Cable Derating Factors: Ambient Temperature, Grouping and Installation Conditions-Instrumentation Cable, Thermocouple, RTD Sensor & Pressure Transmitter Manufacturer

— A Practical Guide for Engineers, EPCs, and Project Teams

Cable current-carrying capacity tables published in IEC 60364-5-52, IEC 60502, and other standards are calculated under specific reference conditions: a defined ambient temperature, a single circuit in isolation, and—for buried cables—a defined soil thermal resistivity and burial depth. Real installations almost never match these reference conditions exactly. Cables are grouped in trays, run through hot process areas, buried in dry soil, or installed at depths greater than the reference.-4 -26

When installation conditions are less favourable than the reference, the cable must be derated—its tabulated current rating reduced—to prevent overheating, insulation degradation, and premature failure. This guide explains the derating factors that engineers must apply, how they compound, and the practical consequences of getting them wrong.-8

1. The Derating Equation

The effective current-carrying capacity of a cable under actual installation conditions is:

$I_{z} = I_{t a b} \times C_{a} \times C_{g} \times C_{s} \times C_{d} \times C_{i}$

Where:

Symbol	Factor	Applies When
$I_{z}$	Effective (derated) current rating (A)	—
$I_{t a b}$	Tabulated rating at reference conditions (A)	—
$C_{a}$	Ambient temperature correction factor	Air or ground temperature ≠ reference
$C_{g}$	Grouping (mutual heating) factor	Multiple loaded circuits in proximity
$C_{s}$	Soil thermal resistivity factor	Buried cables; soil resistivity ≠ reference
$C_{d}$	Depth of burial factor	Buried cables; depth ≠ reference
$C_{i}$	Thermal insulation factor	Cable in contact with building insulation

The critical point: these factors multiply. Two factors of 0.80 each produce a combined factor of 0.64—not 0.60. Individually modest derating can compound to require a substantially larger cable.-26

The cable must be selected such that its tabulated rating meets or exceeds the required value:

$I_{t a b} \geq \frac{I_{n}}{C_{a} \times C_{g} \times C_{s} \times C_{d} \times C_{i}}$

Where $I_{n}$ is the protective device rating.-8

2. Ambient Temperature Correction Factor ( $C_{a}$ )

Why it matters: A cable's current rating is limited by its maximum conductor operating temperature—typically 70°C for PVC insulation and 90°C for XLPE. If the ambient temperature is higher than the reference, there is less temperature “headroom” for the cable's own I²R heating, so it must carry less current.-26

2.1 Reference Temperatures by Standard

Standard	Reference Ambient (Air)	Reference Ambient (Ground)
IEC 60364-5-52	30°C	20°C
BS 7671	30°C	20°C
AS/NZS 3008	40°C	25°C
NEC	30°C	—

Source: ECalPro comparison of reference conditions.-8

2.2 IEC 60364-5-52 Table B.52.14 (Air)

Ambient Temp (°C)	PVC (70°C max)	XLPE (90°C max)
10	1.22	1.15
20	1.12	1.08
30	1.00	1.00
40	0.87	0.91
50	0.71	0.82
60	0.50	0.71

Source: IEC 60364-5-52 Table B.52.14.-4

Interpretation: At 50°C ambient, a PVC-insulated cable retains only 71% of its 30°C rating. A cable rated 100 A in the table delivers only 71 A in practice. XLPE, with its higher maximum conductor temperature, retains 82%—but still loses nearly one-fifth of its capacity.

2.3 The Underlying Formula

The ambient temperature factor is derived from the allowable temperature rise:

$C_{a} = \sqrt{\frac{T_{m a x} - T_{a c t u a l}}{T_{m a x} - T_{r e f e r e n c e}}}$

Where $T_{m a x}$ is the maximum conductor temperature (70°C for PVC, 90°C for XLPE), $T_{a c t u a l}$ is the actual ambient temperature, and $T_{r e f e r e n c e}$ is the reference ambient temperature (30°C air, 20°C ground).-8

Engineering consequence: The square-root relationship means that derating accelerates as ambient temperature approaches the insulation limit. A cable that appears adequately sized at 30°C may be severely overloaded at 55°C.

3. Grouping Correction Factor ( $C_{g}$ )

Why it matters: When multiple loaded cables share the same route—bunched in conduit, stacked in a tray, or buried side by side—they heat each other. The mutual heating reduces each cable's ability to dissipate its own heat, so the group must be derated.-4

3.1 Grouping Tables in IEC 60364-5-52

IEC 60364-5-52 provides four separate grouping tables depending on the installation arrangement:

Table	Arrangement
B.52.17	Cables bunched in conduit or enclosed in trunking
B.52.18	Single layer on a wall or floor (cables touching)
B.52.19	Single layer on a perforated or solid tray
B.52.20	Buried cables — multiple circuits

Source: ECalPro.-4

3.2 Illustrative Grouping Factors

For cables bunched in air (touching, on a surface, embedded, or enclosed), typical factors are:

Number of Circuits	Correction Factor
1	1.00
2	0.80
3	0.70
4	0.65
5	0.60
6	0.57

Source: IEC 60364-5-52 Table B.52.17, as reproduced in multiple references.

Interpretation: Six circuits bunched together retain only 57% of their individual rating. A cable that would carry 100 A alone carries only 57 A when grouped with five others. This is one of the most consequential derating factors in industrial installations, where cable trays are routinely densely packed.

3.3 Important Caveats

Grouping factors apply only to loaded circuits. A cable carrying no current does not contribute to mutual heating and should not be counted.
Spacing matters. Cables with spacing between them derate less severely than cables in direct contact. The tables distinguish between touching and spaced arrangements.
Buried cables have separate grouping tables. Direct-buried circuits derate differently from cables in air.

4. Soil Thermal Resistivity Factor ( $C_{s}$ )

Why it matters: For buried cables, the soil is the heat sink. Soil thermal resistivity (measured in K·m/W) determines how effectively heat is conducted away from the cable. Dry, sandy, or rocky soil has high resistivity (poor thermal conductivity) and requires more derating than moist, loamy soil.

4.1 Reference Conditions

Standard / Source	Reference Soil Thermal Resistivity
IEC 60502-2	2.5 K·m/W (some tables) or 1.5 K·m/W (others)
Prysmian MV rating	1.5 K·m/W
ABB	2.5 K·m/W

Caution: The reference value varies between standards and manufacturers. Always verify which reference applies to the specific tabulated rating being used.-21

4.2 Illustrative Soil Resistivity Factors (Direct Buried)

For a reference resistivity of 1.5 K·m/W, typical factors for single-core cables are:

Soil Resistivity (K·m/W)	Factor (16 mm²)	Factor (50 mm²)
0.7	1.29	1.32
1.0	1.15	1.16
1.5	1.00	1.00
2.5	0.82	0.81
3.0	0.75	0.75

Source: Prysmian MV cable rating factors.-21

Interpretation: At 2.5 K·m/W (a common value for dry soil), the cable retains approximately 81–82% of its reference rating. At 3.0 K·m/W (very dry or rocky soil), it retains only 75%.

Best practice: For critical cables, use the worst-case (driest) soil condition unless the cable is installed in selected backfill with known thermal properties.

5. Depth of Burial Factor ( $C_{d}$ )

Why it matters: The reference burial depth for IEC 60502-2 and many manufacturer tables is 0.8 m. At greater depths, the thermal resistance of the soil path to the surface increases, reducing heat dissipation and requiring derating.-21

5.1 Illustrative Depth Factors

Depth of Laying (m)	Single-Core (<185 mm²)	Three-Core
0.5	1.04	1.04
0.8	1.00	1.00
1.0	0.98	0.98
1.5	0.95	0.95
2.0	0.93	0.93
3.0	0.90	0.90

Source: Prysmian MV rating factors, IEC 60502-2.-21

Interpretation: At 2.0 m depth, the cable retains 93% of its 0.8 m rating. The effect is less severe than temperature or grouping, but it still compounds with the other factors.

6. Thermal Insulation Factor ( $C_{i}$ )

When a cable is in direct contact with building thermal insulation, its heat dissipation is severely restricted. IEC 60364-5-52 provides a factor (Table B.52.21) for this condition. In practice, this factor is often 0.5 or lower—a cable embedded in insulation may carry only half its free-air rating. This is a common cause of overheating in roof voids and insulated walls.-4

7. How the Factors Compound: A Worked Example

Consider a PVC-insulated, 4 mm², 3-core cable with a tabulated rating of 32 A at 30°C in free air (single circuit, no grouping).

Actual installation conditions:

Ambient temperature: 45°C (hot process area) → $C_{a} = 0.79$
Grouped with 3 other circuits in a tray → $C_{g} = 0.65$
No soil factors (air installation) → $C_{s} = 1.00$ , $C_{d} = 1.00$
Not in contact with insulation → $C_{i} = 1.00$

$I_{z} = 32 \times 0.79 \times 0.65 = 32 \times 0.5135 = 16.4 A$

Result: The cable carries 16.4 A—just over half its tabulated rating. If the load is 20 A, the 4 mm² cable is inadequate and a larger size is required.

The engineering lesson: The compounding effect is not intuitive. Each factor alone appears manageable; together they can halve the effective capacity of a cable.

8. Common Mistakes to Avoid

Mistake	Consequence	Prevention
Applying only one factor	Significant overestimation of capacity	Apply all applicable factors multiplicatively
Using the wrong reference temperature	Incorrect $C_{a}$ ; cable overloaded	Verify whether the tabulated rating is based on 30°C, 40°C, or another reference
Counting unloaded cables in grouping	Over-derating; unnecessarily large cable	Count only circuits carrying current
Using a single soil resistivity for all routes	Inaccurate derating	Measure or obtain route-specific soil data; use worst-case
Ignoring burial depth	Overestimation of capacity in deep trenches	Apply $C_{d}$ where depth ≠ 0.8 m
Forgetting thermal insulation	Severe overheating in insulated walls/roofs	Apply $C_{i}$ ; route cables outside insulation
Assuming XLPE and PVC derate identically	Incorrect factor selection	Use insulation-specific tables; XLPE derates less than PVC at high temperature
Not documenting derating calculations	No traceability; errors undetected	Record $I_{t a b}$ , each factor, and $I_{z}$ for each circuit

9. Why Choose Anhui Tiankang for Cable Selection?

Anhui Tiankang (Group) Co., Ltd. has nearly five decades of experience in industrial cable manufacturing. We provide the data and engineering support that allow EPCs to apply derating factors correctly and select cables with confidence.

Support	Detail
Clear tabulated ratings	Reference conditions stated explicitly on datasheets
Construction-specific derating data	PVC, XLPE, LSZH, armoured, and fire-resistant cables with applicable correction factors
Engineering assistance	Derating calculations, cable sizing verification, and installation condition assessment
Complete product range	Power, instrumentation, control, and fire-resistant cables with consistent documentation

Core advantages:

Complete certifications: CCC Ex, ATEX, IECEx, SIL, CCS marine
CNAS-accredited laboratory: full electrical and thermal performance testing
Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects
One-stop supply: from cables to glands to installation accessories

10. Conclusion

Cable derating is not a single calculation—it is a chain of multiplicative factors that reflect the reality of the installation. Ambient temperature, grouping, soil thermal resistivity, burial depth, and thermal insulation each reduce the effective capacity of the cable. Applied together, they can halve—or worse—the current a cable can safely carry.

Key takeaways:

Factor	Symbol	When It Applies	Typical Range
Ambient temperature	$C_{a}$	Air or ground temp ≠ reference	1.22 (cold) to 0.50 (hot, PVC)
Grouping	$C_{g}$	Multiple loaded circuits	1.00 (single) to 0.57 (6 bunched)
Soil resistivity	$C_{s}$	Buried cables; soil ≠ reference	1.29 (wet) to 0.75 (dry/rocky)
Burial depth	$C_{d}$	Depth ≠ 0.8 m	1.04 (shallow) to 0.90 (3 m)
Thermal insulation	$C_{i}$	Cable in contact with insulation	Often ≤0.5

The most important rule: Never select a cable from a table without confirming that the reference conditions match the actual installation. If they do not—and they rarely do—apply every applicable factor, and apply them multiplicatively.

Remember: A cable that is correctly derated for its installation conditions will deliver its rated life. A cable that is selected from a table without derating may appear to work at commissioning—and fail months or years later, when the insulation has been degraded by sustained overheating that was never visible on the surface.

Contact Us

For cable derating calculations, installation condition assessment, or project quotations, please contact:

Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/

Anhui Tiankang – Your partner for reliable industrial cable solutions.

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