Current Carrying Capacity Of Cables: A Quick Guide

Introduction

Overheating of cables is a major safety concern in electrical projects. Projects often overlook derating factors or the margin of safety to save cost, which can be disastrous for the safety of an electrical system.

I have spoken to hundreds of electrical consultants and architects. They wanted to understand the factors affecting the current rating of a cable. They also felt that more designers and clients need to understand the importance of derating factors.

I couldn't find an article that explained these topics in a simple manner. So, I wrote this quick guide for anyone wanting to understand cable rating factors.

It is important to understand the core principles of current carrying capacity:

• How much heat is generated in the cable?
• How quickly can the heat dissipate?
• How much heat can the insulation bear without melting?

The idea is to reduce the heat generated and increase the heat dissipation, which ensures longer-lasting insulation.

Based on this principle, some key factors affect the current carrying capacity of electrical cables. I'm listing them below:

Factors Impacting Current Carrying Capacity Of Electrical Cables

Material Factors (Conductor and Insulation)

One of the most critical factors is the conductor. The bigger the conductor, the more current it can handle, as the resistance is lower. Lower resistance translates to lower heat generated. The material of the conductor is also important. Copper has lower resistivity than aluminium. Hence, copper would generate lower levels of heat for conductors of the same size.

Then there's the insulation. While thicker insulation offers better protection, it can also limit heat dissipation, which impacts current carrying capacity. The type of insulation matters, too. For example, XLPE insulation handles heat better than PVC, allowing XLPE to support higher current.

Environmental Factors

Beyond the material factors, where we install the cable makes a big difference as well:

• Ambient Temperature: Higher temperatures can reduce a cable's capacity as the insulation’s base temperature is already high. As a result, it requires less heat to reach its melting point. Higher temperatures also increase the conductor’s resistance, which generates more heat.

• Installation Method: Installation in areas where the possibility of heat dissipation is higher will allow for a higher current carrying capacity compared to an area where the possibility of dissipation is lower. This is the reason for the difference in current carrying capacity in conduits, cable trays, ground, and air.

• Grouping and Spacing: More heat is generated when cables are packed close together, which can lower their capacity. Proper spacing helps prevent overheating and keeps everything running smoothly.

Understanding Derating of Cables

To accommodate these installation factors, any cable rating the manufacturer provides must be derated to determine the current the cable can withstand in the expected installation conditions.

Derating is a critical step; ignoring it can often lead to system failure. Manufacturers provide derating factors based on various conditions, which must be applied to calculate the current carrying capacity.

For instance, if a cable is rated at 100 amps but we factor in temperature with a derating factor of 0.85 and grouping with a derating factor of 0.9, its effective capacity drops to 100*0.85*0.9, i.e. 76.5 amps.

Additional Considerations

There are a few other things to keep in mind:

• Short-circuit current: Cables must handle high currents during short circuits, at least for the time it takes the protection systems to act.
• Harmonics: Harmonic currents can create extra heat, affecting capacity.
• Margin of Safety: No matter how well you design a system, you cannot foresee change in its dynamics in the future. Thus, you must maintain a margin of safety over and above all the other factors that we account for.

Conclusion: To Wrap Up

By considering these factors, you can select the right cables for your projects that optimize performance and safety.

I hope this article helped you understand the factors affecting the current carrying capacity of electrical cables.

If you'd like me to write about any other topic related to electrical cable design or electrical cables, please leave a message in the comments.