Why Correct Derating Factors Are Essential in Cable Sizing Calculations?

When engineers and electricians size cables, they often start by looking up the manufacturer’s current-carrying capacity charts. These charts are a useful starting point—but they’re based on ideal reference conditions. In the real world, cables rarely operate in those perfect conditions. This is where derating factors come in.

Derating factors are multipliers applied to the cable’s rated capacity to account for installation and environmental conditions that reduce the cable’s ability to carry current safely. Using the correct derating factors is not just a matter of best practice—it’s critical for safety, compliance, performance, and cost control.

1. Safety Comes First

Cables generate heat as current flows through them. In ideal laboratory conditions, the heat dissipates into the surrounding air or ground easily. But when cables are bundled together, installed in conduits, run in hot climates, or buried in soil with poor thermal properties, the heat has fewer escape paths.

Without correct derating, the chosen cable may be undersized for these real-world conditions. That leads to excessive operating temperatures, which can cause:

• Insulation degradation over time, leading to short circuits or ground faults.

• Fire hazards if the insulation fails catastrophically.

• Injury or damage to equipment connected to the overloaded cable.

By applying accurate derating factors, you ensure the cable will run within its safe temperature range, greatly reducing these risks.

2. Compliance With Electrical Standards

Industry standards such as IEC 60364, BS 7671, and the National Electrical Code (NEC) mandate that derating factors be applied whenever installation conditions differ from reference values. These conditions include:

• Ambient temperature higher or lower than standard test conditions (often 30 °C for air, 20 °C for ground).

• Grouping of cables, where multiple circuits run close together and heat each other.

• Installation methods, such as in conduit, buried directly, in cable trays, or suspended.

• Soil thermal resistivity for underground cables.

Ignoring these requirements means your installation is non-compliant, and it may fail inspection, invalidate warranties, or even result in legal liability if a fault occurs.

3. Long-Term Reliability

Cable failure is rarely instantaneous—it’s often the result of years of overheating that accelerate insulation aging. A cable consistently running above its design temperature will have a much shorter service life.

For example, a PVC-insulated cable rated for 70 °C might last decades when operated correctly, but if it’s routinely run at 90 °C because of poor sizing and no derating, its lifespan could be cut dramatically. Proper derating ensures that cables will perform reliably over their intended life span, avoiding premature replacement and costly downtime.

4. Avoiding Oversizing and Wasted Cost

Some installers, fearful of undersizing, choose to oversize cables dramatically. While this avoids overheating, it increases project costs unnecessarily—not only in the extra copper or aluminium, but also in larger conduits, trays, and supports.

Applying the correct derating factors lets you choose the optimal cable size: big enough to handle the load safely under actual conditions, but not so oversized that it wastes money and space.

5. A Quick Example

Imagine you have a cable rated for 100 A in free air at 30 °C. If the ambient temperature is actually 45 °C, and the cable is run in a group of three, two derating factors apply:

• Temperature factor ≈ 0.87 → 100 A × 0.87 = 87 A.

• Grouping factor ≈ 0.9 → 87 A × 0.9 = 78.3 A.

Your cable can now only carry 78 A safely. Without derating, you might load it to 100 A—risking serious overheating.

Final Thoughts

Derating factors bridge the gap between textbook cable ratings and the realities of an electrical installation. They protect people, property, and equipment by ensuring cables are correctly sized for the actual conditions they will face.

Ignoring them is a gamble with high stakes—ranging from costly downtime to catastrophic failure. Applying them correctly means your installation will be safe, reliable, efficient, and compliant. In short, derating factors aren’t optional—they’re essential.