What size cable is needed for a 47kW distribution box

For a 47 kW three-phase distribution box at 400V with a power factor of 0.8, a typical copper cable size would be around 16–25 mm², depending on installation conditions and cable length.

Calculating Load Current

To determine the cable size, first calculate the load current using the formula for three-phase systems:

I = (kW × 1000) / (√3 × V × PF)

For a 47 kW load at 400V three-phase with a power factor of 0.8:

I = (47 × 1000) / (√3 × 400 × 0.8) ≈ 84.7 A

This is the continuous current the cable must carry .

Selecting Cable Size

Cable sizing must consider:

• Current-carrying capacity (ampacity): The cable must safely carry the calculated current without overheating.
• Voltage drop: Typically limited to 3–5% for power circuits. Longer cable runs require larger conductors to maintain voltage within limits.
• Installation method and ambient temperature: Cables in conduit, buried, or grouped with others may require derating .

For a load of ~85 A:

• 16 mm² copper XLPE/SWA cable can carry approximately 80–90 A under standard conditions.
• 25 mm² copper XLPE/SWA may be recommended for longer runs or higher ambient temperatures to reduce voltage drop and provide safety margin .

Practical Recommendations

• Verify the installation method (e.g., conduit, tray, buried) and apply the appropriate derating factors.
• Check the one-way cable length to ensure voltage drop does not exceed limits. For longer distances, increase the cable size.
• Always select the next standard size above the calculated requirement to ensure safety and compliance with IEC 60364-5-52 or BS 7671 standards .
• Consider future load expansion; slightly oversizing the cable can improve reliability and reduce heating.

In summary, for a 47 kW three-phase distribution box at 400V, a 16 mm² copper cable is generally sufficient for short runs, but 25 mm² is safer for longer distances or higher ambient temperatures, ensuring compliance with voltage drop and current-carrying requirements.