Precise selection: thermal stability + corrosion allowance

Copper-clad steel selection guide, part 6 of 8

IEEE 80-2013 and GB/T 50065-2011 agree: the conductor cross-section is set by thermal stability, the copper thickness by the corrosion allowance, and the two are added. The copper layer is both the current-carrying layer and the corrosion allowance – thickness = corrosion loss over the design life + the residual layer still needed for conductivity at end of life.

Step 1　Thermal stability – minimum cross-section

IEEE Std 80-2013 clause 11.3.1 gives two forms: the full equation (37), which yields the section in mm² directly from the material constants (resistivity, temperature coefficient, thermal capacity, fusing temperature), and the simplified form with the Table 2 material constant K_f – note that the simplified form gives kcmil, not mm² (1 kcmil = 0.5067 mm²):

K_f (IEEE 80-2013 Table 2, derived from the Table 1 constants at 1084 °C fusing temperature and 40 °C ambient): copper, annealed soft-drawn 7.00; copper-clad steel wire 40 % IACS 10.45; 30 % IACS 12.06. Worked example, 20 kA for 1 s: copper 20 × 7.00 × 1 = 140 kcmil ≈ 71 mm² (70 mm² in practice); 40 % CCS 209 kcmil ≈ 106 mm²; 30 % CCS 241 kcmil ≈ 122 mm² (a Ø 14 round wire at 154 mm² satisfies it). So for the same fault current, a CCS conductor needs about 1.5–1.7 times the cross-section of copper, depending on conductivity grade. The GB/T 50065-2011 formula S ≥ I·√t / C is the same physical relation in coefficient form, giving mm² directly.

The tabulated constants belong to the construction stated in the table (wire per ASTM B910, copper ≥ 5% of diameter). For CCS of other copper thickness, form or core, the composite thermal capacity and resistivity are calculated from the declared construction per IEEE 80 clauses 11.3.1.1–11.3.1.2, equations (41)–(44) – the thicker the copper, the closer the thermal capacity comes to solid copper. Our assessments are calculated this way.

Common engineering correspondences; Ø16 is the practical limit for bending on site. Conductivity depends on diameter (0.8 mm copper: Ø8 ≈ 42 % IACS, Ø16 ≈ 27 %); a strict equal-withstand match of Ø16 to 120 mm² needs 1.0 mm copper or a calculation from the actual constants. For a specific project the sizing report, calculated from fault current, duration and conductivity, governs.

Step 2　Corrosion allowance – copper thickness

Corrosion allowance = average annual corrosion rate of copper × design life, calculated per DL/T 2094-2020 (as DL/T 2650-2023 cl. 5.1.1.3 directs). Local soil corrosion data take precedence; where none exist, DL/T 1312-2023 clause 4.6, Table 1 gives the rates:

* Extreme: soil rated "high" under the DL/T 1554 multi-factor method and also pH > 8.5 or pH < 4.5 with salt content above 1.5%; extreme soil calls for solid copper.

The 0.6 / 0.8 mm classes of the quick method are simply what this calculation typically yields for a 30–40-year design life. Send us conductor size, route length, soil conditions, design life, fault current and duration, and within 24–48 hours you receive a sizing report for both options (equal diameter and equal short-circuit withstand) with the copper comparison.

8　Standard sizes (continuous-cast; copper 0.25 / 0.6 / 0.8 / 1.0 mm)

The thickness classes follow the anti-corrosion standards (Q/GDW 12015, DL/T 2094) and DL/T 2650 for corrosion, and DL/T 1312 / DL/T 2727 for the product, to simplify thermal and corrosion design. Electroplated 0.25 mm round wire, tape and rods are also supplied. Other thicknesses on request.