Mike Dolt Enterprises
Mike Dolt Enterprises
User: Electric-Light, Location: Massachusetts
Asked on: Wednesday, April 6, 2011 at 4:23 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:52 am EDT
Question: In 120v application, I doubt there's any difference in magnetic trip characteristic between 15 or 20A breaker. The difference in thermal trip provides appropriate protection of conductors from overload.For 100' extension cord, a 15A motor FLA motor load needing a high starting torque will benefit from a 10AWG conductor extension cord, but a 16AWG will do for a 60W work light on a 100' cord.Even with the 16/3 100' if the hot shorts to ground, there's enough current to activate the magnetic trip.Since the ground conductor isn't used to carry current, it really isn't subject to overload. It just provides a bleeder ground or a path to short out so if the hot shorts to case, it will trip the breaker.Why can't we have something like 10/2, 16-ground extension cords?
Magnetic Trip Characteristic Similarity
You are correct that in a 120V application, there is generally no significant difference in magnetic trip characteristic between a 15A and 20A circuit breaker. Both breakers are designed to trip quickly in the event of a short circuit or overcurrent fault.
Thermal Trip Characteristic Difference
The primary difference between a 15A and 20A circuit breaker is their thermal trip characteristic. A thermal trip is used to protect the circuit from sustained overloads. The thermal trip element heats up when current passes through the breaker and trips when the temperature reaches a predetermined level. A 20A breaker has a higher thermal trip setting than a 15A breaker, meaning it can handle more current before tripping.
Conductor Protection
You are also correct that a 10AWG conductor is more appropriate for a 100' extension cord with a 15A motor load due to the higher current draw of the motor, especially during starting. A 16AWG conductor would be sufficient for a lower power load, such as a 60W work light.
Ground Conductor Overload
While the ground conductor typically does not carry current during normal operation, it can carry current during a fault condition, such as when the hot wire shorts to the ground. In this case, the current flowing through the ground conductor could be enough to activate the magnetic trip of the breaker.
10/2, 16-Ground Extension Cords
It is not common to have 10/2, 16-ground extension cords for the following reasons:
NEC Requirements
The National Electrical Code (NEC) requires that the ground conductor be at least as large as the circuit conductors. For a 10AWG circuit, this would require a 10AWG ground conductor.
Cost Considerations
Using a 16AWG ground conductor would reduce the cost of the extension cord, but it would not provide the same level of protection as a 10AWG ground conductor.*
Limited Visibility: A 16AWG ground conductor may not be as easily visible as a 10AWG conductor, making it more difficult to identify in an emergency. Flag for review