The cable size for a level 3 distribution box depends on the load current, voltage, distance, and conductor material, but typical high-capacity feeders often use 50 mm² to 150 mm² copper or equivalent aluminum conductors.

Determining Cable Size

The correct cable size is not fixed and must be calculated based on several factors:

• Load Current (Amperage): The cable must safely carry the full load current without overheating. For example, a 150 A feeder may require a 50 mm² copper cable for a 3-phase system at 415 V .
• Voltage Level: Higher voltages allow smaller conductors for the same power. Level 3 distribution boxes typically operate at medium voltage (e.g., 415 V three-phase), which affects sizing .
• Distance and Voltage Drop: Longer cable runs increase resistance and voltage drop. NEC recommends a maximum of 3% voltage drop for branch circuits and 5% for feeders .
• Conductor Material: Copper has higher conductivity than aluminum. Aluminum cables must be upsized compared to copper to achieve the same ampacity .
• Insulation and Installation Conditions: Armored cables (PVC or XLPE) and conduit fill affect heat dissipation and ampacity .

Practical Examples

• A 75 HP, 415 V, 3-phase motor located 75 m from the panel with a 150 A MCCB may use a 50 mm² 4-core PVC armored copper cable .
• For residential or commercial service entrances, 1/0 AWG copper or 4/0 AWG aluminum is common for 200 A service .

Recommendations

1. Calculate the full-load current of all connected equipment.
2. Select a cable that meets or exceeds the ampacity for that current, considering voltage drop and distance.
3. Use NEC or IEC tables for ampacity and derating factors based on installation conditions .
4. Consider future expansion by adding a 25% safety margin to the calculated cable size .
5. Verify insulation type and conduit compatibility for mechanical protection and heat dissipation. In summary, there is no single cable size for all level 3 distribution boxes. The proper conductor size must be calculated based on load, voltage, distance, and installation conditions, with typical medium-voltage feeders using 50 mm² to 150 mm² copper or equivalent aluminum cables for high-current applications .