Coiled Cable Derating

Cable left wound on a drum or reel can't shed its heat, so its safe current drops sharply with every layer. Check a flexible cable on a drum, in a spiral or on an extension reel against your load and protective device, and see how many layers you can leave wound.

Guidance only. Results are estimates based on the figures you enter and published tables. All electrical work must be designed, checked and tested by a qualified electrician to the regulations that apply where you are (e.g. BS 7671, NEC).

Cable

Include sun on a dark drum.
A
A
MCB / fuse feeding this cable.

How the Cable Is Laid

Extension reels to BS EN 61242 are marked with two ratings: one for fully unwound and a much lower one for wound. Enter what the label says. A reel with a BS 1363 plug is limited to 13 A however it is used.

Cable drum diagram

Derated Capacity

Tabulated rating (It)
—
Factors
—
Derated capacity (Iz)
—
Max layers left wound
—
Load Ib ≤ Iz
—
Device In ≤ Iz (overload protection)
—
Laid as Factor Iz Load

Notes & References

  • Method. Iz = It × Ca × Cw. The load must not exceed Iz, and for the cable to be protected against overload the device rating must not exceed it either (BS 7671 Regulation 433.1.1: Ib ≤ In ≤ Iz).
  • H07RN-F ratings (It): DIN VDE 0298-4:2003-08 Table 13, rubber-sheathed flexible cable in free air, 30 °C ambient, 60 °C conductor, as reproduced in Lapp catalogue Table T12-9. Single-phase uses the two-loaded-core column (twin cable); three-phase uses the three-loaded-core three-core column, the lowest of the multicore columns. 1 mm²: 15 / 12.5 A; 1.5: 18.5 / 15.5; 2.5: 25 / 21; 4: 34 / 29; 6: 43 / 36; 10: 60 / 51; 16: 79 / 67; 25: 105 / 89 A.
  • Flexible cord ratings: BS 7671 Table 4F3A (60 °C rubber and PVC flexible cords), which agree with DIN VDE 0298-4 Table 11: 0.75 mm² 6 A; 1.0: 10 A; 1.25: 13 A (single-phase only); 1.5: 16 A; 2.5: 25 / 20 A; 4: 32 / 25 A.
  • Ambient (Ca): 60 °C conductor column of BS 7671 Table 4B1 / DIN VDE 0298-4 Table 17. Below 30 °C we use 1.00 rather than taking credit for a cool day.
  • Wound cable (Cw): DIN VDE 0298-4:2003-08 Table 27 — 1 layer 0.80, 2 layers 0.61, 3 layers 0.49, 4 layers 0.42, 5 layers 0.38; a single-layer spiral 0.80. No factor is tabulated beyond five layers. Other sources quote different series (for example NEMA/ICEA WC 8 for reels, and a NIOSH study found published factors can underestimate heating on reels), so treat any cable left wound with caution.
  • Extension reels to BS EN 61242 carry separate wound and unwound ratings. Typical 1.25 mm² reels are marked 3120 W (13 A) fully unwound, with wound ratings commonly between about 720 W and 1200 W; read the label on yours. Many have a thermal cut-out, which is a last resort and not a reason to leave them wound.
  • Coiled or heaped on the floor. Cable piled in loops, flaked tightly or covered (by matting, set or audience) behaves like cable on a drum or in thermal insulation and has no tabulated factor. For temporary event power, good practice (BS 7909) is to fully unwind drums and lay out spare cable in loose, open loops or figure-of-eight flakes that aren't stacked or covered.
  • Related tools: Cable Gauge Calculator (fixed wiring sizes and voltage drop), Power Calculator.
  • This is guidance only. It does not replace BS 7671, BS 7909, the cable manufacturer's data or the judgement of a competent person.