Why Most Cable Sizing Calculations Underestimate Temperature Rise in Grouped Cables

Every cable sizing standard provides derating factors for grouped cables. The principle is straightforward: cables in proximity share the heat each produces, raising the effective ambient temperature and reducing the permissible current-carrying capacity. The standard tables are:

• IEC 60364-5-52:2009+A1:2011, Table B.52.17 — reduction factors for groups of more than one circuit or multicore cable
• AS/NZS 3008.1.1:2017, Table 22 — derating factors for groups of enclosed or bunched cables
• BS 7671:2018+A2, Table 4C1 — grouping rating factors
• NEC Table 310.15(C)(1) — adjustment factors for more than three current-carrying conductors

These tables share a common assumption: all cables in the group are equally loaded and of equal size. The derating factor is derived from a thermal model where every cable contributes the same heat flux. Under this assumption, every cable in the group experiences the same temperature rise, and a single derating factor is valid for all positions.

The rigorous alternative is IEC 60287-2-1:2023, which calculates the thermal resistance of each cable individually, accounting for its specific position within the group, the thermal resistivity of the surrounding medium, and the heat contribution from every neighboring cable. This method produces a different permissible current for each cable position — but it requires detailed thermal modeling that most engineers do not perform for routine installations.

The IEC 60287 thermal circuit model treats each cable as a heat source embedded in a thermal resistance network. The key variables are:

• T1 — thermal resistance of insulation between conductor and sheath
• T2 — thermal resistance of bedding between sheath and armour
• T3 — thermal resistance of outer covering
• T4 — thermal resistance of the surrounding medium (air, soil, tray)

For a single isolated cable, T4 depends on the cable’s ability to dissipate heat to its surroundings. For grouped cables, each cable’s T4 is modified by the presence of neighboring heat sources. The modification is position-dependent:

The temperature rise of each cable is proportional to its own losses multiplied by its own total thermal resistance, plus the mutual heating contribution from every other cable in the group. For a center cable in a 4×3 arrangement (12 circuits, 4 columns, 3 layers), the mutual heating contribution from 11 neighboring cables can exceed the cable’s own self-heating.

The simplified table method collapses this position-dependent calculation into a single worst-case factor. The factor is derived from the hottest cable in the group (the center cable), meaning edge cables are over-derated while center cables are derated by exactly the correct amount — but only if all cables are equally loaded.

During a shutdown inspection at a mineral processing plant, we conducted thermal imaging of a 600 mm perforated cable tray carrying 12 three-phase XLPE/SWA circuits (all 35 mm² copper, rated 90°C). The tray was installed in an enclosed cable tunnel with an ambient temperature of 38°C. All 12 circuits were feeding identical 22 kW pump motors running at approximately 85% of full load.

Measured sheath temperatures by position (cables arranged in 2 layers, 6 per layer):

The center cable in the top layer reached 76°C — a 38°C rise above the 38°C ambient. The coolest edge cable (bottom layer, left outer) was at 61°C — a 23°C rise. The temperature difference between the hottest and coolest cable was 15°C.

The AS/NZS 3008.1.1:2017, Table 22 derating factor for 12 cables in a group on a perforated tray is 0.70. Applied to the 35 mm² cable’s base rating of 126 A at 40°C ambient (per Table 13, Column 17), the derated capacity is 88 A. The actual load current was approximately 34 A per phase. At this loading level (39% of derated capacity), none of the cables were in danger of overheating. But the temperature distribution pattern is instructive for what would happen at higher loading.

The simplified derating tables assume all cables carry the same current. In practice, cable trays frequently carry a mix of heavily loaded feeders and lightly loaded control or instrumentation cables. When loads are unequal, the uniform derating factor can be simultaneously too conservative for lightly loaded cables and insufficient for heavily loaded ones.

Consider a tray carrying 9 circuits: 3 heavily loaded motor feeders at 95% of derated capacity and 6 lightly loaded circuits at 20% of derated capacity. The IEC 60364-5-52, Table B.52.17 derating factor for 9 circuits is 0.73. This factor was derived assuming all 9 circuits are at 100% load.

In reality, the 6 lightly loaded cables contribute minimal heat. The 3 heavily loaded cables are essentially operating in a group of 3 (not 9) from a thermal standpoint, which would have a derating factor of 0.82. The uniform 0.73 factor forces the motor feeders to be oversized by approximately 12% relative to what a rigorous thermal analysis would require.

Conversely, if 6 of the 9 cables were heavily loaded and only 3 were light, the center cables would experience more mutual heating than the 0.73 factor accounts for, because the factor assumes the total heat output is distributed uniformly.

IEC 60364-5-52, Clause B.52.4, Note 2 acknowledges this: “Where cables in the group are not all loaded to their grouped rating, the rating factors may be modified accordingly.” However, no prescriptive method is given, leaving engineers to apply engineering judgment or resort to the full IEC 60287 calculation.

The derating table approach can produce unsafe results in specific scenarios that are more common than most engineers realize:

1. High fill ratio cable trays in hot environments. A perforated tray at 90% fill in a 45°C ambient (common in equatorial industrial plants and mining operations) presents a thermal environment significantly worse than the test conditions underlying the derating tables. The tables were derived for 30°C or 40°C ambient in well-ventilated spaces.
2. Cables in enclosed trenches or tunnels without forced ventilation. The derating tables for “enclosed” installations assume natural convection. If the enclosure is long (more than 5 m) with limited air circulation, the local air temperature around the cables can rise 10–20°C above the nominal ambient, compounding the grouping effect.
3. Mixed cable sizes in a group. A 240 mm² feeder cable next to 2.5 mm² control cables has a drastically different thermal time constant. The large cable acts as a massive heat source that elevates the temperature of the small cables disproportionately. The small cables may exceed their rated temperature while the large cable remains well within its limit.
4. Cables on closed (non-perforated) trays. AS/NZS 3008.1.1:2017, Table 22 provides separate derating factors for perforated and non-perforated trays. Non-perforated tray factors are lower (more restrictive), but even these can underestimate the thermal penalty when the tray is horizontal and cables sit in a pool of stagnant hot air above the tray surface.

Based on field measurements across multiple industrial facilities and the underlying thermal physics, the following practices reduce the risk of thermal issues in grouped cable installations:

1. Limit cable tray fill to 50% of cross-sectional area for power cables. This is more conservative than the typical 40–50% fill permitted by AS/NZS 3000:2018, Clause 3.9.7 and NEC 392.22, but it provides thermal margin that accounts for future additions. Document the design fill ratio and maximum permissible fill in the cable schedule.
2. Place highest-loaded cables at the edges of trays. Edge cables have the best heat dissipation path. Center positions should be reserved for the most lightly loaded circuits. This simple arrangement change can reduce peak cable temperature by 5–10°C without changing any cable sizes.
3. Use perforated trays wherever fire regulations permit. The temperature difference between perforated and solid trays is 5–15°C for the same cable loading, depending on tray fill and ventilation.
4. Apply the IEC 60287 method for any tray carrying more than 20 power cables. The simplified derating tables were validated for moderate group sizes. For large groups, the cumulative mutual heating exceeds what the tables were designed to handle.
5. Conduct thermal imaging surveys on critical cable runs within 6 months of commissioning. This validates the design calculations against actual thermal performance and establishes a baseline for monitoring future cable additions.
6. Maintain spacing between layers in multi-tier tray systems. IEC 60364-5-52, Clause 522.8.5 recommends a minimum of 300 mm between tray tiers. Increasing this to 450 mm significantly reduces inter-layer thermal coupling.

Standards referenced: IEC 60287-2-1:2023, IEC 60364-5-52:2009+A1:2011, AS/NZS 3008.1.1:2017, AS/NZS 3000:2018, BS 7671:2018+A2, NEC/NFPA 70:2023.