Reference Installation Method C (Clipped Direct) Cable Sizing — Method
IEC Reference Method C — cables clipped direct to a wall, ceiling or unperforated cable tray — gives ~12 % higher ampacity than Method B1 (in conduit) for the same cable, because heat dissipates directly to the ambient air without a conduit barrier. The most common installation method for surface-mounted cables in commercial and industrial plant rooms, switchrooms and on cable racks.
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 C 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:
- Cable Sizing Calculator (universal) — the seed page covering all standards in one tool
- Cable Sizing — Installation Method A — IEC Reference Method A · Insulated Wall
- Cable Sizing — Installation Method E (Buried / In Ground) — IEC Method D1 · In Ground
- IEC 60364 Cable Sizing Calculator — IEC 60364-5-52 · International
- All Electrical Engineering Calculators →
Frequently Asked Questions
Method C: a single cable (single-core or multi-core, sheathed) clipped direct to a wall, ceiling or non-perforated cable tray. The cable is in still air with one surface against the support — heat radiates and convects from the other ~3 sides freely. Reference temperature 30 °C ambient; the cable surface temperature reaches the insulation rating (70 °C PVC or 90 °C XLPE) at the tabulated Iz.
Method C ampacity is ~12 % higher than Method B1 for the same cable, because there is no conduit thermal barrier. Example (IEC Table B.52.4, XLPE Cu): 16 mm² B1 = 85 A vs C = 94 A. For long runs, Method C also has lower I²R losses since the cable runs at a lower steady-state temperature for the same current.
Perforated tray is Method E (single cable in free air on a tray) or F (group), giving even higher ampacity than Method C — ~5–10 % more for the same cable. Use Method E values when the tray is genuinely perforated and air can pass below the cable. Method C applies to unperforated tray, ladder rungs, or direct clipping to a solid surface.
If cables are spaced at least one cable diameter apart and not bunched, no grouping derating applies (Cg = 1.0). When clipped touching in groups, apply Cg from IEC Table B.52.17: 0.85 for 2 cables, 0.79 for 3, 0.75 for 4, 0.73 for 5 — same multipliers as Method B.
No — buried cables use Method D1 (in conduit in ground) or D2 (direct buried). The thermal model is completely different (soil thermal resistivity dominates) and the reference ambient is 20 °C not 30 °C. For a transition (e.g. a riser from underground to wall-clipped), apply Method C to the above-ground section and Method D1/D2 to the buried section, then use the lower ampacity.
Per IEC 60364-5-52 Table B.52.4 (XLPE / EPR Cu, 30 °C ambient, no grouping): 6 mm² Method C = 51 A. PVC equivalent (Table B.52.2) = 41 A. After Ca 0.87 (40 °C) × Cg 0.80 (3 circuits): working ampacity ≈ 36 A (XLPE) — adequate for a 32 A protective device.