How to Select and Calculate the Cable Size Required for Your Project

I. Introduction

Cables aren’t decided on a whim—in engineering projects, cables are like the blood vessels in the human body, responsible for transmitting energy. If you choose a cable that’s too small, at best it’ll trip the circuit breaker; at worst, its insulation could melt, sparking a fire. But if you go with a cable that’s too large, not only will the procurement costs double, but the excessive weight will also place extra strain on cable trays and installation work.  At its core, choosing the right cable size is a delicate balancing act between “safety” and “total lifecycle costs.”

II. Core Fundamentals: The Four Key Factors Determining Cable Size

Calculate the current (load size): This is the starting point for selecting the right equipment. The power rating indicated on your device’s nameplate directly determines the base current that the cable must be able to handle. For a three-phase 380V motor, there’s a remarkably handy mental calculation formula:

1 kW is approximately equal to 2 A of current.

This can help you quickly estimate on-site.

Installation Environment (Heat Dissipation Conditions): Whether the cable has good heat dissipation significantly affects its current-carrying capacity. Is the cable installed in conduit, buried underground, routed through cable trays, or exposed to the air? What is the ambient temperature surrounding the cable? The poorer the heat dissipation, the greater the reduction required in the cable’s carrying capacity (current rating). When cables are directly buried, the current rating is typically multiplied by a correction factor ranging from 0.8 to 0.9; and when multiple cables are laid side by side, the correction factor can be as low as 0.7 to 0.9.

Line Length (Voltage Drop): This is the factor most easily overlooked by beginners. Cables themselves have resistance, and the longer the distance, the greater the voltage drop at the end of the line. When the supply distance exceeds 200 meters, the issue of voltage drop begins to become prominent; motors，An excessively low terminal voltage can lead to insufficient torque, overheating, and even burnout. Construction sites are mandated to ensure that the voltage drop does not exceed 5%.

Mechanical Strength and Short-Circuit Thermal Stability: In addition to carrying normal current, cables must also be able to withstand unexpected situations.

First, regarding physical pulling and burying: for directly buried cables with steel tape armor, the copper core cross-sectional area should not be less than 25 square millimeters.

Second, when a short circuit occurs in the system, the enormous current surge generates extremely high heat. Therefore, the cable cross-section must be large enough to ensure that the insulation layer does not melt.

III. Practical Exercise: The 5-Step Cycle for Cable Sizing Calculation

When dealing with medium- and high-power equipment, please follow the five steps below strictly.

Step 1: Calculate the rated current of the load

This is the most raw input data.

Single-phase purely resistive load (such as heaters, lighting):

Step 2: Introduce the correction factor and calculate the corrected current-carrying capacity(I_z)

The “safe current-carrying capacity” specified in the manufacturer’s manual is typically measured under standard environmental conditions—such as air at 30°C or soil at 25°C, with cables laid individually. In actual operating conditions, however, the situation is much more complex, so correction factors must be introduced.

Temperature Correction (k₁): For every degree Celsius increase in ambient temperature, cable heat dissipation slows down slightly, and the current-carrying capacity must be reduced accordingly.

Parallel Laying Correction (k₂): When multiple cables are bundled together in a cable tray or conduit (commonly referred to as “tying them together”), the heat accumulates, so a further derating is required.

Step 3: Initial Selection of Cable Cross-Section Area (Initial Selection)

Take the I_z value calculated in Step 2 and refer to the “Safe Current-Carrying Capacity Table” provided by the cable manufacturer. Locate the corresponding installation method column and select the standard cable cross-section that is greater than and closest to & (e.g., 4, 6, 10, 16, 25, 35 mm², etc.).

Step 4: Voltage Drop Check —— The Step Most Easily Overlooked

Cables themselves have resistance. The longer the distance, the greater the voltage drop along the cable.

Standard requirement: Chinese regulations generally require that the voltage drop at the end of industrial and residential power distribution systems be controlled within 3% to 5%.

Voltage drop ΔU estimation formula for a three-phase balanced line:

If the calculated voltage drop percentage exceeds 5%, unfortunately, even if the cable doesn’t overheat, you must forcibly increase the cable cross-section by one or several sizes until the voltage drop meets the specification.

Step 5: Verify short-circuit thermal stability

In medium- and large-scale projects or when selecting equipment for high-voltage applications, it is essential to verify whether the cable insulation layer will melt instantaneously due to the high temperatures generated during a short-circuit fault before the protective devices operate—in most cases, within a few tenths of a second. The formula for determining the minimum cross-sectional area that meets thermal stability requirements is:

IV. Advanced Selection: How to Choose the Right Cable Material and Model

Choosing the right size is only half the battle—selecting the right material and model is what ensures the project’s long-term stability and durability.

Example of Cable Model Identification (using YJV22 as an example):

1. Copper core (Cu) vs. Aluminum core (Al)

Copper-core cables (such as YJV): low resistivity, high current-carrying capacity, excellent ductility, antioxidant properties, and stable connections. Although expensive, they are the preferred choice for modern buildings and core equipment.

Aluminum-core cables (e.g. YJLV): Highly competitive pricing (about one-third the cost of copper) and lightweight. Suitable for overhead power transmission outdoors, temporary construction power supply, or large-span non-critical external lines where budget sensitivity is extremely high.

2. Insulation materials: PVC vs XLPE

PVC (polyvinyl chloride, such as VV type): Maximum operating temperature 70°C. Inexpensive and gradually being phased out; commonly used in low-end or temporary applications.

XLPE (Cross-linked Polyethylene, such as YJV type): Maximum operating temperature 90°C. It boasts high heat resistance, greater current-carrying capacity, and longer service life, making it the absolute mainstream choice today.

3. Sheathing, Armor, and Special Requirements

Direct burial installation: It is essential to use cables with armor (such as YJV22) to protect against compression from underground soil and, even more importantly, to prevent rodent damage.

High-occupancy venues (shopping malls, high-rise buildings, subways): Low-smoke, halogen-free, flame-retardant cables (such as WDZ-YJV，LSZH) must be specified to ensure that no highly toxic, dense smoke is produced in the event of a fire.

 V. Case Study: Full-Process Calculation of a Typical Factory Project

Step 1: Calculate the current

It is known that p = 30,000  and u= 380. Referring to the motor nameplate, its power factor cosφ is approximately 0.85, and its efficiency η is approximately 90%.

Step 2: Consider the correction factor

According to the table,

when the YJV cable is laid in air at 35°C, the temperature correction factor k₁ is= 0.94;

When multiple cables are laid side by side in cable trays, the installation factor k₂ = 0.85.

Step 3: Preliminary Section

Refer to the current-carrying capacity table for YJV-0.6/1kV three-core cables:

A 10 mm² copper-core cable has a current-carrying capacity of approximately 65 A in air (less than 74.6 A, thus eliminated).

The current-carrying capacity of a 16 mm² copper-core cable in air is approximately 85 A (which is greater than 74.6 A, confirming the initial selection as successful).

Step 4: Verify the voltage drop

According to the table, the AC resistance of a 16 mm² copper cable is approximately 1.15 Ω/km, and the inductive reactance can be neglected. The line length L = 0.1 km.

Finalize the design

For this project, the final selection is: YJV-0.6/1kV 3×16+ 1×10 (or 3×16+2×10) copper-core cable.

VI. Pitfall Guide: 4 Common Mistakes Made by Engineering Staff

❌ Misconception 1: Looking at Power but Ignoring Distance

“Back in the day, a 30kW device worked fine with a 16-square-millimeter cable!”—but that was only because the distance was short. If you extend the distance to 500 meters, a 16-square-millimeter cable will suffer from excessive voltage drop, making it impossible for the device to even start up—and worse yet, all that electrical energy could be wasted as heat, burning out the cable itself.

❌ Misconception 2: Ignoring Future Expansion

In the early stages, to save money, you rigidly specify cable sizes without considering future expansion plans. Two years into the project, when production needs to be increased, you realize that the cables have become the biggest bottleneck, forcing you to tear up the ground again for a second round of construction—resulting in a net loss. It’s recommended to leave a capacity margin of 15% to 20% for core trunk lines.

❌ Misconception 3:Ignoring Tray Fill Ratio

Ignoring the fill rate of cable trays—stuffing dozens of cables tightly and densely into a closed tray, leaving barely enough space even for a single palm to fit in. This is tantamount to “covering the cables with a huge quilt,” preventing heat from dissipating and making the interior highly susceptible to localized aging and breakdown.

❌ Misconception 4:Buying “Low-price Non-standard” Cables

Purchasing “low-priced, non-standard” cables. Some unscrupulous manufacturers on the market use copper-clad aluminum or reduce the actual cross-sectional area of the copper core (e.g., a 10-square-millimeter cable may actually have only 8.5 square millimeters). Before using scientific formulas for calculation, it is essential to ensure that the cable itself complies with national standards (GB) quality specifications.

VII. Summary

Selecting the right cable size scientifically is a process of “precise customization tailored to specific needs.” By keeping in mind the closed-loop procedure—calculating current, checking corrections, matching cross-sections, and verifying voltage drops—you not only ensure your project smoothly passes acceptance tests and operates safely for decades, but also save real money for your project team during bidding and audits by presenting clear and transparent calculations.