06 April 2021
What is the temperature of a molded case breaker?
The acceptable operating temperature of a circuit breaker is defined by UL in the UL489 standard (June 2011), which is listed below.
Table 1: Summary of temperature rise and maximums for a standard rated breaker (breakers are calibrated in 40 deg. C ambient)
- Terminations for standard rated breakers: UL 489 Paragraph 184.108.40.206.2 says the temperature rise on a wiring terminal at a point to which the insulation of a wire is brought up as in actual service shall not exceed 50°C (122°F).
- Terminations for 100% rated breakers: UL489 Paragraph 220.127.116.11.3 says the temperature rise on the termination shall not exceed 60°C (140°F).
- Handles, knobs, and other user surfaces: UL489 Paragraph 18.104.22.168.6 says the maximum temperature on handles, knobs, and other surfaces subject to user contact during normal operation shall not exceed 60°C (140°F) on metallic and 85°C (185°F) on nonmetallic surfaces.
|Surface||Temp. Rise above ambient||Temp. Max at 40 deg. C ambient (104 F)|
|Termination on standard rated breaker||50°C (122°F)||90°C (194°F)|
|Termination on 100% rated breaker||60°C (140°F)||100°C (212°F)|
|Handles, knobs, other user contact surfaces - Metallic||N/A||60°C (140°F) Maximum|
|Handles, knobs, other user contact surfaces - Nonmetallic||N/A||85°C (185°F) Maximum|
The heat that is generated from inside the breaker comes from the contacts and the bimetal, which are typically the hottest parts within the breaker, and the temperature in the current path gets cooler the closer to the bus or cable, which function as heat sinks. Therefore, the sides or bottom of the breaker can be hotter than the terminations. Each breaker family or frame will have its own heating characteristics due to different designs. Smaller frames tend to run hotter on the side of the breaker than do larger frames, since the side of the breaker is much closer to the heat generators within the breaker.
If your breaker is operating within the temperature rise and maximum values in Table 1, then the breaker should be operating normally.
If you still believe the breaker is suspect and are concerned that the breaker may nuisance trip, then I recommend you hire Schneider Electric Field Services (Square D Services) to primary injection test the breaker to determine whether or not the breaker performs per its Characteristic Tripping Curve.