Re: Victor connect to 30 A instead of 40 A
 

I did sort of diverge a bit. My point was and still is the breakers are there for more than protecting the wires. Stalling motors or a bad motor controller is probably the most common way to trip the snap action breakers on the robot.

I heard tell last year that some of the 6 cim drives were tripping the main breaker last year.

Re: Victor connect to 30 A instead of 40 A
 

Guys,
The breaker vs. wire size tables are lifted from the NEC (National Electrical Code) recommendations for open frame DC wiring based on temperature rise in the wire. A little adjustment has been made based on short time periods instead of continuous loaded currents. Snap action breakers do not limit current. That is to say, a 30 amp breaker will pass 100 amps as well as it will pass 30 amps. The only difference will be the length of time it will continue to pass that current.

Re: Victor connect to 30 A instead of 40 A
 

I thought the whole point of a breaker was to limit current? Better to say how fast they trip depends on % overload?

Re: Victor connect to 30 A instead of 40 A
 

Frank,
All of these breakers, the main included, can withstand several seconds of 600% overload without trip. It is the sustained current over rated current that trips the mechanism. The auto reset limits current in a fashion as the reset features restores current flow rather quickly. In a sustained over current situation these may actually "buzz" as they open and close rapidly. This in effect limits current by changing the average current passing to the load. Not what I would call predictable or effective current limit.

Re: Victor connect to 30 A instead of 40 A
 

Point taken. You can see substantially more current than the number on top of the breaker. The trip time for the Snap Action is in the <.1S range for 600% overload. 1.5 to 3.9 S for 200% overload.

Re: Victor connect to 30 A instead of 40 A
 

What's "not quite right"? I don't know about you, but I choose my wire based on the motor and its associated load, and select a breaker current limit accordingly. You really do it the other way around?

With that logic I feel like this would be a real conversation:

Student: "This CIM will give us all the torque we need to run our shooter!"
Mentor: "Yeah but we already cut and crimped this awesome 18 gauge wire, so pick a different motor compatible with it"
Student: "But we won't get enough torque from anything pulling under 20 amps"
Mentor: "I guess your CIM solution was destined to fail then"

Re: Victor connect to 30 A instead of 40 A
 

My previous statement was, in effect, you choose the wire size by the current draw of the load (motor, etc.) and then you choose the breaker by the wire size.

Breakers (and fuses) do not limit current. they open the circuit when the bimetallic surfaces get too hot. In the case of auto-resetting breakers they then close the circuit when the bimetallic surfaces cool.

Again, size the breaker (or fuse) for the wires connected to it.

Re: Victor connect to 30 A instead of 40 A
 

Granted they don't limit current in the same way as certain power supplies do (like the jag), but they do limit current. That is pretty much their function.

Re: Victor connect to 30 A instead of 40 A
 

I can see how it's possible to choose definitions so that your viewpoint is valid. But I think the common definition of "limit current" in this context disagrees with you.

The function of a circuit breaker is to interrupt the current, not to limit it. A 20 amp fuse or circuit breaker does not limit the current to 20 amps. If you try to draw more than 20 amps for long enough to trip the breaker, the current is stopped. That's not what current limiting means in my book.

Re: Victor connect to 30 A instead of 40 A
 

I make recommendations based on motor design. If your design goal is based on getting a full 12 volts to the motor, then you need to increase the size of the wire feeding the controller and the motor and then make them as short as possible. The rule of thumb I came up with is the "wire foot". 1 foot of #10 wire passing 100 amps will drop 0.1 volt. So if you have a CIM motor that you expect to run in the high current part of the motor curve, and feed the motor controller with 1 foot wires and then the CIM motor has 1 foot wires, that is a total of 4 feet of wire. (Remember the red and black wires carry the same current.) So that is 4 wire feet, 0.4 volt drop. Add 4 wire feet for a Jag or Talon, 6 for a Victor and your design curve CIM is now running at 11.2 volts on a good day neglecting any loss in the #6 and PDB. So as these losses start to add up, you can guess why some electrical designs cause robots to drive anything but straight. 4 feet of wire on one drive train compared to a few inches on the other side, unbalances the current sent to both sides. You cannot drive straight unless you compensate for the imbalance.