Derating a Circuit Breaker for High Altitude Operation
Why does altitude affect circuit breaker operation?
Circuit breakers work on the thermo-dynamic properties of metal. In other words, the heating and cooling of the bi-metallic strip inside a breaker heats up and literally bends. This bending is what causes a breaker to trip. So the question then becomes, why does altitude affect this bi-metallic strip and it's function? The answer has to do with air, air pressure, and air's ability to dissipate heat.
The atmosphere has air in it. As you rise from sea level, the density of that air reduces, and fairly dramatically. By the time you get 1 mile high, 5,280 feet above sea level, sources say there is roughly 40% of the oxygen by volume. This is why runners train at altitude, the get used to using less air to run further. Breakers on the other hand need that air to pass heat.
Reduced air density at altitudes greater than 3300 ft. (1000 meters) affects the ability of a molded case circuit breaker
to transfer heat and interrupt faults compared with sea level. Most circuit breakers are designed for out-of-the-box operation up to 4500 feet. However at altitudes over 6600 feet (2000 meters) the rare air up there really starts to make a difference. The heat generated by the strip can not be dissipated as efficiently at altitude and the breaker trips while under normal operation. WHY? The air thickness and it's lack of ability to abosorb heat.
Where does this actually happen?
If you are installing breakers in a cabin, in the mountains above 7,000 feet, or similar application, please derate your circuits accordingly. If you fail to derate the circuits, your breakers will trip routinely. By using a larger circuit breaker, the system produces less heat and leads to fewer false interrupts. To build according to code, make sure to upsize all associated components as well.