Cable Ampacity Derating Example: 6 mm², 40°C and Grouped Circuits

How many amps can a 6 mm² copper cable carry?

There is no single ampacity for every 6 mm² cable. You need the conductor material, insulation type, number of loaded conductors and installation method. A copper XLPE cable in conduit can use a different reference table from a cable in free air or on a tray.

Under the Korean KEC/KS example supported by this tool, 6 mm² copper XLPE with three loaded conductors, B2 installation, 30°C ambient and one circuit has a reference ampacity of 44 A. That number belongs to those conditions, not every 6 mm² cable product.

Worked correction: 44 A at 30°C to about 32 A

Keep the 6 mm² copper XLPE conductor and B2 method. If the air temperature is 40°C rather than the 30°C reference, the XLPE ambient correction factor is 0.91. Two bunched circuits in the selected arrangement have a grouping factor of 0.80.

Multiplying the factors gives 44 × 0.91 × 0.80 = 32.032 A. The corrected figure shows the effect of the selected ambient and grouping, but does not include every design constraint or cable manufacturer's special installation conditions.

• Reference ampacity: 44 A
• XLPE air-temperature factor at 40°C: 0.91
• Two bunched circuits factor: 0.80
• Corrected result: about 32.03 A

Which installation conditions must be checked first?

First identify the applicable jurisdiction and wiring method, then the conductor and insulation, the number of loaded conductors and the correct reference ampacity. After that, evaluate ambient-temperature and circuit-grouping corrections. This is the sequence explained by the Korean Electrical Association for KEC-based reference tables.

Avoid correcting the temperature twice. Some published tables already assume a different baseline, while this specific tool uses the 30°C air reference in its Korean examples. Never borrow the 0.91 or 0.80 factors from this particular layout without verifying your own cable type and grouping.

Why do trays, conduits and grouped bundles differ?

Airflow and the thermal interaction between neighboring circuits differ by cable arrangement. A bunched group and a spaced single layer on a cable tray may have distinct grouping factors. A cable that can dissipate heat in one arrangement cannot automatically carry the same current in another.

The tool's Korean reference supports stated copper B2/C cases. Additional installation methods such as other tray arrangements, buried wiring, or manufacturer-specific cable designs require the corresponding approved standard and product data rather than a change of units alone.

Does corrected ampacity determine the breaker size?

No. The ampacity is a current-carrying estimate for a conductor under the stated thermal conditions. Breaker and conductor protection require assessment of the load, relevant code provisions, voltage drop and protective-device coordination. For a load-selected cable and candidate breaker, use the related cable-size-by-load tool and check its assumptions.

The US option uses a separate NEC 2023 copper THHN/THWN-2 reference and AWG/kcmil units, including termination limitations. The NEC version adopted by the installation jurisdiction may differ. Do not substitute this Korean example for a US NEC ampacity table.

• Identify the jurisdiction, wiring method and real cable specification
• Confirm ambient and grouping conditions before multiplying factors
• Review terminal temperatures, voltage drop and protection separately