Derating Factors of Cables: How to Calculate Safe Ampacity

When you’re sizing electrical cables, it’s not enough to just pick a cross-section from a manufacturer’s catalog. Every cable has a base current-carrying capacity (ampacity), but that number assumes perfect reference conditions: mild temperatures, well-ventilated installation, and no other cables nearby.

In reality, cables are laid in different ways—direct in the ground, in ducts, or in free air—and each method affects how much heat the cable can safely dissipate. This is where derating factors come in. By applying them, you adjust the nominal ampacity to reflect real-world conditions and ensure safety, reliability, and compliance with IEC standards.

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Calculating Base Current (Ampacity)

The starting point is always the manufacturer’s tables. These tables give the maximum continuous current a cable can carry under standard reference conditions. You’ll usually see three main scenarios:

• Laid in free air: Cables installed on ladders, trays, or cleats. These have the best cooling and therefore the highest ampacity.
• Laid in ducts: Cables pulled through conduits or ducts in air. The cooling is reduced compared to free air, so ampacity is lower.
• Laid direct in ground: Buried cables surrounded by soil. Here the soil’s thermal properties and temperature play a big role.

Once you have the base ampacity for your installation method, you move to the next step: applying derating factors.

0.6/1 (1.2) kV Multi-core Cables, Stranded Copper Conductors, PVC Insulation, PVC Sheathed

Two-Core Cables

Table 1

Three-Core Cables

Table 2

ACO Code	Nominal Cross Section (mm²)	Max. Conductor Resistance at 20 °C (Ω/km)	Voltage Drop (mV/A/m)	Current Rating (A) Laid in Ground	Current Rating (A) Laid in Duct	Current Rating (A) Laid in Free Air	Approx. Diameter (mm)	Approx. Weight (kg/km)
CX1-T103/U04	1.5	12.1	29	21	21	21	12.1	181
CX1-T103/U06	2.5	7.41	18	28	27	27	13.2	234
CX1-T103/U08	4	4.61	11	37	36	36	14.4	332
CX1-T103/U10	6	3.08	7.3	48	45	45	15.8	432
CX1-T103/U12	10	1.83	4.4	66	61	61	19.4	700
CX1-T103/U17	16	1.15	2.8	87	79	79	22.0	1020
CX1-T103/U19	25	0.727	1.8	115	103	103	25.0	1500
CX1-T103/U23	35	0.524	1.3	144	128	128	28.0	1700

Four-Core Cables

Table 3

ACO Code	Nominal Cross Section (mm²)	Max. Conductor Resistance at 20 °C (Ω/km)	Voltage Drop (mV/A/m)	Current Rating (A) Laid in Ground	Current Rating (A) Laid in Duct	Current Rating (A) Laid in Free Air	Approx. Diameter (mm)	Approx. Weight (kg/km)
CX1-T104/U04	1.5	12.1	29	26	23	22	13.0	218
CX1-T104/U06	2.5	7.41	18	35	26	32	14.1	275
CX1-T104/U08	4	4.61	11	45	36	41	15.5	367
CX1-T104/U10	6	3.08	7.3	57	45	50	17.0	474
CX1-T104/U12	10	1.83	4.4	75	60	68	21.2	754
CX1-T104/U17	16	1.15	2.8	97	75	89	23.7	1105
CX1-T104/U19	25	0.727	1.8	128	102	120	27.0	1705
CX1-T104/U23	35	0.524	1.3	155	120	145	30.0	2210
CX1-T104/U27	50	0.387	1.0	185	145	179	33.0	2860
CX1-T104/U29	70	0.268	0.7	220	180	225	36.5	4070
CX1-T104/U33	95	0.193	0.5	265	210	268	40.0	5145
CX1-T104/U37	120	0.153	0.4	305	245	310	43.5	6370
CX1-T104/U39	150	0.124	0.35	335	275	352	47.0	7890
CX1-T104/U41	185	0.099	0.30	375	310	404	51.0	9795
CX1-T104/U45	240	0.075	0.25	435	365	483	56.0	10190

CX1-T104/U45

300

0.0601

0.061

490

405

562

60

12460

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What Are Derating Factors?

A derating factor is simply a multiplier applied to the base ampacity to adjust for conditions that make the cable hotter. For example, if a cable is rated at 100 A in free air but your site has a higher ambient temperature, you may need to multiply by 0.87. The new safe ampacity becomes 87 A.

In formula form:

Final Ampacity = Base Ampacity × (Product of Derating Factors)

Derating factors ensure your cables don’t overheat, insulation doesn’t degrade prematurely, and protective devices operate correctly.

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Types of Derating Factors According to IEC

IEC standards define several correction factors that reflect real site conditions. Here are the most important ones you’ll encounter:

1. Ambient Temperature (Air Installations)

Cables in free air or ducts are rated at a reference of about 35 °C. If the actual ambient is higher, the ampacity must be reduced. For example, at 40–50 °C, derating becomes significant, especially for PVC insulation.

Table 4

Ambient air temp (°C)	55	50	45	40	35	30	25
PVC	0.65	0.76	0.85	0.93	1.00	1.07	1.13
XLPE	0.80	0.85	0.90	0.95	1.00	1.04	1.09

2. Ground Temperature (Buried Installations)

When cables are buried, the reference soil temperature is usually 20 °C. Warmer soil, such as in hot climates, reduces heat dissipation, so you apply a correction factor.

Table 5

Soil temp (°C)	55	50	45	40	35	30	25
PVC	0.71	0.89	0.95	1.00	1.08	1.15	1.22
XLPE	0.84	0.89	0.90	1.00	0.90	1.10	1.14

3. Depth of Burial

Deeper burial often means less airflow and slower heat dissipation. Cables buried at 1.5 m may have a slightly lower ampacity compared to those at 0.8–1.0 m.

Table 6

Depth of Burial (cm)	Up to 70 mm²	Up to 240 mm²	Above 300 mm²
50	1.00	1.00	1.00
60	0.99	0.98	0.97
80	0.97	0.96	0.94
100	0.95	0.93	0.92
125	0.94	0.92	0.89
150	0.93	0.90	0.87
175	0.92	0.89	0.86
200	0.91	0.88	0.85

4. Soil Thermal Resistivity

Soil type is critical. Wet or compact soil conducts heat well, while dry or sandy soil resists heat transfer. IEC uses a reference resistivity of 2.5 K·m/W. Lower values (wet soil) give you a bonus factor >1, while higher values (dry soil) reduce ampacity.

Table 7

Soil Thermal Resistivity (°C·cm/W)	90	100	120	150	200	250
Correction Factor	1.17	1.12	1.07	1.02	0.91	0.80

5. Grouping Factors in Air (Horizontal vs. Vertical)

When multiple circuits run together, they heat each other. IEC distinguishes between arrangements:

• Horizontal touching: The worst case, as cables are side-by-side with limited airflow.
• Vertical single layer: Better cooling, so the derating factor is less severe.

Table 8

Number of Loaded Circuits on the Support	2	3	4	5–6	More than 9
Horizontal Grouping Factor	0.85	0.78	0.75	0.72	0.70
Vertical Grouping Factor	0.80	0.73	0.70	0.68	0.66

6. Grouping Factors Underground

Buried circuits are also affected by how close they are:

• Touching cables: Strong mutual heating, lowest factor.
• 15 cm spacing: Improved cooling, factor increases.
• 30 cm spacing: Almost like single circuits, factor close to 1.

Table 9

No. of Circuits	Spacing 30 cm (Trefoil)	Spacing 30 cm (Flat)	Spacing 15 cm (Trefoil)	Spacing 15 cm (Flat)	Touching (Trefoil)	Touching (Flat)
2	0.91	0.91	0.87	0.87	0.81	0.81
3	0.82	0.84	0.76	0.78	0.69	0.70
4	0.77	0.81	0.72	0.74	0.64	0.65
5	0.73	0.78	0.68	0.70	0.60	0.61
6	0.70	0.76	0.66	0.67	0.54	0.56

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Putting It All Together

The process is straightforward once you understand the logic:

1. Look up base ampacity from the correct installation method (air, duct, ground).
2. Check the conditions at your site: actual ambient, soil type, burial depth, grouping.
3. Select the correction factors from IEC tables for each condition.
4. Multiply them together with the base ampacity to get the final safe current.

For example, a cable rated 200 A in ground might be installed in 35 °C soil with thermal resistivity of 3.0 K·m/W and in a trench with three touching circuits. After multiplying the factors, the final ampacity could fall to around 140 A.

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Examples:

Example 1

Question:
What is the maximum current that a PVC cable with a cross-sectional area of 4 core cable of 95 mm² can carry if the ambient air temperature is 50 °C?

Solution:
From Table 3, we find that the natural (reference) ampacity of a 95 mm² cable in air at 20 °C is 268 A.

From Table 4, the correction factor for 50 °C ambient is 0.76.

Therefore, the corrected Thermal Rating for this cable at 50 °C is: 268 x 0.76 = 203 A

So the cable can safely carry 203 A, not 268 A as shown in the base table.

Example 2

Question:
Calculate the maximum current capacity of a PVC cable, 240 mm², buried in soil with a temperature of 50 °C at a depth of 80 cm, next to two other cables.

Solution:
From table 5, The correction factor for soil temperature = 0.89
From table 6, The correction factor for burial depth = 0.96
From table 7, The correction factor for grouping of cables (assuming touching cables) = 0.70
From table 3, the natural (reference) current capacity of a 240 mm² PVC cable = 435 A

Therefore, the corrected thermal rating of this cable under the given conditions is: 435 x 0.70 x 0.96 x 0.89 = 260 A

So, The maximum current the 240 mm² PVC cable can carry under the given conditions is 260 A.

Example 3

Question:
We need to lay 18 cables in the two arrangements shown in the following figure. Compare the two methods and find the correction factor for each case.

Solution:
Case 1: Cables arranged in two horizontal groups (as in left figure):
From table 8
Correction factor for 9 cables horizontally = 0.70
Correction factor for 2 layers = 0.80
Therefore, the total correction factor = 0.7 x .8 = 0.56

Case 2: Cables arranged in two vertical groups (as in left figure):
From table 8
Correction factor for 9 cables vertically= 0.66
Correction factor for 2 layers = 0.85
Therefore, the total correction factor = 0.66 x 0.85 = 0.56

Conclusion:
There is no difference between the two arrangements. In both cases, the cables must be derated to 56% of their natural ampacity.

Example 4

Question:
It is required to select a PVC cable suitable for carrying a load current of 300 A (Actual Ampacity), given that the cable will be buried in soil with a temperature of 50 °C, at a depth of 80 cm, and installed alongside two other cables.

Solution:
From table 5 Soil temperature correction factor = 0.82
From table 6 Depth of burial correction factor = 0.96
From table 9 Grouping correction factor (assuming touching cables) = 0.70

Therefore, the rated current of the selected cable can be calculated as:
I_cable = 300 / (0.82 x 0.96 x 0.70) = 501 A

Since a single cable cannot carry this current, the load must be split over two parallel cables. In that case, the number of cables in the trench becomes Four (instead of three).

From Table 9, the grouping factor for 6 touching cables = 0.63
Thus, the required cable rating is recalculated:
I_cable = 300 / (0.82 x 0.96 x 0.63) = 557 A

This means we need to select two parallel cables, each carrying:
557 / 2 = 279 A

From Table 2, we find that a 120 mm² PVC cable has a natural current capacity of approximately 280 A.

Therefore, the solution is to use:
2×(3×120+70)mm²

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Why Derating Factors Matter

Ignoring derating can lead to undersized cables, overheating, nuisance tripping, or even fire hazards. By applying IEC correction factors, you design installations that:

• Extend cable life
• Improve energy efficiency
• Guarantee safety and compliance
• Prevent costly downtime

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Final Thoughts

Derating isn’t just a calculation—it’s about translating lab conditions into field reality. By combining base ampacity tables with the correct IEC derating factors for temperature, soil, depth, and grouping, you ensure that your cables are sized to handle real-world conditions safely.

If you’re publishing these methods on your site, you can enrich the article by adding tables of factors and worked examples for your audience. This helps engineers, electricians, and contractors quickly see how theoretical numbers become practical designs.