Power losses in LV cables and how to control them

1. What are power losses?

Power losses in an LV distribution system are the electrical energy dissipated as heat during transmission from the transformer substation to the connected loads. This phenomenon cannot be fully eliminated, but it can be controlled within design limits.

Power losses fall into two categories: technical losses and non-technical losses (commercial losses). Technical losses arise from the resistance and reactance characteristics of the cable run, governed by the power loss formula ΔP = I² × R. Non-technical losses include metering equipment errors, unauthorized connections, and operational management gaps.

2. Causes of power losses in LV cables

The equation ΔP = I² × R points to two primary ways to reduce power losses: lower the cable run's resistance R, or reduce the current I flowing through the cable. For LV cable runs, losses can be influenced by six distinct groups of causes. Each group operates through its own mechanism and when multiple factors occur simultaneously, power losses can increase substantially.

• Resistance R and Reactance X exceeding design limits: The conductor has resistance R due to the opposition it presents to electron flow in the metal. Reactance X arises from the magnetic field that builds around the conductor when alternating current flows through it. Together, these quantities produce impedance Z = √(R² + X²), which directly affects power losses along the cable run. The longer the run, the higher R and X become — with greater impact on losses and transmission efficiency.
• Undersized conductor cross-sectional area relative to actual load: When cross-sectional area S is smaller than required, conductor resistance increases according to the formula R = ρ × L / S. Halving the cross-sectional area, for example, doubles the resistance and can increase I²R power losses fourfold. This situation commonly occurs when cable selection is driven primarily by material cost without fully accounting for current carrying capacity.
• Poor contact at joints and cable lugs: Every joint or cable lug that is not installed to standard can introduce additional contact resistance R_tx. This resistance concentrates heat at the contact point and over time, can accelerate aging of the surrounding XLPE/PVC insulation and increase the risk of failure.
• Phase load imbalance: When single-phase loads are distributed unevenly across phases A, B, and C, current in the neutral conductor can increase. This generates additional losses on the neutral core of 4-core cables and causes voltage imbalance between phases, which can affect the operation of certain electrical equipment.
• Low power factor (cosφ): Inductive loads such as motors, transformers, and magnetic ballast lighting require reactive power, which forces current to rise in order to deliver the same useful power output. According to the formula I = P / (√3 × U × cosφ), when cosφ decreases, current I increases. Higher current means greater I² × R losses along the cable run.
• Cable run length exceeding the appropriate service range: In a 0.4 kV distribution system, an excessively long cable run can increase voltage drop at the end of the line. Extending the run without increasing conductor cross-sectional area or raising the distribution voltage can cause voltage drop ΔU to exceed the permissible limit, resulting in end-of-line voltage that does not meet operating requirements.

3. Technical impacts on M&E systems

Power losses in LV cables produce several technical effects on M&E systems:

• End-of-Line voltage drop: This is one of the most visible signs of an overloaded or incorrectly designed cable system. When voltage at the end of the run falls below the required level, motors may need to draw more current to maintain torque. Heat then accumulates in the windings, reducing motor efficiency and shortening its service life.
• Cable service life and operating temperature: When conductors operate at elevated temperatures over an extended period, insulation ages faster. PVC-sheathed cable, rated for continuous operating temperatures of approximately 70°C, is therefore more susceptible to early degradation than XLPE-sheathed cable, rated at approximately 90°C, when both operate under high-load conditions.
• Heat buildup and fire risk at joints and cable lugs with high contact resistance (R_tx): Poor contact concentrates heat in a small area. If this condition persists, temperatures can rise high enough to degrade or damage the surrounding insulation and significantly increase the risk of failure.

4. Engineering solutions to minimize power losses in LV cables

Voltage drop, thermal aging, and fire risk each require simultaneous intervention at three levels: network design, material selection, and operations and maintenance procedures.

• Three-phase load balancing during the design phase distributes loads evenly across phases, reducing neutral current and losses on 4-core cables. For large loads, 3-phase supply typically reduces per-phase current, improves voltage drop, and lowers power losses compared to single-phase supply.
• Conductor cross-sectional area selection must be based on calculated current, installation conditions, ambient temperature, and the applicable correction factors per the relevant standard. An appropriate capacity margin for future load growth is recommended, but it should be determined from project-specific data rather than a fixed percentage.
• Prioritizing materials with strong thermal and electrical characteristics reduces losses. High-quality copper conductors have low resistivity, and XLPE withstands heat better than PVC — making it generally more suitable for continuous loads or high-temperature environments.
• Installing power factor correction capacitors at the distribution panel reduces reactive current on the cable. When the power factor improves, total current decreases, which in turn reduces losses and lightens the load on supply equipment.
• Periodic maintenance using infrared thermal cameras enables early detection of hot spots at cable lugs, joints, busbars, and switchgear. Inspection intervals should be determined based on system criticality and operational risk, not by applying a fixed schedule to every project.

5. Summary and technical recommendations

This analysis has clarified four key areas that M&E engineers need to understand in order to control power losses in LV cables:

• The nature of power losses depends primarily on current I and resistance R according to the formula ΔP = I² × R. When either I or R increases, losses increase substantially.
• The six primary causes of losses are: cable run resistance and reactance, undersized cross-sectional area, joints with high contact resistance R_tx, phase load imbalance, low cosφ, and excessive cable run length.
• The consequences of power losses include end-of-line voltage drop, heat buildup, accelerated insulation degradation, and increased risk of failure at joints.
• The control strategy requires a combined approach — selecting the correct cross-sectional area, balancing three-phase loads, improving cosφ, ensuring joint quality, and implementing periodic maintenance — rather than applying any single measure in isolation.

In addition, compliance with technical standards such as TCVN 6610 and IEC 60502-1 helps control conductor quality and resistance throughout the system's service life. Contact Ngoc Lan Cable today for solutions tailored to your project.