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Electrical & Wiring·Lesson 10 of 34

The Main 120A Breaker

The single most important safety device on the robot: the main breaker that acts as the master switch.

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What the main breaker does

Between the battery and the power distribution device sits the 120-Amp main circuit breaker. It serves two jobs at once:

  1. Master power switch - flipping it off cuts all power to the robot. Rule R612 requires it to be quickly and safely accessible from the exterior of the robot (not behind an access panel or next to moving parts), and it is the only 120A breaker allowed on the robot.
  2. Overcurrent protection - it is a thermal breaker, so it trips on a combination of current magnitude and duration. A brief spike above 120A is fine; a sustained overload trips it and protects the wiring and battery.

The specific part

Rule R609 names the exact legal parts: a Cooper Bussmann CB185-120, CB185F-120, CB285-120, CB285F-120, or CB285120F, or an Optifuse 153120 or 253120. (Vendors and WPILib sometimes refer to these by the short form '185120F'.) Buy the genuine part - knockoffs can fail closed under load.

Wiring it

The main breaker is wired in line with the positive (red) side of the main circuit (R609):

  • Battery (+) -> 120A breaker -> PD (+), all with 6 AWG copper wire.
  • The battery negative goes directly to the PD negative input - the breaker is only on the positive side.
  • Crimp terminal lugs sized for the breaker studs onto the 6 AWG wire, then bolt them down (a star/lock washer helps) for a solid, low-resistance connection.

Rule R609 allows nothing else on this main circuit except the listed devices (with narrow exceptions like R625 monitoring circuitry and approved SB-50-to-SB-120 adapters). Per R621, the only load that may share a branch breaker is a device downstream of a Kraken X60 Powerpole adapter board (WCP-1380, RF-4003).

Common mistakes

  • Loose lug bolts - a loose 120A connection heats up and can melt insulation or weld itself shut. Torque it properly and re-check often.
  • Mounting it where you can't reach it - if field staff can't kill power fast, you fail inspection (R612).
  • Wrong wire gauge - anything thinner than 6 AWG on the main path is both unsafe and illegal.

Sources

Key takeaways

  • The 120A main breaker (Cooper Bussmann CB185-120/CB285120F or Optifuse 153120/253120) is both the master switch and overcurrent protection.
  • It is wired on the positive side: battery(+) -> 120A breaker -> PD(+) with 6 AWG wire (R609).
  • Rule R612 requires it to be quickly accessible from outside the robot; it is the only 120A breaker allowed.
  • Loose lug connections are dangerous - crimp proper lugs and torque the studs tight.

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.What are the two roles of the 120A main circuit breaker on an FRC robot?

2.On the 120A main breaker, what connects to the terminal labeled 'BATT'?

3.What wire gauge must connect the 120A main breaker to the power distribution board?

Answer every question to submit.