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:
- 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.
- 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 'CB285120F'.) 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#
the part worth keeping
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.
Electrical & WiringPower: Battery, Breakers, and the Main Circuitlesson 2 of 5
Keep going
Take the quiz+10 XP with an accountMore in Power: Battery, Breakers, and the Main Circuit
where this came from
Sources and corrections
This lesson is AI-assisted: drafted from primary sources, then reviewed and edited by hand. Errors still get through. When one is reported we fix it and write down what changed, in public, in the corrections log.
sources and further reading
- docs.wpilib.orgWPILib Introduction to FRC Robot Wiring
clipped to this lesson
Articles that go further on this
The lesson gets you through the topic. These go wider on it, and they read in one sitting.
- 19 min readFRC PDH: The Power Distribution Hub Explained (Channels, Fuses & Wiring)A complete guide to the REV PDH (Power Distribution Hub): all 24 channels, breaker and fuse sizing, clean wiring, the switchable channel, CAN telemetry, and PDH vs PDP./blogread it
- 7 min readFRC LED Lights: Wiring, Power Budgeting, and the WPILib AddressableLED APIHow to wire addressable LEDs off the PDH's switchable channel, budget their current draw, and use WPILib's AddressableLED API to signal robot state./blogread it
- 5 min readHow to Wire an FRC Robot: A Beginner Electrical GuideA beginner-friendly, accurate guide to wiring an FRC robot: battery, 120A breaker, PDH, roboRIO, radio, CAN bus, and the wire-gauge rules that pass inspection./blogread it
answer sheet
Lesson quiz
All 3 right completes the lesson. Miss one and only that question comes back, anything you already answered correctly stays banked.
0 of 3 answered
01What are the two jobs the 120A main circuit breaker does on an FRC robot?
02Where in the main circuit is the 120A breaker wired, per rule R609?
03What wire gauge must connect the 120A main breaker to the power distribution device?
Answer every question to submit.
All 35 lessons in Electrical & Wiringopenclose
01 / prerequisites
02 / control-system-components
03 / power-battery-breakers
04 / motor-controllers-can-bus
05 / connections-rules-troubleshooting
06 / worked-examples-mini-projects
- Not read yet:Mini-Project 1: A Single-Motor Test Stand from Battery to Spin
- Not read yet:Mini-Project 2: Current-Limited Drivetrain (CTRE and REV)
- Not read yet:Mini-Project 3: A Live Power-Monitoring Dashboard
- Not read yet:Mini-Project 4: A Switchable Channel for Lights and Vision
- Not read yet:Mini-Project 5: CAN Device Bring-Up with Tuner X and the Hardware Client
07 / common-mistakes-troubleshooting
08 / advanced-techniques-case-studies