If this question were in a physics exam, I would answer it as follows; whether this is a sensible idea in practice is entirely another matter. One would have to be pretty certain that no fault state could leave current flowing for more than two seconds.
We know from the specification of the wire the resistance per metre R and mass of copper per metre M. Given the current, I, we know that the power dissipated in the wire is I^2 R per metre. The total heat energy dissipated per metre of wire is therefore E=I^2 R t, where t=2 seconds is the time that the current is active for. We (conservatively) estimate that negligible heat leaves the copper wire during this 2 seconds, and therefore the rise in temperature T is given by
T=E/(MC) = I^2 R t / (MC)
where C is the specific heat capacity of copper. A wire needs to be chosen with R and M such that this temperature rise T is acceptable.
Three things of note:
The Maximum Amps for Chassis Wiring is also a conservative rating,
The AWG guidelines are very CYA. It says 16A, but that's probably anywhere from 60% to 90% of what it can actually handle.
but is meant for wiring in air, and not in a bundle.
Multiple 18 gauge wires tied together will have a lower (derated) capacity than a single 18 gauge wire. And in a pipe less than in free air.
For short lengths of wire, such as is used in battery packs you should trade off the resistance and load with size, weight, and flexibility.
But the biggest thing is really your application! Length is minimal, probably 1 meter (plus return path, so 2 meters), so the wire resistance is minimal, which means very little voltage droop.
But how many power wires are there? Is each motor individually cabled?
Is the full amperage running the full length of the cables? Is the 5 Amps for the computer actually going over the arm, or is it at the base? Can it be individually powered?
What duty cycle is to be expected? How often are the motors going to be on? How long will the entire setup be on?
For the most part, you need a wiring schematic. A plan on how everything is connected. Without that, you cannot really know what you need in terms of wire size. A schematic for the robot arm would be the best place to start.
Best Answer
The
[Wikipedia article][1]on American wire gauge suggests that the fusing (melting) current for 24-gauge copper is 29 amps. That would put you within the acceptable range.I would normally worry about voltage drop with such a current in a small gauge wire. From the same article, 24 gauge is about 25 milliohms per foot, you'll have two feet of wire, so that's 50 milliohms, and at 5 amps you'll drop about 0.25 volts. That won't have a measureable effect on the igniters. Based on that voltage and the current, you can calculate the wattage and, based on the heat capacity of copper and the mass of the copper in the wire, figure out how much hotter it's going to get. If you have a limit, it's much more likely to be the temperature tolerance of the insulation rather than the copper conductor.
If I were thinking about this, I'd just give it a try and see how hot the wire got.
The big drawback of cat5 in this application - I assume you're using it to connect to an igniter for the engine - is that the individual wires are solid (and not very flexible) and the flexibility is even worse with multiple wires. You will also run the risk of breaking wires as they flex over time. I've used this sort of cable to do connections in the past and hated my choice.
If I were building this, I'd look for something in the 12-14 gauge stranded - something like speaker wire would be fine. If I needed the pairs to be color coded, I'd do it with a Sharpie.