Cross-section calculator
Select the thermal cross-section of an earthing conductor. Compare five materials, weight and cost.
Conductor operating conditions
One set of data for five materials.
This calculator is illustrative. All materials are calculated at once; click a card to see the details of one.
Material and cost comparison
| Material | Min. thermal [mm²] | Tape [mm] | Mass [kg] | Price [EUR/kg] | Cost [EUR] |
|---|---|---|---|---|---|
| Copper-bonded steel | 155.26 | 40.14 × 4.14 | 130.00 | — | |
| Tinned copper-bonded steel | 155.26 | 40.14 × 4.14 | 130.00 | — | |
| Copper | 70.76 | 25.13 × 3.15 | 67.00 | — | |
| Stainless steel 304 | 302.58 | 60.14 × 5.14 | 240.00 | — | |
| Hot-dip galvanized steel | 287.79 | 60.14 × 5.14 | 242.00 | — |
Cost = length × mass per metre × your price per kilogram. Excludes labour and mark-ups. An empty price field means no costing; outside the table range no product is selected.
Illustrative calculator. Thermal calculation using the formula and constants from the CBM calculator; the formula accounts for the conductivity of the coating relative to copper. The selection covers the CBM size table; it does not account for additional mechanical or corrosion requirements or the connection method. The result is for preliminary calculations and must be checked in the design and by measurement. It does not confirm that the installation complies with standards.
Formula, material properties and scope
A = I / √[TCAP · 10⁻⁴ / (t · α · ρᵣ) · ln((K₀ + Tₘ) / (K₀ + Tₐ))]
I — fault current in kA, t — fault duration in seconds, A — cross-section in mm². We use the constants from CBM's working application (including ρᵣ = 9.67 for Fe/Cu and K₀ = 242 for Cu). Some values in their descriptive table differ from the code. For Cu, the reference to the previous column has been corrected: the numerator uses its own K₀, in line with the stated formula. Comparing all materials requires an initial temperature below 419°C. Scope of this tool: faults up to 5 s.
| Material | TCAP | α | ρᵣ | K₀ [°C] | Tₘ [°C] |
|---|---|---|---|---|---|
| Fe/Cu | 3.85 | 0.00378 | 9.67 | 245 | 1084 |
| Fe/Cu/Sn | 3.85 | 0.00378 | 9.67 | 245 | 1084 |
| Cu | 3.42 | 0.00381 | 1.78 | 242 | 1084 |
| INOX | 4.03 | 0.0013 | 72 | 749 | 1400 |
| Fe/Zn | 3.93 | 0.0032 | 20.1 | 293 | 419 |
The size is selected without rounding the result down. Table limits are inclusive, and the actual width × thickness area must be at least equal to the thermal result. “Out of range” means the required cross-section exceeds the largest tape in the CBM table; we do not invent larger sizes.
CBM tables and documentation (in Polish)