What protection device is most effective for a circuit rated at 20 amperes but loaded to 21 amperes?

The most effective protection device for a circuit rated at 20 amperes but loaded to 21 amperes is a magnetic circuit breaker. Magnetic circuit breakers are designed to trip almost instantaneously when exposed to overcurrent conditions that exceed their rated capacity. This quick response time is crucial in preventing damage to the electrical system and minimizing hazards such as potential fires or equipment damage.

When a circuit is loaded beyond its rated limit, as in this scenario, the magnetic mechanism within the breaker will sense the surge in current and interrupt the circuit promptly. This is particularly valuable in scenarios where overcurrent can be momentary but potentially damaging.

Comparatively, other options like a fused disconnect might require a fuse replacement after tripping which could lead to delays in restoring power. A Ground Fault Circuit Interrupter (GFCI) is focused on detecting ground faults rather than overcurrent conditions, while a thermal overload relay reacts to sustained overloads which may take longer to trip compared to the magnetic mechanism. Thus, in the specific case of immediate response to a slight overcurrent condition, the magnetic circuit breaker stands out as the most effective option.