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Originally Posted by squirrel
I guess I'm pretty conservative...
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A) Not really... for 10ga wire they suggest limiting it to 15A for power transmission, and only go up to 55A when describing it as unbundled and for a short length. Since your estimate is for car wiring... which runs through insulated areas and for longer lengths, I'd say your estimates actually match up pretty well.
B) It's not like being conservative with wire diameter and load is a bad thing! Especially when wiring a car or a house. Igniting a 3lb robot is quite different from igniting a 3000lb Buick!
Adam,
For this application, however, I would also consider looking at where the "bottleneck" in the system is going to come. What will happen to the voltage of the LiPo's at their maximum rated draw? Their internal resistance at full draw may drop your voltage down too low for your microcontroller to keep running. Likewise you may find the LiPo's start to heat up... and you really do not want to overheat LiPo's. Although I don't have first hand experience with overheating them, I understand they are both expensive to replace, and somewhat combustible when abused.
What will happen to the motors? Keep in mind that if you use a 10ga wire to deliver current to your motor, that current still has to "fit" through a much smaller wire inside the motor that is wrapped tightly on all sides by other small wires carrying the same amount of current and giving off the same amount of heat. This can often be sustained while the motor is turning (due to the dynamic convection cooling of air moving over the coils) but at stall, or even low RPMs, small motors can let out their magic smoke fairly quickly.
An example is the FP motor in the KOP. Although it's stall is rated something like 55A, I find they smoke if you run them for more than a few seconds at anything above about 10A (okay, maybe 15 or 20... but I'm being conservative, too). Keep in mind that just because a motor has a rated "stall current" does not mean that the motor can handle being stalled (or close to stalled) for more than a second or two.
Practially you may need a wire diameter capable of carrying much less than 27A, as the motors may act as a fuse and go up in smoke long before the wire (which is likely larger diameter than the motor wires and has air on all sides to cool it) does... which isn't really a good thing.
On the other hand, the narrower you make the wire, the higher the voltage loss in the wire, and the less power reaching your wheels. That is a performance trade off, not a safety trade off, and one only you can decide.
It may make sense to hook up your 2 LiPo's in series, to get 14.4V, which should give you higher RPM on your motors, then gear the motors down (roughly by a factor of two) to get increased torque at a given current, while keeping your top speed the same. Of course the gearing adds weight, too, but it is just a thought. Cramming voltage down a wire (up to a point) is quite painless in terms of power loss, while current gives off heat... P=I(squared)R, after all. Motors will also tend to be more efficient at higher voltages as well. Perhaps you are already doing that, I couldn't tell from the post whether your batteries were in series or parallel.
So keep your wire runs as short as possible, do some practice driving, that will simulate what you expect from competition and feel the wires. If they are heating up, then they are too small. Oh... and feel the motors, too. Warm to the touch (in this application) isn't a problem... hot to the touch is.
Jason