Ask any electrician what a 15-amp breaker does, and they’ll probably say it trips when the load exceeds 15 amps. That’s not really true. Circuit breakers don’t trip because of a specific number of amps. They trip because of heat. That means you can run 16, 17, even 18 amps through a 15 amp breaker, and it may hold for minutes, hours, or indefinitely, depending on the conditions. If the panel is cold, like in a garage in winter, it may take a long time for the breaker to heat up enough to trip. But if the panel is sitting in the hot Florida sun, the exact same breaker might trip within minutes.
Most breakers used in homes and businesses are thermal-magnetic. They have two separate tripping mechanisms. One is a thermal element that responds to heat buildup over time. The other is a magnetic coil that trips instantly on a dead short. The thermal part uses a bimetal strip that bends as it gets hot, and that bending motion releases the latch. The higher the current, the faster it heats up and the faster it trips. This is called inverse time behavior. But slightly overloaded breakers can run for a long time before they trip.
This is also exactly why motors can start without tripping the breaker, even if the inrush current is five to eight times higher than the rating. Inrush current only lasts a second or two, and that is not long enough to heat up the thermal element. So if a motor pulls 60 amps for one second on a 20 amp breaker, the breaker sees it as a brief pulse and holds. If breakers tripped immediately on inrush, HVAC systems, compressors, and well pumps would constantly go down.
Let’s look at what the manufacturers say. Square D says their QO and QOB breakers are calibrated at 40 degrees Celsius. That is 104 degrees Fahrenheit. At that temperature, a 15-amp breaker carrying 135 percent of its rated load, which is about 20.25 amps, can take up to one hour to trip. If it sees 200 percent or 30 amps, it trips in under two minutes. If it sees 300 percent or more, it trips almost instantly using the magnetic section. If the panel is colder than 40 degrees Celsius, those times get longer. If the panel is hotter, the trip times get shorter.
Siemens says the same thing. Their QP series breakers also follow the same inverse time curves. They can carry 135 percent of their rating for at least an hour. Their magnetic trips kick in around 240 to 400 amps, depending on the model and pole count. Siemens also tests their breakers at 40 degrees Celsius. In cold environments, the bimetal strip takes longer to reach its bend point. In hot environments, that process speeds up.
This is why troubleshooting breaker trips requires more than just checking the amp draw. A 20-amp breaker that is feeding 22 amps on a hot day may trip fast if it is outside in the sun. The exact same load in a climate-controlled panel indoors may run for hours without a problem.
Bottom line is this. A 15-amp breaker does not trip at 15 amps. A 20-amp breaker does not trip at 20 amps. They trip when enough heat builds up over time. That time varies with the level of overload and the ambient temperature around the breaker. Every major manufacturer publishes this data in the time-current curves. They all show the same principle. Current alone does not trip a breaker. Time and heat do.
This is also why motor inrush does not trip breakers. It is a feature, not a flaw. And if you are teaching electrical concepts or doing plan review or field inspections, it is important to understand how these devices actually work. The number on the handle is only one part of the story.
References
Schneider Electric. Ambient Temperature Rerating for Thermal Magnetic Breakers.
Schneider Electric. QO and QOB Time Current Curves.
Siemens. Time Current Curves for QP Series Breakers.
Siemens. Residential and Commercial Breaker Overview.
Most breakers used in homes and businesses are thermal-magnetic. They have two separate tripping mechanisms. One is a thermal element that responds to heat buildup over time. The other is a magnetic coil that trips instantly on a dead short. The thermal part uses a bimetal strip that bends as it gets hot, and that bending motion releases the latch. The higher the current, the faster it heats up and the faster it trips. This is called inverse time behavior. But slightly overloaded breakers can run for a long time before they trip.
This is also exactly why motors can start without tripping the breaker, even if the inrush current is five to eight times higher than the rating. Inrush current only lasts a second or two, and that is not long enough to heat up the thermal element. So if a motor pulls 60 amps for one second on a 20 amp breaker, the breaker sees it as a brief pulse and holds. If breakers tripped immediately on inrush, HVAC systems, compressors, and well pumps would constantly go down.
Let’s look at what the manufacturers say. Square D says their QO and QOB breakers are calibrated at 40 degrees Celsius. That is 104 degrees Fahrenheit. At that temperature, a 15-amp breaker carrying 135 percent of its rated load, which is about 20.25 amps, can take up to one hour to trip. If it sees 200 percent or 30 amps, it trips in under two minutes. If it sees 300 percent or more, it trips almost instantly using the magnetic section. If the panel is colder than 40 degrees Celsius, those times get longer. If the panel is hotter, the trip times get shorter.
Siemens says the same thing. Their QP series breakers also follow the same inverse time curves. They can carry 135 percent of their rating for at least an hour. Their magnetic trips kick in around 240 to 400 amps, depending on the model and pole count. Siemens also tests their breakers at 40 degrees Celsius. In cold environments, the bimetal strip takes longer to reach its bend point. In hot environments, that process speeds up.
This is why troubleshooting breaker trips requires more than just checking the amp draw. A 20-amp breaker that is feeding 22 amps on a hot day may trip fast if it is outside in the sun. The exact same load in a climate-controlled panel indoors may run for hours without a problem.
Bottom line is this. A 15-amp breaker does not trip at 15 amps. A 20-amp breaker does not trip at 20 amps. They trip when enough heat builds up over time. That time varies with the level of overload and the ambient temperature around the breaker. Every major manufacturer publishes this data in the time-current curves. They all show the same principle. Current alone does not trip a breaker. Time and heat do.
This is also why motor inrush does not trip breakers. It is a feature, not a flaw. And if you are teaching electrical concepts or doing plan review or field inspections, it is important to understand how these devices actually work. The number on the handle is only one part of the story.
References
Schneider Electric. Ambient Temperature Rerating for Thermal Magnetic Breakers.
Schneider Electric. QO and QOB Time Current Curves.
Siemens. Time Current Curves for QP Series Breakers.
Siemens. Residential and Commercial Breaker Overview.