When we were being inspected at our last competition, I happened to mention that we used solid core wire on our robot. The Inspector immediately stopped what he was doing and warned me, quite sternly, that it was illegal to use solid core wire on the robot. I did not get the feeling that he was messing with me. I asked him to produce a rule in the manual that stated this, however he never got around to it with the hustle of competition. I did my own search, and despite my best efforts found nothing about the legality of solid core wire. In fact the words “solid core” or “Stranded” never even appear. Can I get a confirmation that solid core wire is indeed legal?
I am aware of its pros and cons and use scenarios, yet I still fail to see how it would come to be outlawed.
Nothing about solid core wire in the manual about being illegal, as long as the gauge is correct. I’ve encountered some inspectors that tend to get overly alarmed (or maybe confused with their own engineering projects?) about certain design decisions or robot features that are actually legal. If an inspector is holding you back because of an issue they are questioning that is not thought to be outlawed in the rules, don’t be afraid to ask for the lead robot inspector to come in and give a final determination.
Thank you seg9585, he passed us that time but basically said we should avoid it next year and if we were to move on past that event that we may have to re-do the wiring in order to pass inspection there.
Valkonn, I appreciate the response. To satisfy your curiosity: I am using solid wire on our static removable wiring board. This wire, at its longest length, is less than a foot and zip tied along its entire length to sturdy expanded aluminium. The rest is stranded; all wires that even have a possibility of moving (with in reason) are stranded.
Solid wire is simply more rigid, where stranded wire provides better flexibility. At the same AWG they have the same rated current capacity.
For simple point to point wiring, solid wire is fine. If you had wire going to something flexible (some rotating element) than you should use stranded.
In your home, the entire electrical system is solid core wire. The appliance cords are all stranded.
I have never heard that rule through my time in FIRST nor did I find any mentions of that in the game manual online. My team uses stranded wire, and while we’ve been told about how we should have/haven’t soldered wires on certain connections, I have never heard about the restrictions of type of wire. It could just be their misconception about types just because of flexibility of stranded over solid core.
However, when I was a sophomore in high school a robot inspector who was on one of the host teams at a week 0 scrimmage try to trick me because he knew I was new to being lead of electrical. He tried to convince me that standard wires are made out of aluminum and asserted that until one of my mentors had to step in. The fact that you said he didn’t seem to be messing with you reminded me of that.
Thank you every one for your replies, I got the answer I was looking for. I just needed reassurance that the stress of the build season hadn’t caused me to go crazy!
While there is no rule prohibiting solid wire, for most applications on an FRC robot using stranded wire is ideal for connections involving the WAGO terminals on the PDP, any of the Weidmuller connectors, or screw terminals. This is because these connection interfaces rely on being able to compress the wire for a larger contact surface. This is the same reason why it is not recommended to solder/tin the ends of stranded wires going into these types of connectors.
Solid is not the way to go !! D.C. current likes to flow on the outer edge of wire. Solid only has one edge where super fine strand has well as many strands that make up the wire, so will flow the current easier.
That and solid wire can break in a high vibration environment.
Hunter,
There is no rule specifying stranded wire or solid wire. However, the terminals on the PDP are designed for stranded wire. The terminal is rated by the contact area of the stranded wire and the shape of the inner contacts. ** If you use solid wire, you must derate the current carrying ability by at least 2-4 AWG.** A solid #14 wire is only able to handle the current of a #16 solid wire when used with the the PDP. This specification is contained deep in the Weidemuller on line catalog/product sheets. If you are using 20 amp breakers, #14 solid wire would be appropriate for that branch. If you were using a 30 amp breaker, you would have to use a #12 wire to be properly protected by the 30 amp breaker.
Additionally, solid wire is not a good choice for objects that move. It is brittle and will stress fail on your robot if your robot is moving. Try bending and rebending a piece of solid wire. It will eventually break, that is the kind of failure you would expect. There is no real way to predict the failure but it will occur at some point.
If you would like to ask your LRI to add to the discussion, he or she can call me or text me in the morning.
Skin effect varies with voltage and frequency. As the potential is increased, the magnetic action (current passing through a conductor produces a magnetic field) tends to push current to the outside of the conductor. As frequency is increased, similar action takes place. It is for this reason that high voltage wiring is usually a composite of a strong but poor inner conductor with an outer coating of a highly conductive material. At high frequency, again the current tends to move to the outside of a conductor. High frequency at a high voltage is the worst case. High power coax used in transmission for instance, has an inner conductor that is actually a copper pipe with a hollow core. So little current flows in the core, it is not efficient to manufacture a solid conductor.
Ham radio wire antennas will often be made of a product called Copper Weld, a copper clad, steel wire. Little current flows in the center so using steel makes the wire strong enough to survive wind load without raising the electrical loss.
This was something taught long ago in a previous life when I was in the car audio industry. I never questioned it as it seemed plausible. But after your post I looked around a bit and stand corrected.
My vibration cracking holds true even if unlikely.
As stated above, the connectors make a big difference. Insulation displacement works better with solid core.
For long high-frequency signal applications (where the cable doesn’t move much), solid-core would be preferred over stranded, but the distances would be much greater than you’re going to find on a robot. An example would be when trying to run 1000baseT Ethernet for a distance near or longer than the specification (100 m). This isn’t because of skin effect, but because each little conductor is a slightly different length so signals arrive at the end of each at a slightly different time, making for a less-sharp signal. This is why you run solid-core through the walls (where they don’t bend much) and stranded from the wall to your device (shorter lengths, and need to flex).