Branch circuits#
Every output channel of the PD is a branch circuit. Rule R610 requires every circuit (except the roboRIO and radio circuits covered by R615/R617) to be protected by a breaker or fuse, and R621 requires each branch to be protected by one and only one breaker/fuse sized per Table 8-3. These auto-resetting breakers protect both the wiring and the motor controller from sustained overcurrent.
Legal breaker types (R619)#
Only specific breakers may go in a PD:
- Snap-Action VB3-A series or AT2-A (terminal style F57), 40A or lower
- Snap-Action MX5-A or MX5-L series, 40A or lower
- REV Robotics ATO auto-resetting breakers, 40A or lower
- CTR Electronics ATO breakers, 40A or lower (added for 2026)
- ATM breakers up to the value permitted for fuses per R620
These are auto-resetting thermal breakers: when they trip from overcurrent they cool down and reset themselves automatically rather than needing a manual reset.
Sizing by Table 8-3 (R621)#
The maximum protection depends on what the channel feeds:
- Motor controllers: up to 40A (one breaker each).
- Custom circuits: up to 40A (no quantity limit).
- Spike relay / Automation Direct 25A relay, PCM/PH with compressor, Servo Power Module/Servo Hub: up to 20A.
- Additional non-radio VRM / non-compressor PCM/PH: up to 20A (three total).
- roboRIO and radio: a 10A fuse/breaker on a dedicated channel (R615/R617).
A common mistake is putting a 40A breaker on a circuit wired with thin wire - the wire would melt before the breaker tripped. The breaker must match both the device and the wire gauge.
Fuses vs breakers (R620)#
Low-current channels often use fuses instead of breakers. R620 limits fuses to automotive blade types: ATM fuses for the PDP, sized to match the value printed on that channel's own fuse holder; ATM fuses up to 15A for the PDH (with a single 20A exception for a PCM/PH compressor); and ATC/ATO fuses (10A or lower) for the PDP 2.0. Unlike breakers, a fuse does not reset - it blows once and must be replaced, so keep spares in your pit kit.
Practical tips#
- Label each channel so you know which breaker feeds which device.
- Carry a full set of spare breakers (40A, 30A, 20A) and 10A fuses to every competition.
- A breaker that trips repeatedly is a symptom - investigate the mechanism or motor for a jam or stall; do not just up-size the breaker.
Sources#
the part worth keeping
Key takeaways
- Each branch circuit needs exactly one breaker/fuse sized per Table 8-3 (R610/R621).
- Legal breakers are Snap-Action VB3/AT2/MX5, REV ATO, CTR ATO (2026), or ATM - all 40A or less (R619).
- Match the breaker to BOTH the device and the wire gauge; a 40A breaker on thin wire is dangerous.
- Motor controllers/custom circuits get up to 40A; relays/PCM/PH/servo up to 20A; roboRIO and radio use 10A. Fuses don't reset, so carry spares.
Electrical & WiringPower: Battery, Breakers, and the Main Circuitlesson 3 of 5
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sources and further reading
- docs.wpilib.orgWPILib Hardware Component Overview
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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
01Under R619, what is true of the breaker types that are legal to install in a Power Distribution device?
02Why is putting a 40A breaker on a circuit wired with thin wire a common mistake?
03What is a key advantage of the auto-resetting thermal branch breakers used in a PD?
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