Cable Pulling & Conduit Fill Lab
Check that the cables fit the conduit or duct, then plan the pull: tension along every straight and bend, sidewall pressure on the bends, bend radius and the best direction to pull from. Works for LV power cables in ducts, conduits and sleeves on site.
How to use: list the cables (or keep the example of three 240 mm² singles), pick the conduit, then build the route from straights and bends. Tap i for help.
1. Inputs
2. Fill and pull
Cross section of the conduit, the route with tension at each point, and tension along the pull from both ends.
3. Checks
4. Route sections, best direction
5. Fill check
6. How it works
Conduit fill
Fill is the total cable area over the conduit area. The common limits (NEC Chapter 9 Table 1, used widely in the Gulf) are 53% for one cable, 31% for two and 40% for three or more. BS 7671 uses cable and conduit factors, and AS/NZS 3000 asks that cables can be drawn in without damage. Leave spare space for future cables where you can.
Jam ratio
With three cables, if the conduit inside diameter is 2.8 to 3.2 times the cable diameter, the cables can wedge side by side in a bend and jam. Pick a size outside that band.
Tension
On a straight, tension rises by weight × friction × length (more uphill, less downhill). In a bend it multiplies by e^(μθ), so bends near the far end hurt most. That is why pulling from the end nearer the bends usually needs less force. The weight correction factor allows for cables pressing on each other.
Sidewall pressure and bend radius
Sidewall pressure is the tension divided by the bend radius. Too much crushes the insulation against the inside of the bend. Larger radius bends and pulling from the other end both reduce it. Keep the bend radius above the cable maker's minimum (often 8 to 12 times the overall diameter during pulling).
Learning tool. Use the cable maker's weights, diameters and pulling limits, and a pulling calculation from the installer for long or heavy pulls.