TL;DR

Overheating inside a QO panel is a fire precursor. Use a thermal camera to locate the hot component, identify whether the cause is undersized wire or high load current, and correct it before returning to service.

What you might see

panel cover warm above ambient temperature
burning or hot-plastic smell
thermal camera shows hot spot at a breaker or lug
breaker trips after running warm for 30 minutes

Likely causes

Tap to mark as ruled out

Undersized conductors carrying current near or above their ampacity in a hot enclosure
High ambient temperature around the panel reducing breaker trip thresholds
Loose connection with elevated contact resistance generating heat
Panel loaded above 80% of its main breaker rating for extended periods

Required tools

Thermal camera or IR thermometer
Clamp ammeter
Torque screwdriver
NEC wire ampacity reference
Arc-flash PPE
LOTO kit

Safety first

Thermal scanning requires the panel to be energized with the door open. This is energized work. Use arc-flash PPE and an energized work permit.
Do not return a panel to service if any component exceeds 75 degrees Celsius above ambient. Investigate and correct the root cause first.

Procedure

9 steps · ~45 min

1
Obtain an energized electrical work permit and put on arc-flash PPE before opening the panel door.
2
Open the panel door. Scan the entire interior with a thermal camera, including all breaker faces, the neutral bar, and the main breaker.
Warning: The thermal scan must be done with the panel energized and under load. Keep your body clear of the bus bars.
3
Note every component showing more than 10 degrees Celsius above the ambient inside the panel. Record temperatures and photograph.
4
For each hot breaker, clamp an ammeter on the load wire and record current vs. the breaker rating.
5
Open the main breaker, lock out the feeder, and remove the dead-front cover.
6
At each hot component, check the wire gauge in the NEC ampacity table for the ambient temperature and conduit fill. Confirm the wire is not undersized for the current it carries.[1]
7
Re-torque any loose connections at hot components.
8
If a breaker slot is hot due to harmonic or continuous load, consider upgrading to a Square D QO 100% rated breaker for continuous-duty circuits.[1]
9
Restore power, re-scan after 15 minutes under load, and confirm the temperature has returned to normal.

Sources

Square D QO and I-Line Load Center Installation Instructions, Square D (Schneider Electric)
Square D QO Load Center installation instructions, ampacity derating and continuous-load breaker selection (general)
View source

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