Current-Carrying Capacity (CCC) refers to the maximum amount of electrical current a cable can safely carry without exceeding its temperature limits or causing damage. It is an important factor in electrical installations to ensure safety, efficiency, and longevity of the cables used.

There are nine main factors that influence the ampacity (CCC) of cables, either increasing or decreasing it.

1. Ambient Temperature

The CCC of a cable is affected by the temperature difference between the surrounding environment and the cable’s maximum temperature limit.

Therefore, cables installed in cooler environments can carry more current than those in hotter areas.

Each country has different standard ambient temperatures for cable rating, which can be found in IEC 60287-3-1 for specific regions.

2. Burial Depth

The depth at which a cable is buried impacts its ability to dissipate heat, making it a significant determinant of current rating. Cables buried deeper tend to get hotter, necessitating a lower rating.

For low-voltage cables, a common burial depth is 0.5 meters (or 0.7 meters according to BS 7671).

3. Soil Thermal Resistivity

For buried cables, one of the primary factors affecting current ratings is soil thermal resistivity, which varies based on soil composition, moisture content, seasonal weather patterns, and the heat generated by the cable load.

Typical soil thermal resistivity is 1.2 K.m/W but can range from 0.8 K.m/W in clay or peat to around 2.5 K.m/W in dry, well-drained soil under heavy cable load.

4. Cable Load Variation

Cable standards for low-voltage systems typically assume continuous loading. If the actual load is intermittent, the cable’s current-carrying capacity can be increased.

5. Thermal Insulation

The presence of thermal insulation around the cable restricts its ability to dissipate heat, which reduces its current-carrying capacity.

6. Direct Sunlight Exposure

Exposure to direct sunlight significantly affects current ratings, with the general rule being that sunlight can increase the cable temperature by up to 20 degrees Celsius.

7. Third Harmonic Currents

The presence of third harmonic currents causes additional joule losses in the cable, leading to increased heating and a reduced current rating.

8. Grouping of Parallel Circuits

When multiple cables are placed close together, whether in the air (on cable trays) or buried underground, they generate heat collectively.

This mutual heating must be considered with a derating factor for circuit grouping.

9. Enclosures

Installing cables inside enclosures results in extra heat buildup, effectively lowering the current rating.

If multiple cables are placed in the same enclosure, the derating effect becomes more pronounced.

When Can Derating Factors Be Avoided?

Interestingly, there are situations where CCC derating factors do not need to be applied. The AS/NZS 3008.1 standard provides examples, including:

(a) Mineral-insulated metal-sheathed (MIMS) cables without other nearby cables or enclosures.
(b) Limited grouping lengths not exceeding:
(i) 1 meter for conductor sizes < 300 mm² (aluminum) or < 150 mm² (copper);
(ii) 3 meters for conductor sizes ≥ 300 mm² (aluminum) or ≥ 150 mm² (copper);
(iii) Half the cable length.
(c) Cables operating at less than 35% of their rated capacity.

It is worth noting that the current rating tables for all mentioned low-voltage cable standards are derived using the standard IEC 60287 methods.

Therefore, ratings and derating factors across different standards can generally be used interchangeably, but assumptions about standard conditions should be handled with care.

For example, the standard ambient temperature in Australia is higher than in the United Kingdom.

For expert consultation, reach out to PT Media Kontrol Utama. Our experienced professionals are ready to help you maintain the reliability and safety of your electrical distribution system.

Reference :

• Current-Carrying Capacity (CCC)
  Electrotechnik Australia. Accessed 19 October 2024.