Understanding Cable Derating — Why the Same Cable Carries Less Current in Hot Weather

A cable’s current-carrying capacity is not limited by the copper or aluminium conductor — metals can handle enormous temperatures. The limit is the insulation wrapped around the conductor. Every insulation material has a maximum continuous operating temperature beyond which it degrades, becomes brittle, cracks, and eventually fails, exposing live conductors.

The conductor heats up because of I²R losses: current squared times resistance. The generated heat must escape through the insulation, through any surrounding air or soil, and ultimately to the ambient environment. The steady-state conductor temperature is reached when heat generation equals heat dissipation. If the conductor reaches the insulation’s maximum operating temperature at a given current, that current is the cable’s rated capacity.

Think of it like a bathtub with the tap running and the drain open. The water level (temperature) stabilises when inflow (I²R heating) equals outflow (heat dissipation). If you increase the tap flow (more current), the water level rises. If you partially block the drain (poor heat dissipation), the water level also rises. Both make the cable hotter.

Derating is the process of reducing the allowable current when conditions make heat dissipation harder than the reference conditions assumed in the standard cable ratings.

Cable current ratings in standards are published for a specific reference ambient temperature — typically 30°C for cables in air (BS 7671, IEC 60364) or 40°C (AS/NZS 3008) or 25°C for some North American standards. “Ambient temperature” means the temperature of the air (or soil) surrounding the cable, with the cable not energised.

If the ambient temperature is higher than the reference, the cable starts closer to its insulation limit before any current flows. There is less “thermal headroom” for I²R heating, so the cable must carry less current.

The derating factor for ambient temperature (C_a) is calculated from the temperatures involved:

C_a = √((T_max − T_amb) / (T_max − T_ref))

For example, a 90°C XLPE cable in 45°C ambient (reference 30°C):

• C_a = √((90 − 45) / (90 − 30)) = √(45/60) = √0.75 = 0.87

The cable can only carry 87% of its rated current. For a cable rated at 200 A at 30°C, this means only 174 A at 45°C.

An 18% reduction might not sound dramatic, but consider that 45°C ambient is common in:

• Roof spaces in tropical and subtropical climates
• Engine rooms and boiler plants
• Enclosed switchboard compartments
• Cables installed near heat-producing equipment
• Desert and Middle Eastern climates where ground surface temperatures exceed 50°C

Conversely, in colder climates (below 30°C), the derating factor exceeds 1.0, meaning the cable can carry more than its published rating. However, most engineers do not apply this “up-rating” factor because it leaves no margin for unusually warm days.

When multiple cables share the same route — in conduit, on a cable tray, in trunking, or bundled together — each cable’s heat adds to the thermal environment of its neighbours. The result is that every cable in the group runs hotter than it would if installed alone, and each must be derated.

The grouping derating factor (C_g) depends on the number of circuits and the arrangement:

Note: Values are indicative. Exact factors vary by standard and specific installation arrangement. See AS/NZS 3008.1.1:2017 Table 22, BS 7671:2018 Table C.4, or IEC 60364-5-52 Table B.52.17.

The grouping factor is the most aggressive derating in many installations. Six circuits in a conduit reduces each cable’s capacity by 43%. This is why large cable installations use open cable trays with spaced cables wherever possible — the improved heat dissipation means smaller cables for the same current.

A common mistake is to count only the number of cables rather than the number of circuits. In a three-phase circuit, the three phase conductors (and neutral if present) count as one circuit for grouping purposes, because the magnetic fields partially cancel and the total heat output is that of one circuit, not three or four separate conductors.

Derating factors are multiplied together, not added. This is because each factor independently reduces the cable’s capacity, and their effects compound:

I_derated = I_rated × C_a × C_g × C_i × ...

Where C_i is the thermal insulation factor (for cables surrounded by thermal insulation), and additional factors may apply for soil thermal resistivity, depth of burial, and other conditions.

The multiplication means that each additional derating factor hits harder in absolute terms. Consider:

• C_a = 0.87 alone means the cable carries 87% of rated current
• C_g = 0.57 alone means the cable carries 57% of rated current
• C_a × C_g = 0.87 × 0.57 = 0.50 — the cable carries only 50% of rated current

This is not an unusual scenario. Six circuits in a conduit (C_g = 0.57) in a 45°C roof space (C_a = 0.87) is a perfectly common installation. A cable rated at 200 A under reference conditions can now only carry 100 A. To get 200 A of actual capacity, you need a cable rated for 400 A under reference conditions — a substantially larger, heavier, and more expensive cable.

This cascading effect is why experienced engineers try to improve installation conditions rather than simply using larger cables. Moving cables from conduit to an open cable tray might change C_g from 0.57 to 0.72 — a 26% improvement that can eliminate the need for upsizing. Keeping cables out of roof spaces avoids the ambient temperature penalty entirely.

Let us follow a specific cable through a derating calculation to see the practical impact.

Given:

• Cable: 50 mm² copper, 4-core, XLPE insulated (X-90)
• Installation: 6 circuits grouped in a cable ladder (single layer, touching), installed in a plant room
• Ambient temperature: 45°C
• Reference conditions (AS/NZS 3008.1.1:2017): 40°C ambient for enclosed cables, single circuit

Step 1: Base current rating

From AS/NZS 3008.1.1:2017, Table 13, Column 17 (4-core XLPE cable on cable ladder): I_rated = 155 A

Step 2: Ambient temperature derating

From Table 27: for XLPE cable (90°C max) at 45°C ambient (reference 40°C): C_a = 0.95

Step 3: Grouping derating

From Table 22: 6 circuits on a single-layer cable ladder, cables touching: C_g = 0.72

Step 4: Apply derating

I_derated = 155 × 0.95 × 0.72 = 155 × 0.684 = 106 A

Result: The 50 mm² cable that carries 155 A under reference conditions can only carry 106 A in this real-world installation — a 32% reduction. If the circuit load is 120 A, this cable is undersized and a 70 mm² cable is needed.

This is why cable sizing cannot be done by simple rules of thumb. The installation conditions can reduce a cable’s capacity by a third or more, and the only way to get the right answer is to apply the correct derating factors from the applicable standard.

While ambient temperature and grouping are the most common derating factors, others can be significant in specific installations:

• Depth of burial (C_d): Cables buried deeper than the standard reference depth (typically 0.5 m for direct burial) experience higher soil temperatures and reduced heat dissipation. A cable at 1.0 m depth may require a 5–10% derating compared to 0.5 m.
• Soil thermal resistivity (C_s): The standard reference soil resistivity is typically 2.5 K·m/W. Dry, sandy soil can have a resistivity of 3.0–5.0 K·m/W, significantly impairing heat dissipation. Wet clay or chalk has lower resistivity (1.0–1.5 K·m/W) and allows higher cable ratings. This factor is critical for HV cable installations where the thermal environment dominates the design.
• Thermal insulation (C_i): Cables passing through thermally insulated walls or ceilings have severely reduced heat dissipation. AS/NZS 3008 applies a derating of 0.50 for a cable completely surrounded by thermal insulation for 0.5 m or more. This halves the cable’s capacity and is often the dominant derating factor in domestic installations where cables run through insulated ceiling spaces.
• Harmonic currents: In systems with significant third-harmonic content (LED lighting, computer loads, VFDs), the harmonic currents add in the neutral conductor of a three-phase four-wire system. The neutral may carry up to 1.73 times the phase current in extreme cases, requiring additional derating. See BS 7671 Table C.3 for harmonic derating factors.

Cable derating is not optional — it is a mandatory part of cable sizing under every major standard. Getting it wrong means either an unsafe installation (cable too small) or a wastefully expensive one (cable too large).

The ECalPro Cable Sizing Calculator automatically applies the correct derating factors for your chosen standard, installation method, ambient temperature, grouping arrangement, and any special conditions. It shows each factor separately so you can see exactly how the rated current was derived and verify it against the standard tables.

For a complete overview of the cable sizing process, including how derating fits into the eight-step methodology, read Cable Sizing for Beginners.