Cable Ampacity Derating — Temperature Correction and Grouping Factors

What Is Cable Ampacity?

Cable ampacity is the maximum continuous current a cable can carry without exceeding its rated conductor temperature. It is not a fixed property — it depends on the cable's insulation temperature rating, the installation method, the ambient or ground temperature, and how many other loaded cables share the same installation space.

A cable sized purely on its tabulated base ampacity without applying derating factors can overheat in service — degrading insulation, shortening cable life, and in severe cases causing fire. Correct ampacity calculation is one of the most fundamental steps in cable schedule preparation and distribution system design.

Base Ampacity — The Starting Point

Base ampacity is the current-carrying capacity of a cable under reference conditions — a specific ambient or ground temperature and a specific installation method, with no other loaded cables nearby.

The two main reference tables used in practice are:

• NEC Table 310.16 — ampacity of insulated conductors rated up to 2000V in conduit or cable, in free air, at 30°C ambient. Values are given for 60°C, 75°C, and 90°C insulation ratings, for copper and aluminium conductors.
• IEC 60364-5-52, Annex B tables — ampacity by installation method (Reference Methods A1, A2, B1, B2, C, D1, D2, E, F, G) at a reference ambient temperature of 30°C for air installation and 20°C for ground installation. Values are in mm² rather than AWG/kcmil.

The base ampacity from these tables is a ceiling, not a design value. In practice, rarely do all reference conditions apply simultaneously — derating is almost always required.

Derating Factor 1 — Ambient Temperature Correction

Both NEC and IEC base ampacity tables assume a specific reference ambient temperature. When the actual installation temperature differs, a correction factor must be applied.

The correction factor is derived from the thermal model of the cable:

• NEC formula (Table 310.15(B)(1)): CF = √((T_r − T_a) / (T_r − 30))
• IEC formula (IEC 60364-5-52 Table B.52.14): CF = √((T_r − T_a) / (T_r − T_ref))

Where T_r is the rated conductor temperature (°C), T_a is the actual ambient temperature (°C), and T_ref is the reference temperature (30°C for NEC air; 20°C for IEC ground).

For example, a 75°C-rated copper cable in a 45°C ambient:

• CF = √((75 − 45) / (75 − 30)) = √(30/45) = 0.816
• A cable with base ampacity of 230A derated to: 230 × 0.816 = 188A

This single factor can reduce effective ampacity by 20–30% in hot climates or installations near heat sources. Outdoor installations in tropical or desert environments routinely see ambient temperatures of 40–50°C, making this correction critical.

Derating Factor 2 — Grouping and Bundling

When multiple loaded cables share a conduit, cable tray, or trench, they generate heat collectively. The centre cables in a group have less ability to dissipate heat than a single isolated cable — their effective ampacity is reduced.

The grouping factor depends on the number of current-carrying conductors (CCCs) and the installation method:

• NEC Table 310.15(C)(1) — adjustment factors for more than 3 current-carrying conductors in a conduit or cable:
  • 4–6 conductors: 0.80
  • 7–9 conductors: 0.70
  • 10–20 conductors: 0.50
  • 21–30 conductors: 0.45
  • 31–40 conductors: 0.40
  • 41+: 0.35
• IEC 60364-5-52 Table B.52.17 — grouping factors for cables in conduit, tray, or bunched in free air. Factors range from 0.80 for 2 cables down to 0.41 for 12+ cables, with separate tables for different installation methods.

An important NEC clarification: the neutral conductor of a 3-phase, 4-wire circuit counts as a CCC if it carries significant harmonic current (typically from non-linear loads like VFDs, computers, and electronic ballasts). Per NEC 310.15(E), if more than 50% of the neutral current is third-harmonic or higher, the neutral must be counted.

Derating Factor 3 — Soil Thermal Resistivity (Buried Cables)

For cables installed in underground duct banks or direct burial, the surrounding soil's ability to conduct heat away from the cable is the limiting factor. Soil thermal resistivity (ρ, in °C·m/W or °C·cm/W) varies significantly by soil type and moisture content:

• Wet clay or saturated soil: ρ ≈ 0.5–1.0 °C·m/W — good heat dissipation
• Moist sand or average soil: ρ ≈ 1.0–2.0 °C·m/W — typical design assumption
• Dry sand or crushed rock: ρ ≈ 2.5–3.0 °C·m/W — poor heat dissipation, significant derating

The precise calculation method for underground cables is the Neher-McGrath method (IEEE 835, adopted into NEC Annex B) for North American practice, and IEC 60287 for international projects. Both methods calculate the steady-state temperature rise of the conductor based on load current, cable construction, installation depth, conduit material, and soil thermal resistivity.

For duct bank installations specifically — where multiple cables are grouped in concrete-encased conduits — mutual heating between cables is the dominant effect. The more loaded circuits in a duct bank, the greater the thermal interaction. This is why duct bank design must account for ampacity from the outset, not as an afterthought.

Combining Derating Factors

When multiple derating conditions apply simultaneously, all correction factors are multiplied together:

• Derated Ampacity = Base Ampacity × CF_temp × CF_group

A worked example — 4/0 AWG copper THWN-2 (90°C) in conduit, 8 current-carrying conductors, 40°C ambient:

• Base ampacity (NEC Table 310.16, 90°C column): 260A
• Temperature correction (40°C ambient, 90°C rated): CF = √((90−40)/(90−30)) = √(50/60) = 0.913
• Grouping factor (8 CCCs, NEC Table 310.15(C)(1)): 0.70
• Derated ampacity: 260 × 0.913 × 0.70 = 166A

This is a 36% reduction from the base value. A designer who ignored derating and sized for 260A would have a cable operating dangerously above its thermal limit in service.

Note that NEC Art. 310.15(A)(2) permits using the 90°C ampacity for derating calculations even when the termination temperature rating limits the final operating ampacity to 75°C — this is a commonly misunderstood provision that allows the higher base value to be derated down, as long as the termination rating is still respected.

Installation Method and Its Effect on Ampacity

The installation method significantly affects base ampacity because it determines how efficiently heat can escape from the conductor:

• Free air (exposed) — highest ampacity; convection and radiation cool the cable efficiently. NEC Table 310.17; IEC Method E/F.
• In conduit in air — moderate ampacity; the conduit restricts airflow around the cable. NEC Table 310.16; IEC Method B.
• In conduit buried in concrete or soil — lowest ampacity; heat must conduct through conduit wall, air gap, concrete, and soil. NEC Annex B / Neher-McGrath; IEC Method D.
• Cable tray (ladder or ventilated) — similar to free air for single-layer installations; reduce significantly for multi-layer or covered trays.

The same 4/0 AWG copper conductor can have a base ampacity ranging from 230A (in conduit, NEC 75°C) to 285A (free air, NEC 75°C) — a 24% difference based purely on installation method. This distinction matters when routing cables: a short section through a cable tray versus a long conduit run should ideally be sized for the worst-case installation condition along the entire route.

Where Standards Diverge

• Reference temperatures — NEC uses 30°C as the ambient reference for all above-ground installations. IEC uses 30°C for air and 20°C for ground installations. This means IEC-derated values for underground cables will differ from NEC even when using the same basic formula.
• Neutral conductor counting — NEC has specific rules on when neutrals count as CCCs (third-harmonic loading threshold). IEC 60364-5-52 takes a simpler approach — any conductor that normally carries current in operation counts.
• Conductor cross-section units — NEC uses AWG and kcmil; IEC uses mm². When adapting a calculation from one system to the other, use the actual resistance value rather than trying to find an equivalent size — conductor properties at nominally equivalent sizes can differ by 5–10%.
• Underground calculation methods — Neher-McGrath (NEC Annex B / IEEE 835) and IEC 60287 produce equivalent results for simple cases but differ in how they handle mutual heating in duct banks. For large or critical underground installations, both methods should be checked or a specialist cable sizing software used.

Practical Design Rules of Thumb

• Always apply derating before selecting the cable size — not after. Size up to the next standard conductor that satisfies the derated ampacity requirement.
• For preliminary sizing in duct banks with 4+ loaded circuits, assume a group derating factor of 0.70–0.80 pending detailed thermal calculation.
• For hot ambient environments (>40°C), temperature correction alone can reduce ampacity by 15–25% — factor this in early or cable sizes will need to increase significantly at detailed design.
• Size the entire cable route for the worst-case installation condition. A cable that passes through both a cable tray and a conduit section must be sized for the conduit.
• For MV/LV distribution feeders, apply a utilization factor of 0.8 on top of derating to allow for future load growth without cable replacement.

Summary

• Cable ampacity is not fixed — it depends on insulation rating, installation method, ambient temperature, and cable grouping
• Derated Ampacity = Base Ampacity × Temperature Correction Factor × Grouping Factor
• NEC reference: 30°C ambient, Table 310.16 base ampacity, corrections per Tables 310.15(B)(1) and 310.15(C)(1)
• IEC reference: 30°C air / 20°C ground, IEC 60364-5-52 Annex B tables and correction factors
• Underground and duct bank installations require Neher-McGrath (NEC) or IEC 60287 methods for precise calculation
• Always size for the worst-case installation condition along the full cable route