12 AWG or 10 AWG?

Life’s biggest question, really. I know FIRST states the minimum wire gage for 40 AMP motors (i.e. big SIMs) is 12 AWG, but I’ve always been weary of this. Here is a current limit vs wire diameter standard chart:

http://www.powerstream.com/Wire_Size.htm

The maximum current for chassis wiring using 12 AWG is 41 amps, which I guess is okay. But with current spikes and the likes…I don’t know.

Is the voltage drop on 12 gage wire not that big a deal? What’s your take? What does your team use and why?

Thanks for any and all input.

A short spike in current won’t melt the wiring or otherwise damage it unless you’ve really abused the wire (left nicks and gouges in it, etc). Between the breakers and the natural voltage drop of the battery under load, there’s nothing to be concerned about. That said, using thicker gauge wire than required does give you a bit of a voltage advantage for a slight weight penalty.

For short runs (under a foot long) 12 would be fine, but for longer runs you should consider using 10 gauge. The motor wires are 14 gauge…you can also cut them shorter and splice 10 gauge to them, if you’re really concerned about voltage drop.

We usually use 12, but this year we had a bit longer runs from the PDB to the speed controllers, so we went with 10. I doubt it would make a noticeable difference on most robots, though.

There are 2 major considerations when sizing wire: Heat and Resistance Loss.

In a typical FRC robot, #12 will easily handle anything the robot can put on it, since there are 40A breakers on the PDB max. There is more than enough cooling air around the wires to prevent them from overheating. Plus, they are used for such short periods of time (~2 min).

But resistance is not your friend. A wire with 1.588 mOhms per ft, running at 40 Amps, will drop 0.064 volts per foot (and have to dissipate 2.5 watts per foot). From PDB to Jaguar/etc and then CIM might be 4 feet, so there’s 8 feet of wire (4 black, 4 red), so you lose 1/2 volt there, all the time.

#10 wire would lose 2/3 of that, or about 0.3 volts.

Does 0.2 volts matter? Maybe. Depends. But that’s how the numbers work.

1676 always uses #10 for CIMs, and the legal wire for every other circuit.

As others have mentioned you will have more voltage drop with the 12ga vs the 10ga but the only time it will matter is if you are sending full power to the motor. At anything less than full power you can compensate for the increased voltage drop by commanding more power to the motor.

We’re gonna go with 10 AWG for the drive train’s SIMs to try and maximize our power efficiency there, especially since the wires are a bit long. Thanks for the help!

If the run is long enough to make a significant difference between 12ga and 10ga wire, then you ought to be worrying about the extra weight as well.

You’ll save both resistance and weight by thinking about your layout and keeping those high-current runs short. Keep the motor controllers near the motors they control and wire lengths to a minimum.

I bought a roll of 10ga wire for our team a couple of years ago and we haven’t used more than a few feet of it. I don’t think any of it ever stayed on a robot.

We use 12 on all of our heavy amp draw motors, never had a problem with current spikes or damaging cables due to amps. Keep in mind that 10 gauge is very heavy stuff and depending on what kind you use, it may be hard to bend around tight areas also.

Lior,
The FRC rules do not relate to motor current. They merely size the breaker rating to the wire it is being used with. The chart you link to is based on continuous duty in an enclosed chassis and is specified for the voltage drop across the wire. (This is actually a specification that is derived from aircraft needs.) At 100 amps, a #10 wire will drop 0.1 volt per foot while a #12 with the same current will drop almost twice that. Since the load would remain the same, the available current (at the motor) will decrease a small amount with the #12 wire.