Why a 40% fill ratio at most?
Cables must be able to slide inside the conduit and shed heat. Above 40% pulling force rises quickly, cables snag in bends and core temperature rises, reducing current capacity. For moving runs use 30% so cables can slide past each other.
From area to nominal size
Per cable compute A = π/4 × d². Sum, divide by the fill ratio and compute the required inner diameter with Di = √(A_total × 4 / π). Compare that to the stated Di of the nominal size — not to the NW number itself, since NW is a system designation, not an exact diameter.
Conduit length including bends and reserve
Measure straight runs centre to centre, add π × R / 2 per 90° bend (R = the applied bend radius) and add 5% for alignment. Remember the conduit seats to the stop in the gland: allow 10 to 20 mm insertion length per gland.
When do you step up a size?
Step up one nominal size with more than three bends, a pull-in length over five metres, cables with a tacky PUR sheath, or when you want to add a cable later. The extra cost is small compared with re-routing the conduit.
| Nominal size | Di (mm) | Free at 40% (mm²) | Gland |
|---|---|---|---|
| NW 10 | 9,8 | 30 | M12 |
| NW 12 | 11,5 | 42 | M16 |
| NW 17 | 16,7 | 87 | M20 |
| NW 23 | 22,6 | 160 | M25 |
| NW 29 | 24,3 | 185 | M32 |
| NW 36 | 35,4 | 393 | M40 |
| NW 48 | 47,3 | 703 | M50 |
Frequently asked questions
- May I fill to 50% when the conduit does not move?
- Only on a short, straight run without later expansion. Above 40% pulling gets hard and heat dissipation poor; with power cables you then have to derate.
- How do I calculate with cables of different diameters?
- Always add areas, never diameters. Two 10 mm cables do not fill 20 mm but 157 mm², equal to one 14.1 mm cable.
- Can you determine the nominal size for me?
- Yes. Send cable diameters, run length, number of bends and whether the run moves; you receive a complete proposal with conduit, glands and mounting material.
Read next
- Which cable protection conduit to choose: PA, PP or metal?Material, profile and gland system together decide whether a conduit lasts for years. Choose in three steps.
- What do IP ratings mean for cable protection conduits?The two digits explained, plus why a conduit only achieves an IP rating together with its gland.
- How do you install a protective conduit without leaks?Cut square, seat to the stop, tighten correctly and clamp: the installation sequence that delivers a seal.