So I am building a large VEX tank that can move myself. Video https://www.youtube.com/watch?v=4X7j_ireIsI
However, after turning on the spot for about 10 seconds, 1 side or both sides shut down. I am running 16 motors, 4 power packs and a cortex, this thing has the power to move me easily!
How do I identify if it’s my motors PTC’s or my cortex / powerpack PTC shutting down?
your load needs to be spread out evenly. There are PTC’s in ports 1-5 and 6-10 on the cortex, as well as a larger one in the power expander. You want the entire left side in 1-5 and the right side in 6-10, or vice versa. Also, evenly distributing the power expanders will help as well… if that does not help, check your friction and even further maybe lower the speed and increase your torque.
That’s exactly what I have done, each power expander runs 2 motors (A and D)
Only 2 motors are actually running of the cortex (port 5 and 6) it’s now running 6 power expanders.
I am definitely missing something
Your weight is bending the base, causing additional friction .
Did you make sure that no wheel is traction wheels? Because if you do have traction on the corners, it can definitely cause some issues with decreased inefficiency when turning. I suggest all omnis except having traction on the middle wheels.
Put some wheels closer to the center that are non powered
You should bring this to worlds and help teams carry their robot to their matches 
Just add a lift to automatically put the robot in the field then there ya go!