Circuit Protection & Panel Requirements

Types of Overcurrent

Overcurrent is any current that exceeds the rated current of conductors or equipment. There are three types:

• Overload - current that exceeds the normal full-load rating, typically 1.1 to 6 times rated current. Develops gradually and can last minutes or hours. Example: a motor running with a seized bearing.
• Short circuit - an unintended low-resistance path between conductors. Produces extremely high current (thousands of amps) that must be cleared in milliseconds.
• Ground fault - current flowing through an unintended path to ground. Can range from a few milliamps (GFCI level) to thousands of amps.

Overcurrent, Ground Fault & Arc Fault Protection

Modern electrical systems use multiple layers of protection to address different types of hazards:

Overcurrent protection is the foundation of circuit safety. Fuses and circuit breakers detect current exceeding the conductor's rated ampacity and interrupt the circuit before conductors overheat or catch fire.

Ground fault protection detects current flowing through unintended paths to ground. At the personnel level, a GFCI trips at approximately 5 mA to prevent electrocution. At the equipment level, ground fault protection of equipment (GFPE) is required for services rated 1000 amps or more at 150V to ground or higher, and trips at much higher thresholds to protect against arcing damage.

Arc fault protection detects dangerous arcing conditions in wiring and connections. An arc fault circuit interrupter (AFCI) monitors the circuit for characteristic signatures of arcing - such as those caused by damaged insulation, loose connections, or pinched wires. Unlike overcurrent devices, AFCIs can detect low-level arcing that would not trip a standard breaker but can ignite surrounding materials and cause a fire.

The 80% Continuous Load Rule

The maximum continuous load on a circuit breaker should not exceed 80% of its rating. A continuous load is defined as a load where the maximum current is expected to continue for 3 hours or more.

Example: A 20A breaker with a continuous load must be limited to 20A x 0.80 = 16A maximum. If the load exceeds 16A continuously, the breaker must be upsized to 25A or 30A.

Some circuit breakers are rated for 100% continuous loading. These special breakers are listed for continuous operation at their full rating and are more expensive than standard breakers.

Overcurrent Protection Sizing

CEC Section 14 establishes the rules for overcurrent protection. The overcurrent device must be sized to protect the conductor, not the load:

The maximum continuous load on a standard circuit breaker is 80% of its rating. A continuous load runs for 3 hours or more. Overcurrent protection must be sized to protect the conductor, with maximum ratings matched to conductor size (14 AWG = 15A, 12 AWG = 20A).