Practical methods for calculating the cross-sectional area of wires and cables, plus how to determine whether a cable meets standards.

Conversion method:

Knowing the wire's cross-sectional area in square millimeters, calculate the radius using the formula for the area of a circle:

Wire cross-sectional area (square millimeters) = Pi (3.14) × Wire radius (millimeters) squared

Knowing the wire's cross-sectional area in square millimeters allows you to calculate the wire diameter in the same way, for example:

The wire diameter for a 2.5-square-millimeter cable is calculated as follows: First, divide 2.5 by 3.14 to get approximately 0.8. Then, take the square root of 0.8, which equals about 0.9 millimeters. Therefore, the wire diameter of a 2.5-square-millimeter cable is 2 × 0.9 mm = 1.8 mm.

To calculate the cross-sectional area of a wire in square millimeters when you know its diameter, use the same formula used for finding the area of a circle:

Wire cross-sectional area = Pi (3.14) × wire diameter squared / 4

Cable sizes are also specified in square millimeters, and for stranded wires, the total cross-sectional area is simply the sum of the individual wire areas.

The formula for calculating cable cross-sectional area: 0.7854 × (wire diameter in millimeters)² × number of strands. For example, a 1.5-square-millimeter wire with 48 strands (each strand having a diameter of 0.2 mm): 0.7854 × (0.2 × 0.2) × 48 = 1.5 square millimeters.

The conversion relationship between wire gauge (in square millimeters) and current

The safe current-carrying capacity of a 2.5 mm² copper power cable is 28A.

The safe current-carrying capacity of a 4 mm² copper power cable is 35A.

The safe current-carrying capacity of a 6 mm² copper power cable is 48A.

The safe current-carrying capacity of a 10 square millimeter copper power cable is 65A.

The safe current-carrying capacity of a 16 square millimeter copper power cable is 91A.

The safe current-carrying capacity of a 25 square millimeter copper power cable is 120A.

If it's aluminum wire, the wire diameter should be 1.5 to 2 times that of copper wire.

If the copper wire current is less than 28A, using 10A per square millimeter is definitely safe.

If the copper wire current exceeds 120A, use a rate of 5A per square millimeter.

The safe current-carrying capacity of a typical copper conductor is determined based on the allowable maximum temperature of the wire core, cooling conditions, and installation environment.

Generally, the safe current-carrying capacity for copper conductors is 5–8 A/mm², while for aluminum conductors it’s 3–5 A/mm².

[Key Points]

Generally, the safe current-carrying capacity for copper conductors is 5–8 A/mm², while for aluminum conductors it’s 3–5 A/mm².

For example:

The recommended safe current-carrying capacity for 2.5 mm² BVV copper wire is 2.5 × 8 A/mm² = 20 A. For 4 mm² BVV wire,

The recommended safe current-carrying capacity for copper conductors is 4 × 8 A/mm² = 32 A.

2. To calculate the cross-sectional area of copper conductors, use the recommended safe current-carrying capacity for copper wires, which is 5–8 A/mm².

Calculate the upper and lower limits of the selected copper wire cross-sectional area S:

S = [I/(5~8)] = 0.125I ~ 0.2I (mm²)

S – Cross-sectional area of the copper conductor (mm²)

I – Load current (A)

III. Power Calculation: General loads—also known as electrical appliances, such as lights, refrigerators, and more—are categorized into two types: resistive loads and inductive loads.

For resistive loads, the formula is: P = UI

For the calculation formula of fluorescent lamp loads: P = UIcosφ, where the power factor of the fluorescent lamp load, cosφ, is 0.5.

Different inductive loads have different power factors; when calculating the power factor uniformly for household appliances, you can assume cosφ = 0.8.

In other words, if the total power consumption of all appliances in a household adds up to 6,000 watts, the maximum current would be I = P / (U × cosφ) = 6,000 / (220 × 0.8) = 34 (A).

However, in most cases, household appliances aren't used simultaneously, so a common usage factor is applied—typically 0.5.

So, the calculation above should be rewritten as

I = P × Utility Factor / U × cosφ = 6000 × 0.5 / 220 × 0.8 = 17 (A)

In other words, the family’s total current draw is 17A. Therefore, the main circuit breaker cannot be rated at 16A—it should be set to a rating higher than 17A.

How do you determine the cross-sectional area of a wire in square millimeters?

The "several square millimeters" is a nominal value specified by the national standard, while the actual size—measured in square millimeters—is chosen by users based on the load requirements of their wires and cables. In home renovation and electrical work, "wire gauge in square millimeters" is often referred to simply as "several square"—a colloquial term that typically omits the unit, leaving it understood as square millimeters.

The "square" rating of a wire actually refers to its cross-sectional area—the area of the wire's circular cross-section—measured in square millimeters.

1. Each new bundle of wire will have a label on the packaging clearly indicating the wire’s brand, model, and diameter (in square millimeters).

2. Some major-brand wires have the brand name, model number, and wire gauge (in square millimeters) printed directly on the insulation layer.

3. For wires that are not labeled, experienced electricians can determine the wire's gauge—measured in square millimeters—simply by visual inspection.

4. For those without experience, if you need to determine the wire's thickness (in square millimeters), you can use a caliper to measure the wire's diameter and then calculate the cross-sectional area.

To determine the wire's thickness (in square millimeters), you can use a caliper to measure the wire's diameter and then calculate the cross-sectional area.

The formula for calculating the cross-sectional area is: S = radius squared × π (where R is the radius of the wire).

For example, a wire with a diameter of 1.76 would have an area calculated as follows: 1.76 ÷ 2 × 3.14 = 2.76 square units, which rounds to approximately 2.5 square units (using an approximate value).

If you're choosing the right wire size, you typically start by calculating the current based on the power rating of the appliance, and then refer to an electrician's handbook to determine the appropriate wire gauge. This method ensures greater accuracy.

If it’s a single-core cable, you can measure its diameter and then calculate the radius using the formula for area: S = radius × radius × 3.14.

You can then determine the cross-sectional area of the wire. However, some wires are made by twisting multiple individual strands together, which means you can’t calculate it using the single-strand method. Instead, you’ll need to connect the single-core wire to just one of the strands within the multi-strand wire, and then multiply the result by the total number of strands in the wire.

The formula is S = radius × radius × 3.14 × N,

Among them, N is the number of times this wire has been twisted!