Branch Circuit Breakers and Fuses

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

• WPILib Hardware Component Overview
• 2025 Game Manual Section 8