Conduit Bending & Back Box Fill
Conduit bending calculations, set dimensions, and back box capacity for standard accessories
90° bendsSetsOffsetsBack boxes
Conduit Bending Calculations
Accurate bending calculations ensure clean, professional conduit installations that maintain cable pulling capacity.
Offset Depth = Distance Between Marks x sin(Bend Angle)- Offset Depth
- = The perpendicular distance the conduit moves (mm)
- Distance Between Marks
- = Spacing between the two bend points on the conduit (mm)
- Bend Angle
- = Angle of each bend (typically 22.5°, 30°, 45°, or 90°)
Conduit bending geometry
For a standard 45-degree set, the distance between marks equals the offset depth multiplied by 1.414 (the square root of 2). For a 30-degree set, multiply the offset depth by 2.0.
Offset Multipliers
Bend angle multipliers for set calculations
Standard conduit bending practice| Bend Angle | Multiplier | Usage |
|---|---|---|
| 22.5° | 2.6 | Gentle offset, long radius |
| 30° | 2.0 | Standard offset |
| 45° | 1.414 | Common offset angle |
| 90° | 1.0 | Right-angle bend (not for sets) |
Distance between marks = offset depth x multiplier.
Conduit Cable Capacity
Maximum cables in conduit (singles, PVC insulated)
IET On-Site Guide / BS 7671 Appendix 4| Conduit (mm) | 1.5mm² | 2.5mm² | 4.0mm² | 6.0mm² |
|---|---|---|---|---|
| 16 | 3 | 3 | 1 | 1 |
| 20 | 7 | 5 | 4 | 2 |
| 25 | 12 | 9 | 6 | 5 |
| 32 | 20 | 15 | 11 | 8 |
Based on 40% fill factor for straight runs. Reduce by 0.6 for each 90° bend.
Bends Reduce Capacity
Each 90-degree bend in a conduit run effectively reduces the cable capacity. For runs with two 90-degree bends, use the conduit fill tables for "with bends" rather than straight-run figures.
Back Box Depth Selection
Recommended back box depths
Practical installation guidance| Application | Min. Depth | Notes |
|---|---|---|
| Light switch (1 cable) | 16mm | Plate switch only |
| Light switch (2+ cables) | 25mm | Two-way or loop-in |
| Socket outlet (single cable) | 25mm | Minimum for socket mechanism |
| Socket outlet (ring final) | 35mm | Two cables + earth |
| Dimmer switch | 35mm | Deeper mechanism + heat sink |
| Smart switch / timer | 47mm | Electronic module needs space |
Overcrowded Back Boxes
Forcing too many cables into a shallow back box damages insulation, strains connections, and makes maintenance difficult. If a 25mm box is tight, always upgrade to 35mm — it costs pence and saves hours on callbacks.