Proper sizing of fuses and circuit breakers for branch lighting circuits is critical for electrical safety and code compliance. In North America, the National Electrical Code (NEC) provides guidelines to ensure circuits are not overloaded. The first step is determining the total connected load. For lighting circuits, NEC Article 210 specifies that the load should be calculated based on the maximum volt-ampere (VA) rating of all connected luminaires and outlets. As a general rule, a 15-amp breaker on a 120-volt circuit can support up to 1,800 watts (15A × 120V = 1,800W), but the continuous load should not exceed 80% of the breaker rating, meaning 1,440 watts for continuous operation over three hours.

When sizing breakers, consider the wire gauge. For a 15-amp circuit, 14 AWG copper wire is acceptable; for 20-amp circuits, use 12 AWG wire. The breaker must protect the wire from overheating. For fluorescent or LED lighting with inrush current, select a breaker with a time-delay or "C" curve characteristic to avoid nuisance tripping. Fuses are less common in modern panels but are still used in some industrial settings. Time-delay fuses are recommended for lighting circuits with capacitive loads.

Additionally, voltage drop must be considered, especially for long runs. NEC recommends less than 3% voltage drop at the farthest light. If the distance exceeds 100 feet, increase the wire size by one gauge. Finally, always verify that the total load does not exceed the panel capacity. Using a load calculation worksheet or software helps ensure accuracy. By following these sizing rules, you ensure safety, minimize fire risk, and comply with local codes.