Reference Installation Method D1 (In Ground) Cable Sizing — Method
IEC Reference Method D1 (cables in conduit in the ground) and the related Method D2 (direct-buried) cover all underground cable installations — service entrances, sub-mains between buildings, road crossings, garden-feed cables. The reference ambient is 20 °C (soil temperature at 0.5–1 m depth in temperate climates) and the soil thermal resistivity is assumed at 2.5 K·m/W. Hot dry sand can have ρ_th of 3–4 K·m/W and force a derating below the table value.
Where:
- Ib — design current of the circuit (A), from the load calculation
- Ca — ambient temperature correction (1.00 at 30 °C reference)
- Cg — grouping / bunching factor (1.00 for a single circuit)
- Ci — thermal-insulation factor (1.00 if the cable is in free air; 0.50 if fully buried in insulation)
Then pick the smallest cable cross-section in IEC 60364-5-52 Table B.52.4, Method D1 column whose tabulated ampacity Iz ≥ It.
Related cable sizing calculators
Other standard- and method-specific cable-sizing calculators in the same series — same procedure, different reference tables and defaults:
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Frequently Asked Questions
Method D1: cables installed in a conduit (duct) buried in the ground at typical 0.5–1.0 m depth. Method D2: cables direct-buried in the ground without a duct. D1 has slightly lower ampacity than D2 because the air gap inside the duct adds thermal resistance. Reference soil temperature is 20 °C and reference soil thermal resistivity is 2.5 K·m/W.
Soil thermal resistivity ρ_th sets how easily heat dissipates from the buried cable. Wet clay ≈ 0.7 K·m/W (excellent), moist sandy soil ≈ 1.5, the IEC reference 2.5, dry sand ≈ 3–4 (poor). When ρ_th > 2.5, multiply the table Iz by a soil-resistivity correction factor from IEC 60287 (e.g. ×0.91 at 3.0, ×0.83 at 4.0). Worst-case dry-sand backfill is the killer for over-summer ampacity.
IEC 60364-5-52 Table B.52.18 gives depth correction: 1.00 at 0.7 m (reference), 0.99 at 1.0 m, 0.98 at 1.25 m, 0.96 at 1.5 m, 0.95 at 2.0 m. Deeper = slightly lower ampacity because heat takes longer to reach the surface. For depths < 0.5 m, frost protection requirements usually dominate the design.
Soil temperature at 0.7–1.0 m depth in temperate climates is remarkably stable — typically 8 °C in winter and 16 °C in summer. The IEC 20 °C reference covers most of Europe and North America. In tropical countries (soil 25–30 °C at 1 m depth) apply Ca = 0.93 to 0.85 from Table B.52.15.
IEC 60364-5-52 Table B.52.19 (in conduit) / B.52.20 (direct buried) gives Cg by spacing: cables 0.25 m apart in 0.7 m depth get Cg 0.79 for 2 circuits, 0.69 for 3, 0.62 for 4. Touching cables get worse: 0.75 for 2, 0.65 for 3. Always specify the trench cross-section so the derating is calculable.
No — a sleeve through a wall is essentially Method B1 or B2 (in conduit in air on or in a wall). Method D applies only when the conduit is buried in the ground with > 0.3 m of soil cover. A sleeve through a brick wall has air on both ends and very different thermal behaviour.