Branch Circuit Breakers and Fuses
Branch circuits
Section titled “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)
Section titled “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)
Section titled “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-radioVRM / 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)
Section titled “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
Section titled “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
Section titled “Sources”Key takeaways
Section titled “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.
This lesson was adapted from learnfrc.com.