mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
I once had a fault in a 30 amp circuit, my helper at the time thought tightening the clamp on the romex connector meant turn the screws until they don't turn anymore I guess:blink:. Not only did my helper need clean underpants, it tripped the 30 amp breaker as well as the 200 amp main breaker in the panel.
That will happen if the short circuit current exceeds the magnetic trip rating of both the main and branch breaker. Typical molded case breaker trip at about 10x the handle rating instantaneously; so a 30 amp breaker starts at 300amps and a 200 amp breaker about 2,000. Some main breakers even start below 10x. So an 8000 amp utility source can easily trip both if the fault and cable impedance is low enough as 8,000 or even 2,500 is still higher than 2,000.