What do 4-mil and 6-mil wires mean?

In everyday slang within the electrical industry, "4 li" and "6 li" wires typically refer to cables with conductor cross-sectional areas of 4 square millimeters (4 mm²) and 6 square millimeters (6 mm²), respectively. Here, "li" is a simplified, colloquial term that actually corresponds to the internationally recognized unit "square millimeters" (mm²).
1. The Meaning and Conversion of "Li"
The origin of "li":
The colloquial term "li" likely originates from a simplified expression of "square millimeters," for example:
“4 li” = 4 mm² (the conductor cross-sectional area is 4 square millimeters).
“6 li” = 6 mm² (conductor cross-sectional area is 6 square millimeters).
Differences from other units:
Be careful not to confuse this with the unit of length "centimeters (cm)—this refers only to cross-sectional area.
1 square millimeter (mm²) ≈ 0.000001 square meters (m²)—a truly minuscule unit of area.
II. Uses of 4 mm² and 6 mm² Wires
1. 4 mm² wire
Current-carrying capacity:
Copper-core wires (BV/BVR): Approximately 30–40 amps (A), corresponding to a load power of about 6–8 kilowatts (kW) (at 220V, where power = current × voltage).
Common uses:
High-power appliance wiring in the home, such as:
Kitchen appliances such as ovens, microwave ovens, and dishwashers.
Air conditioners with a capacity of 1.5 horsepower or higher (wall-mounted or cabinet models).
Electric water heaters (storage or instant-type).
2. 6 mm² wire
Current-carrying capacity:
Copper-core wires (BV/BVR): approximately 40–55 amps (A), corresponding to a load power of about 8–12 kilowatts (kW).
Common uses:
Ultra-high-power equipment or backbone lines, such as:
More than 3 central air conditioning outdoor units.
The main incoming cable for the whole-house distribution box (some smaller apartments may use 10 mm², but 6 mm² is suitable for medium-sized homes).
Power supply lines for industrial small-scale equipment.