I believe I might just have prevented a potential fire in my #printcave . I was very lucky to be sitting beside the printer when it happened. Melted cables aren’t fun.
I ordered some ATX-extension cables from eBay to use with my 3D printer. By cutting them in half I could plugin my ATX power supply to the printer without modifying or cutting the power supply cables. Twisted a few of the 12V together as well as the grounds to get more cable area for the current to go through. No problem on the first print yesterday but today I also connected the RAMPS fan and that did it.
Alright. So what’s wrong with this picture?
The cable on the left is marked AWG20 and is from a brand power supply. The one in the middle is also marked AWG20 and is from one of my eBay ATX extensions.The right most cable is an AWG26 wire from a Cat5 Ethernet cable.
If you ask me I’d say that the eBay cable is more likely AWG26 than specified AWG20. Chinese crap…
Wires can carry a lot more current than they are rated for. I’ve used 30ga wire to run my hot end and it was just fine (granted, it was also teflon coated). Too many people are of the belief that if it’s over 2A, you need 16, 14ga wire. You don’t. There are circuit boards that use the equivalent of maybe 1 strand of that wire to carry 2.5A+ without issue.
Yeah, it’s probably safer to use a thicker gauge, but the standards are very very conservative.
Should be pointed out that the middle wire has much smaller strands than the left wire, which means the cross-sectional area (which, for stranded wire, is what the AWG refers to) may be larger as there is less space between strands. See http://en.wikipedia.org/wiki/American_wire_gauge#Stranded_wire_AWG_sizes for more info.
Also, as the wire in the middle looks like it may have a higher copper content, it should also be more capable of carrying a higher current, due to it’s inherent lower resistance (unless the left wire has a high proportion of silver, which IMO is unlikely simply due to cost).
@Stuart_Young higher copper content, yes that is possible. Or from what I saw last night it might just as well have been flux coating ,-)
It might not be as obvious from the picture as I thought but I am pretty convinced that the printed specifications are not correct regardless of individual strand thickness and the seller have been notified that it might be the case.
@ThantiK I agree most tables for ampacities are conservative guidelines. They are made that way to give room for less than ideal circumstances and variations beyond control. How I use the recommendations depends on the application but for a machine which is more or less unsupervised for long periods of time and where moderately high currents and temperatures are involved I would be careful to knowingly gamble too much.
One could of course argue that gambling is just what I’ve done this time by not quality checking my eBay purchase from the far east. At least I’ve got RF chained fire sensors in most rooms in the house, fire extinguishers and fire blankets on both floors. It’s not a total protection against disasters but feels like a good investment when playing with 3d printers.
I won’t argue with you on the PCB trace width issue as I only play around in the field and I guess circumstances like the PCB acting as a heat sink, varying acceptance of temperature rise in the application and other factors can make it alright to bend recommendations. Generally I try to the best of my abilities to design things with sensible safety margins especially since someone else might see and use the design.
The fact remains though. The middle wire melted its’ insulation and started to smoke while the left one remained unaffected. They both were in the same cable chain with the middle one doubled. This was not enough to feed the whole printer (motors, fans, hotbed and hotend) as that section of the cable chain totally melted and brittled the metal strands. It also melted the terminal dimensioned for mains wiring that I used to connect the cable sections.
there is an important part which cant be gauged by looking at physical size. the material of conductor. some poorly made cables have higher resistances, that will easily increase the heating in the cable. im sure some of you have heard of china USB cables that use cheap steel wires instead of copper alloys, but in terms of data rate, the steel wires show no degradation because the length used was short and signal degradation wasnt affected (try it, even some SATA cables, poke your magnet on those cables, SOME will STICK haha). however, put that into short lengths required to carry high current (10A? 20A?), what could be a 0.2ohm stretch due to lousy alloy now becomes 0.5ohm? @ 10A–> p= i x i x R, = 50w dissipation (example). so what was warm to touch now becomes nearer to insulation melter, and when that happens, it shorts to probably the ground cable next to it and wala ! poof
with more and more stuff being made in china, and world resource supplies getting kinda short (china wants a piece of spratly islands cos there is potentially undersea mining resources, they are also competing with japan in open ocean resource mining race) … i had the chance of using a large batch of cables (few kilometers) from china, and they were all really really bad, even the insulation crumbles. its like printing filaments, we see the label for what it says, we “trust” the label … sigh … it used to be if we see a cable marked TUV, CL etc etc, it should be safe, but every tom dick harry cable is now printed likewise, we wouldnt know if its really real copper in it. even connector pins, some have gone “steel” instead of copper or bronze or some high wear material, we cant really know by visual inspection sadly
