120 amp breaker?

Hello everybody. I have a question regarding the placement of the main breaker. Our team wants to place the breaker about 3 feet away from the battery. Are there any rules about where the breaker can be placed?

The general rule for the main breaker is to place it close to the main battery and close to the Rockwell block to minimize losses. Your inpsector and the field personnel will be happy if it is easy to get to and it is marked. This benfits you as well if your robot goes haywire, someone can turn it off.

It needs to be where it can be accessed easily. It would be sensible to figure out a way to put the breaker right next to the battery, and put them both right next to the Rockwell Automation power distribution block, and put that right next to the fuse panels, to keep wire lengths to a minimum. This will result in less robot weight (#6 wire is heavy), and shorter wires have less electrical resistance, which helps robot performance.

so if weight is not an issue, and our breaker is easily accessible, then placing it 3 feet away from the battery is okay?

Yes, it’s okay.

“Okay” is a relative term. Putting the breaker so far away from the battery won’t violate any rules, but it’s a lot of weight and power loss to spend for whatever perceived benefit you get for doing it.

A rule of thumb I use is called the Wire Foot. 100 amps flowing through a Wire Foot equals 0.1 volt drop across the wire. The wire foot is based on #10 wire which is 0.001 ohms per foot. 100 amps is near the stall current on a small CIM motor. #6 wire is 1/2WF per foot. Since all the robot current flows through the battery, main breaker and the wiring to the Rockwell block, losses can mount. You need to consider that all the current flowing in this part of the circuit is handled by both wires, red and black. So, if the battery can deliver 600 amps (yes that is what it is capable of, fully charged) and you place three feet of #6 wire in series with the battery you have introduced 6 x 0.5WF x 6 (100 amp) you have the potential for 1.8 volts to be dropped in just the #6 wires. So here are some other WF equivalents to think about.
Bad crimps=2-5WF/each
The critical issue in any robot design is making sure you can play the entire match. When the voltage supplied to the RC falls below 8 volts for a short period of time the RC goes into sleep mode and disables the PWM outputs. Protecting the RC should be a prime consideration in you electrical layout. Inches of wire could mean the difference in a critical score at the end of a match when the battery is low.