Something that has always confused me about the rules is the wire gauge rules. It gives us the minimum wire sizes for every circuit, but what does this mean? Is that the minimum diameter, or the minimum gauge? Can I wire these circuits with wires of greater diameter, or with greater gauge (smaller diameter)? Thanks. I’m just trying to buy wire for my team to use so I can start teaching them the basics, and I want to get this worked out. Since we’re not using pneumatics and the Bridge and cRIO come with the wires they need, can I just buy 18 AWG wire for everything?
Yes there are minimum gauge sizes for the robot. You can go over them (Like 40A minimum is 12 but you can run 10 gauge). Running 18 gauge for everything will not work; you have to run certain gauges for certain amps.
For wire, a higher gauge number results in a smaller diameter, and a lower current-carrying capacity. For 40A breakers, you need to use at least 12 gauge wire. 10 gauge is ok, so is 8. But 24 or 18 gauge would not be legal for the 40A breakers.
I would recommend running your drive train, at e very least, on 40A breakers… As a result, you can not wire the entire robot with 18 gauge.
So if 14AWG is the minimum, you can use 10 or 12AWG. The larger diameter of the smaller gauge numbers means that the wires have a greater current carrying capacity and lower resistance.
When wiring our robot, wherever possible (and particularly for longer wire runs that carried power to a motor) we would try to go one step up on that chart if we could.
And yeah, wire gauge is a really archaic way to measure wires. You’d think FIRST would be helping to inspire science and technology by actually using the units of measurement that have been adopoted by scientists and technologists around the world, eh? Edit: Add smiley here.
The issue with that is going to a store or online and saying you want 6.54 kcmil (#12 AWG) wire. You’re not likely going to find it and they’re going to look at you with an odd look. If you get larger wire (250kcmil) they’re easier to find in the kcmil or mm measurements.
When I think about how the modern units of measurement systems work, and how they’re based on the (arbitrary) number of digits on our limbs, I develop some respect for the thinking of the old guys who came up with such clever things as the number of degrees of a compass, the Fahrenheit thermometer, wire and metal gauges, and all those pesky 16ths on the wrenches in the toolbox. Dividing factors of five in half is a real pain.
That’s perhaps true, but doing mathematical calculations with metric is 1000% more fun than with English units
I do have a fondness for Fahrenheit however. It’s a better unit for describing the temperatures we typically encounter (such as weather and setting thermostats) though it does fall short in science.
Most countries use SI units for engineering, and of course that is the scientific approach.
In my country, we use units that are more convenient for small ratio calculations. American Wire Gauge (AWG) is a good example. 10 AWG wire at standard temperature adds one Ohm per thousand feet to its circuit. Increasing the AWG number by three doubles the per-length resistance; e.g., 13 AWG is two Ohms per thousand feet. This system makes wire size selection much more straightforward than other systems, such as wire diameter in millimeters.
Interestingly, increasing AWG by one unit increases per-length resistance of that wire’s circuit by a factor of the cube root of two (~1.26). Wire used to wind electromagnet coils in motors, actuators, and transformers is typically available in size increments of one-quarter AWG, so the per-length resistance ratio of successive (i.e., +0.25 AWG) wire sizes is the twelfth root of two (~1.05946), which musicians will recognize as the ratio of fundamental frequencies of successive notes on an equitempered chromatic scale – this increment is also called a half-step, or semitone; e.g. stepping from A to A#.
Based on that example and others, I think American engineering units are more like those used by artists, rather than by scientists. I think our system of units promotes creative thought.
The wire minimums exist because wires carrying current will heat up. If a 12 AWG wire is safe to use at 40 Amps, a 14 AWG wire (smaller diameter) is not, it will overheat and may become a fire hazard. 18 AWG is right out.
But, a 10 AWG wire has greater current carrying capacity (known as “Ampacity”) because it is thicker, so you can use that everywhere (except the battery cables, which must be 6 AWG or larger).
BUT, copper weighs a lot, and is expensive. So teams will often use the smallest ALLOWED wire for their wiring.
BUT, the smallest ALLOWED wire may have more loss (resistance) than you want, so some teams use wires larger than the minimums.
The point is, there is a tradeoff: Ampacity versus Weight versus Cost versus Loss. You MUST always use at least the minimum, but larger can have advantages.
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OK, knowing all that, we use #10 for 40A circuits and the minimum size for all other circuits. So we have rolls of #6, #10, #14, #18 and #22. I think we happen to have some #20, but maybe not. All in both red and black.
We buy the highest strand count wire we can find, so it is very flexible. Lower strand counts (like 7 strand THHN) is very stiff and a pain to work with. Home Depot and Lowe’s do not sell wire good for FRC, in my opinion. McMaster, Mouser, and several wire stores online sell good stuff.
That’s a creative approximation with useful implications, but the truth is that AWG isn’t defined that way. Instead of 3√2 ≈ 1.259, the ratio between AWG sizes is 39√92 ≈ 1.229. And the resistance per length depends on using a solid copper conductor in a DC application (no skin effect) under certain environmental conditions—and even then, it’s approximated.
In many practical use cases (FRC included), the distinction between approximation and definition doesn’t really matter. But the fact that approximations are necessary to make the numbering system meaningful substantially dilutes the rationale for adopting AWG as a standard, especially for use cases that don’t derive value from those approximate relationships.
Aside: The ISO paper sizes (A0, A1, A2, A3, A4, etc.) do something similar with 2√2, except that’s actually the ratio between sizes (rounded to a whole number of millimetres).
At least in the states, you got such a large installed base of this archaic stuff that even you could wave your metric wand, engineers that work in the real world would still have to learn both systems. Don’t get me started on pipe sizes. :]
If your wire doesn’t list the gauge on it, you’re using the wrong wire. I strongly recommend wiring the cRIO and breakouts with 18 AWG and the drive motors with 10 AWG.
Accurate and well-reasoned, as always, Tristan. You caught me. :o
(Note to readers: Tristan doesn’t miss much during a robot inspection, either).
As I get older, it is getting harder to be an apologist for ANSI units and related engineering anachronisms. My high school chemistry teacher had me convinced that SI would soon catch on in the US. That was in the mid 1970s. Still waiting …
Yeah, you’d probably get the same sort of looks that the Aussie teams do when they ask for 1/4" bolts. Something like this:
I have to admit, though, you’d have a hard time finding metric-spec wires in Canada. Turns out that although we’re officially metric here, our largest trading partner is… how did you say, “stubborn”? Even my lab is stocked with 20Ga wire and #4 nuts.
This results in some occasional mis-translation… occasionally with interesting outcomes. Most of our furnaces at work are old enough to be in farenheit. I was doing a lab where I wanted to solution heat treat some 6061… so I set the furnace to 775 degrees.
But oops… that was the NEW furnace. Thankfully (safety first!) no one was hurt, and there was only minor damage from the unexpectedly molten aluminum. :o
The new furnace is now labelled “degrees C”, and my students will forever remember the difference between 775F and 775C.
I guess if we can be officially bilingual, we can be unofficially biunital, too.
Thank you for the new word. I suppose I have been biunital for many years now. In my day job, I tend to dimension fixation points (e.g., bolt patterns, pilots, shaft heights) in inches, and bearings, magnets, and (yes, Tristan) wire sizes in millimeters.
