Determining maximum current through 18 gauge wire in robot arm

Question

For a robotics application, I'd like to power some external electronics and sensors off the logic circuit in a robot arm.

The components are a computer (19V, 65W => 5A), 2 cameras (1.2A), 1 light (0.35A), USB2.0 I/O board, and power board. All this on top of current draw from the motor logic, which runs off a 4.3A power supply atm. Therefore, I estimate the upper bound to be 12A.

The logic power runs through an 18 gauge wire. According to this table, 18 gauge permits either 16A as chassis wiring, or 2.3A as power transmission.

Note the robot arm is roughly 0.6m long; obviously, because the logic power runs internally, the wire length is greater than 0.6m.

Will I be able to power my electronics from logic power?

I am willing to test my system empirically.

(BTW, as a CS major, I have zero experience with electronics.)

Answer 1

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.

Answer 2

Though the 18AWG copper wire can support upto 16A, there are certain derating factors which you need to consider while calculating its ampacity. One factor is the ambient temperature. The current carrying capacity of copper changes with temperature and hence some margin needs to be added to your design to accomodate for those losses.

Another factor which you need to consider is the number of wires you are using to carry the required amount of current. This arises from the fact that the current passing through the wire also generates some heat which gets dissipated in the conductors.

Further details about the derating factors and how to apply them to your design can be found here

Now coming to the specific problem which you mentioned here, the worst case maximum power which you have calculated for the system is ~12A. Whereas an 18AWG wire can support upto a maximum of 16A. After you apply the derating factors (which is applicable to your scenario) in the calculation, you may find that the ampacity of the wire is not enough for your design. Then you can decide on either of the two choices -

1. Increase the number of 18AWG wires to increase the current capacity
2. Change the wire gauge to a better value (16AWG or so).

Answer 3

Current capacity of a conductor is basically a question of heat management. If a lot of the load is the motors, and they have a short duty cycle, you can take a much bigger load than the book says - the table you are reading is probably a worst-case value. Or if the conductor is exposed and the machine is in the Arctic you can use more power than if it is in Dubai.

However, it is generally a very bad idea to push electrical equipment to its limits. Databases do not start fires when overloaded, power supplies do. And a robot arm is rather likely to be overloaded at some point.