TL;DR

Repeated breaker trips mean the circuit is overloaded or the breaker is deteriorating. Measure the load current, compare to the breaker rating, and replace any breaker that trips at or below its rated amperage.

What you might see

breaker trips under normal load
breaker trips immediately on reset
breaker handle warm to the touch
downstream outlets or equipment lose power

Likely causes

Tap to mark as ruled out

Overloaded circuit drawing more amperage than the breaker rating
Deteriorating breaker that trips below its rated trip current
Short circuit or ground fault on the downstream wiring
Loose upstream bus connection causing intermittent over-current

Required tools

Clamp ammeter (true-RMS)
Insulated screwdrivers
Torque screwdriver
Arc-flash PPE rated for the panel's incident energy
LOTO kit

Safety first

The QO bus bars are live whenever the main breaker is closed. Opening the panel door exposes you to arc-flash hazard. Wear arc-rated PPE and follow your facility's energized electrical work permit.
Never replace a breaker with a higher ampere rating than the wire gauge can carry. Check the wire AWG before selecting a replacement.

Procedure

8 steps · ~30 min

1
Put on arc-flash PPE and energized-work permit before opening the panel door.
Warning: Square D QO load centers contain live 120/240V conductors at the bus bars. Never work inside the panel without appropriate PPE and a permit.
2
Identify the tripping breaker by label and check the panel schedule for the connected loads.
3
With the panel door open, clamp an ammeter around the load wire at the breaker output terminal and read the current under normal operating conditions.[1]
4
Compare the measured current to the breaker rating stamped on the handle. Sustained current above 100% of rating is an overload.
5
If current is within rating and the breaker still trips, shut off the tripping breaker, move the load wire to a known-good spare breaker of the same rating, and see if the problem follows the breaker or the circuit.
6
If the problem follows the breaker, replace it with a Square D QO-series breaker of the same ampere and pole rating.[1]
7
If the circuit is overloaded, shed loads by splitting them across a new circuit or reduce the total connected load.
8
After any replacement, torque the load wire to 35 in-lb for 12 AWG copper or per the QO breaker label. Close the panel and test under load.

Sources

Square D QO and I-Line Load Center Installation Instructions, Square D (Schneider Electric)
Square D QO Load Center installation instructions, circuit breaker selection and torque specifications (general)
View source

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