with moving the platform more than large ones can). An AC hot end would make me nervous.
@ThantiK The algorithm we use doesn’t work well for heaters that heat up when on full significantly faster than they cool when turned off (or vice versa, for that matter). Marlin has a feature to limit the maximum duty cycle of a heater to decrease the effective heating power to work around this, and I extended the feature to make it possible to use this while still using full power when outside the PID’s functional range, so that you can heat quickly to get near the specified temperature, then limit power to make the PID more effective even with overpowered heaters.
Still, there are all kinds of safety reasons that you should use a heating element that will self-limit its temperature not too far above the operating temperature range, especially since it’s possible for Marlin to crash with the heater stuck on full.
I would avoid mains-voltage hot ends like the plague. Heater cartridges very often short from element to casing. Then your hot end is electrified to mains voltage. Depending on your drivetrain design, you might turn the entire bot into a heater through the chassis, or you might blow a fuse, or you might electrocute yourself.
@Whosa_whatsis The cicoil flex is very nice stuff. If you need larger quantities (100 or 1k feet) digi-key stocks it, with a greater variety and lower cost. If I was building a commercial machine, I’d use it on everything that moves.