Changing cable diameter in a distribution box is permissible only if the smaller cable is adequately protected by a suitable overcurrent device and meets current-carrying and voltage-drop requirements.

Key Considerations

1. Protective Device Requirement When reducing cable size within a distribution box, the smaller cable must be protected by a fuse or circuit breaker rated for its capacity. This ensures that the cable is not overloaded, preventing overheating or fire hazards. Without proper protection, reducing cable size is unsafe and may violate electrical codes (AS/NZS 3000 in Australia) and general safety standards . 2. Current-Carrying Capacity and Voltage Drop The new cable size must be sufficient to carry the expected load without excessive voltage drop. For example, if a main cable is 10 mm² and branches are reduced to 2.5 mm² or 6 mm², each branch must still meet the voltage drop and current-carrying requirements for its load . Calculations should consider the length of the run and the type of insulation. 3. Termination and Mechanical Considerations Cables must be terminated properly in the distribution box. Maintain a minimum bending radius of at least five times the cable diameter to avoid mechanical stress, insulation damage, or conductor breakage . Ensure that connections are tightened to the manufacturer's torque specifications to prevent loose connections and localized overheating. 4. Short Tail Practice In some cases, a short section of smaller cable (short tail) may be used at the end of a larger cable run for termination purposes only. This tail should be as short as possible and rated to carry the full current of the circuit, without relying on voltage drop allowances . 5. Compliance and Safety Always follow local electrical codes and standards. In Australia, AS/NZS 3000 requires that cable size reductions are only allowed if the protective device safeguards the smaller cable. In other regions, similar rules apply, and unqualified personnel should not perform such work due to safety and legal risks .

Summary

• Cable size can change in a distribution box if the smaller cable is protected and meets load requirements.
• Voltage drop and current capacity must be verified for each branch.
• Mechanical installation must respect bending radius and proper termination.
• Short tails are acceptable only for termination and must be minimal in length.
• Always comply with local codes and use qualified electricians for installation. Following these guidelines ensures safe, reliable operation of the distribution system while allowing flexibility in cable sizing within junction boxes.