Helpful video reference. We reference UK electrician Matt's tutorial "Steel Conduit In a House + a New Way of Deburring Conduit" as the companion to this guide. Matt demonstrates the key techniques for working with 20mm steel conduit in a domestic property, including a practical deburring method that protects cable insulation -- the step most tutorials gloss over.
1. Plan the route and measure up
Draw the run on paper or mark it on the wall. Note where bends are needed (changes of direction, drops to accessory boxes) and where couplings will join sections. Measure each length between fittings carefully: steel conduit is sold in 3-metre sticks and waste adds up quickly on a complex run.
Steel conduit is available in two common sizes for domestic work: 20mm and 25mm. Most domestic wiring uses 20mm. Check that your planned cable count and length will fit inside the conduit using the space-factor tables in BS 7671 Appendix 5 before you buy.
2. Cut the conduit to length
Clamp the conduit in a vice or conduit bending stand and cut with a junior hacksaw at exactly 90 degrees. A square cut is important: an angled cut will not seal properly against a box knockout and can create a gap that catches cable insulation. Cut slightly long on your first piece and trim to fit -- it is easier to remove material than add it back.
3. Deburr the cut ends -- inside and out
This is the step that separates a proper installation from one that fails insulation resistance testing. Use a round file to remove all burrs from the inside of the cut end. Run your finger around the inside to confirm it is smooth before proceeding. Deburr the outside too, so the conduit does not cut your hands during handling and does not abrade any exposed conductor at entry points.
The alternative method shown in the reference video uses the corner of a flat file drawn along the inside edge -- quicker for production work on a long run.
4. Form any bends
Short-radius 90-degree bends are made with a conduit bending machine. The former size must match the conduit diameter: a 20mm former for 20mm conduit. Mark the start of the bend on the conduit, set it in the machine, and apply steady downward pressure. Count your pull-throughs: a 20mm conduit with two 90-degree bends and a run over 3 metres starts to get difficult for cable pulling.
Gentle sweeping bends on runs of 3 metres or more reduce pulling friction. Any kink in the conduit -- a bend that collapses the wall of the tube -- means cutting that section out and starting again. Kinked conduit cannot be pulled through safely and cannot be straightened.
5. Fix the saddle clips
Standard heavy-gauge saddle clips fix to the surface and hold the conduit. Space them at 450mm on straight horizontal or vertical runs, at 300mm near bends, and within 150mm of every junction box or fitting. Drill and plug the fixings first, then clip the conduit into the saddles. Do not overtighten: the saddle should grip firmly but not distort the conduit.
At entry into accessory boxes, use a check nut and male bush to terminate the conduit squarely into the knockout. This locks the conduit to the box and leaves a clean, smooth entry point for cable.
6. Thread a draw wire
Before closing up the last section of conduit, push a steel fish wire or nylon cable rod through the full length of the run. Tie off the end inside each accessory box so it cannot disappear back into the tube. This draw wire is what you will attach the cables to when you pull them through.
If the run has more than two 90-degree bends, feed draw wires from each end and join them at the middle section. Trying to push a cable rod around three or more bends in a single push rarely works.
7. Pull cable through
Use single-core PVC-insulated conductors inside conduit -- not twin-and-earth cable. Tape the cable ends to the draw wire with electrical tape, forming a smooth tapered joint so there is no leading edge to snag. Pull the draw wire steadily from the far end.
On runs over about 5 metres, apply a small amount of cable pulling lubricant to the conductors as they enter the conduit. Never use washing-up liquid or engine oil as a substitute: both can attack PVC insulation over time. Purpose-made pulling gel is inexpensive and makes long runs significantly easier.
Leave at least 150mm of tail at each accessory box for termination. Mark each conductor clearly at both ends with cable markers: brown for line, blue for neutral, green/yellow for CPC.
8. Earth the conduit system
A metallic conduit installation must be earthed. Run a separate green/yellow CPC from the consumer unit earth bar to each accessory box and terminate it under the earth terminal. The conduit itself can serve as the CPC only if a continuity test proves the resistance of the conduit path meets the requirements of BS 7671 Table 54.7 -- in practice this means a measured value well below 1 ohm between the far end of the conduit and the main earthing terminal.
For most domestic installations, running a separate CPC alongside the circuit conductors is simpler and provides a more reliable earth path than relying solely on the conduit.
9. Test before energising
Carry out the following dead tests before connecting to the consumer unit: continuity of the CPC (R2 measured at the furthest point), insulation resistance (at least 1 MOhm between each conductor and earth, with all conductors joined together at the far end) and polarity check. Record your readings. These are the tests required by BS 7671 Chapter 61 before any new circuit is energised.
When to call us
Installing conduit as a mechanical protection system for existing cable is something a careful DIYer can attempt on a short, simple run in a garage. The moment a new circuit is involved, Part P notification is required and the work needs to be certified. The testing before energising -- insulation resistance, continuity and polarity -- also needs proper test equipment and the knowledge to interpret the results. If in doubt on any of those points, the job is for an electrician.
Need conduit work done properly in Sandwich?
Richard installs steel and PVC conduit in garages, workshops and outbuildings across east Kent. New circuits are notified under Part P and come with a certificate.
Contact Richard