What causes an FRC robot to brown out?

A brownout happens when battery voltage sags below the roboRIO's threshold (around 6.8 V on the roboRIO 2) because too much current is being drawn at once, often several motors stalling or accelerating at the same instant. To protect itself, the roboRIO sheds load (PWM outputs and some ports cut out), so the robot briefly stops responding.

What is the FRC main breaker current limit?

Every FRC robot runs through a single 120 A main breaker between the battery and the power-distribution board. Sustained draw above 120 A trips it and kills robot power. This tool adds up each mechanism's current so you can see your total against that 120 A ceiling before you're on the field.

How can I prevent brownouts?

Limit peak current: set current limits on your motor controllers, avoid commanding several high-draw mechanisms to full power simultaneously, use a healthy, freshly-charged battery with clean connections, and keep wire gauge adequate. Budgeting your current draw ahead of time, as this calculator does, is the first step.

tools / frc-current-budget

Can it all run at once, or does the robot go dark?

List every motor, set the smart-current limits you actually configured, and see the total against the 120 A main breaker and the roboRIO brownout line before a match finds it for you.

Motor figures come from the FIRST 2017 motor sheet and vendor spec pages. Breaker and brownout figures come from WPILib. Bus-voltage sag is a first-order estimate, not a promise.

the breaker doesn’t care what you meant

start from a scenario

Typical drive is a four-Kraken drivetrain at a 40 A limit with the intake and elevator idle. Full send runs the same robot with every motor stalled at once, which is what a pin against the wall looks like electrically.

verdict / 1 of 3 mechanisms running

Voltage holds, but the main breaker does not.

About 160 A is past the 120 A main breaker, so a sustained pull kills robot power outright.

160amps drawn at once

10.10volts left at the board

40amps over the 120 A main

the pack / four numbers set the lines

Describe the battery and the roboRIO you actually have.

roboRIO version

Sets the brownout line. The 1.0 is fixed at 6.3 V, the 2.0 defaults to 6.75 V.

Brownout threshold (V)

Software-settable on the 2.0 with setBrownoutVoltage().

Battery resting voltage V_oc (V)

12.5 V is a conservative default. A healthy match pack reads about 12.7 to 13.5 V open circuit.

Internal resistance R_int (Ω)

0.015

0.011 is the manufacturer figure, under 0.015 is healthy, over 0.020 means retire the pack. Use your Battery Beak reading.

brownout line: 6.75 V on the roboRIO 2.0
rail droop: 6.8 V, where the 6 V PWM rail starts to sag
sag right now: 12.50 V resting minus 160 A times 0.015 Ω is 10.10 V
margin: 3.35 V above the brownout line
blackout floor: 4.5 V, devices can drop out. Recovery at 7.5 V.

the load list / one line per motor

Every motor, and whether it is moving right now.

Only mechanisms marked running are counted in the simultaneous total. Untick the ones that never move at the same time, because a budget that assumes everything stalls at once is a budget nobody can build to.

Mechanism

Running now

Motor
Drawing
Amps
Breaker
Motor
Drawing
Amps
Breaker
Motor
Drawing
Amps
Breaker
Motor
Drawing
Amps
Breaker

160A total

Mechanism

Running now

Motor
Drawing
Amps
Breaker

30A total, not counted while idle

Mechanism

Running now

Motor
Drawing
Amps
Breaker
Motor
Drawing
Amps
Breaker

80A total, not counted while idle

the check / one breaker, one controller

Every motor measured against its own breaker.

A four-motor drivetrain is four separate 40 A breakers, not one 160 A one. Read the headroom column per motor, then read the total against the main.

160A against the 120 A main

Every motor in the load list with its draw, its own branch breaker rating, and the headroom between them.
mechanism	motor	drawing	amps	breaker	headroom	verdict
Drivetrain	Kraken X60 (Trapezoidal)	smart limit	40 A	40 A	0 A left	within 15% of the breaker
Drivetrain	Kraken X60 (Trapezoidal)	smart limit	40 A	40 A	0 A left	within 15% of the breaker
Drivetrain	Kraken X60 (Trapezoidal)	smart limit	40 A	40 A	0 A left	within 15% of the breaker
Drivetrain	Kraken X60 (Trapezoidal)	smart limit	40 A	40 A	0 A left	within 15% of the breaker
Intake (idle)	REV NEO (V1.1)	smart limit	30 A	30 A	0 A left	within 15% of the breaker
Elevator (idle)	Falcon 500	smart limit	40 A	40 A	0 A left	within 15% of the breaker
Elevator (idle)	Falcon 500	smart limit	40 A	40 A	0 A left	within 15% of the breaker

what goes wrong first

Drivetrain: each of the 4 motors sits near 40 A against a 40 A branch breaker. There is almost no headroom left.
A Intake motor at 30 A sits within 15% of its 30 A branch breaker. There is almost no headroom left.
Elevator: each of the 2 motors sits near 40 A against a 40 A branch breaker. There is almost no headroom left.
Simultaneous draw of about 160 A is past the 120 A main breaker. A sustained overage trips it, and the pack can only source over 180 A for an instant.

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Can it all run at once, or does the robot go dark?

List every motor, set the smart-current limits you actually configured, and see the total against the 120 A main breaker and the roboRIO brownout line before a match finds it for you.

Motor figures come from the FIRST 2017 motor sheet and vendor spec pages. Breaker and brownout figures come from WPILib. Bus-voltage sag is a first-order estimate, not a promise.

Describe the battery and the roboRIO you actually have.

roboRIO version

Sets the brownout line. The 1.0 is fixed at 6.3 V, the 2.0 defaults to 6.75 V.

Brownout threshold (V)

Software-settable on the 2.0 with setBrownoutVoltage().

Battery resting voltage V_oc (V)

12.5 V is a conservative default. A healthy match pack reads about 12.7 to 13.5 V open circuit.

Internal resistance R_int (Ω)

0.015

0.011 is the manufacturer figure, under 0.015 is healthy, over 0.020 means retire the pack. Use your Battery Beak reading.

brownout line: 6.75 V on the roboRIO 2.0
rail droop: 6.8 V, where the 6 V PWM rail starts to sag
sag right now: 12.50 V resting minus 160 A times 0.015 Ω is 10.10 V
margin: 3.35 V above the brownout line
blackout floor: 4.5 V, devices can drop out. Recovery at 7.5 V.

mechanism

motor

drawing

amps

breaker

headroom

verdict

Drivetrain

Kraken X60 (Trapezoidal)

smart limit

40 A

0 A left

within 15% of the breaker

Drivetrain

Kraken X60 (Trapezoidal)

smart limit

40 A

0 A left

within 15% of the breaker

Drivetrain

Kraken X60 (Trapezoidal)

smart limit

40 A

0 A left

within 15% of the breaker

Drivetrain

Kraken X60 (Trapezoidal)

smart limit

40 A

0 A left

within 15% of the breaker

Intake (idle)

REV NEO (V1.1)

smart limit

30 A

0 A left

within 15% of the breaker

Elevator (idle)

Falcon 500

smart limit

40 A

0 A left

within 15% of the breaker

Elevator (idle)

Falcon 500

smart limit

40 A

0 A left

within 15% of the breaker