Start from the cable schedule

A buried conduit or duct is a hole sized to accept a cable under a set of installation limits — not a pipe picked off a rack by nominal bore. Before you can size anything, the electrical design has to give you the cable data:

  • Number of cables sharing the duct — one duct per cable, or several pulled together.
  • Conductor size and insulation — these set the cable’s overall diameter.
  • Single-core or multicore — a multicore cable pulls as one unit; single cores pull as a bundle.
  • Overall cable OD — the dimension the duct must physically accept.
  • Screen, armour, or serving — adds diameter and stiffness to the pull.

The overall cable OD, not the conductor size, is what a duct has to take. The cable count is what drives the fill check. Get both from the cable schedule and the cable maker’s data sheet before you look at duct sizes.

Sizing a conduit: internal diameter and fill

The number that matters is the duct’s internal diameter (or internal cross-section), not its nominal bore. Nominal bore is a trade label: two ducts sold under the same nominal size can have different internal diameters and different wall construction. For a pull calculation, use the internal bore the manufacturer publishes.

A fill limit exists for two reasons:

  • Pullability — cables must be pulled in without the insulation riding on the duct wall or jamming at a bend.
  • Heat — loaded cables dissipate heat through the duct, and a tightly packed duct traps it.

The allowable fill ratio, and any cable derating that goes with it, come from the project’s design standard and the cable maker’s installation data. For conduit systems and low-voltage installation practice in Malaysia, the relevant references are MS IEC 61386 (conduit systems for cable management) and MS IEC 60364 (low-voltage electrical installations); the cable maker’s data sets the cable OD and rating basis. This guide deliberately quotes no percentage — that figure belongs to your project’s design standard, not to a supplier’s page. Confirm it with the electrical consultant before you freeze the duct size.

Bends, pulling and draw points

Cable damage usually happens at bends, not in straight runs. The minimum bend radius is set by the cable maker for the specific cable, and it must be respected over the whole route — the accumulation of several gentle bends through a duct bank is what makes a pull stiff, not any single fitting.

  • Set the bend radius from the cable data, not from what the fitting happens to allow.
  • Count cumulative bends over the run; a series of shallow direction changes adds up.
  • Place draw pits or chambers where cables are pulled in, jointed, or the route changes direction, so the pull can be staged.
  • Leave a draw cord in every duct and spare way.
  • Cap and seal ducts during construction — open ducts fill with silt and water before the cable ever arrives.

Duct banks, spares and cover

A trench rarely carries one duct. Duct banks group several ways in one excavation, and the sizing decision has two more parts beyond the cable pull:

  • Spare ways. Allow spare ducts for future cables, repairs, and the utility’s own additions. Reopening a backfilled trench to add a duct costs far more than the spare duct did.
  • Depth of cover and stiffness class. These are driven by the load above the duct — a road crossing, a haul route, and a landscaped verge are different cases. A duct’s stiffness class must suit the depth and the surface load; confirm the required class with the utility or the authority having jurisdiction.

Duct colour, stiffness class, and jointing method can differ by utility and by distribution versus transmission scope. Treat any figure heard on site as provisional until it is confirmed against the utility’s current requirement for your scope.

Choosing the material: GI conduit vs HDPE cable pipe

Once the size and route are fixed, the material answers a different question — how much protection the cable needs. The short version:

Situation Recommended
Long open-trench run, site ducting HDPE corrugated cable pipe
Under-road or driveway crossing GI where the spec demands crush/impact protection; otherwise high-class HDPE duct
Exposed riser or plant area, mechanical threat GI conduit
Earthing/bonding continuity required GI
Telecom/fibre long-haul Smooth-wall duct per utility spec (HDPE)

Our GI is supplied to BS EN 10255 in the Light/Medium/Heavy series — colour-banded brown, blue, and red — with socket-end 15–100 mm and plain-end 65–350 mm ranges in 6 m lengths. The series, not just the diameter, is what you lock in the submittal; ordering “50 mm GI pipe” without the series is how the wrong wall arrives on site. See the GI pipe spec table and the BS EN 10255 explainer.

The full material comparison — mechanical protection, earthing, installation speed, and the trade-offs — is covered in steel conduit vs HDPE cable pipe. Keep this page on sizing and follow that link for the material decision.

The complete run: pipe + fittings + tape

A buried cable route is only complete as a system:

  • Fittings to match the duct — couplers, bends, tees, and end caps from the same system as the pipe, so the joints seat and the internal bore stays clear.
  • Detectable tape above non-metallic ducts — HDPE and other plastic ducts are invisible to a cable locator; detectable tape gives the next contractor something to trace. The logic is in detectable tape vs plain tape.
  • Draw pits and chambers at pulls and joints, sized to the cable’s bend radius.

Order pipe, fittings, and tape together so they arrive on the same delivery — a missing coupler or end cap holds up a pull just as surely as a missing duct.

Malaysian compliance context

For buried cable work in Malaysia, three things vary by project and must be confirmed, not assumed:

  • Utility requirements. TNB, Telekom, and local authority scopes can each specify duct colour, stiffness/ring class, wall type, and jointing method. Confirm the requirement for your scope with the utility or the authority having jurisdiction before ordering.
  • The conduit standard. GI conduit is specified to BS EN 10255, which superseded BS 1387. Tender documents still cite “BS 1387 Class B / Class C”; confirm with the consultant that the equivalent BS EN 10255 series is accepted before substituting.
  • Threading. Where threaded GI joints are used, threads are cut to BS EN 10226-1 (BS 21).

The wider compliance picture is on the Electrical & Telecom Protection page, and the data-centre procurement angle is in our DC electrical conduit and cable containment guide.

  • Cable schedule issued — cable count, conductor size, and overall cable OD stated
  • Duct internal diameter and allowable fill confirmed against the project design standard
  • Cable derating basis confirmed with the cable maker’s data
  • Minimum bend radius and cumulative bend on the route agreed
  • Draw pits / chambers and draw cords provided
  • Depth of cover and duct stiffness class confirmed with the utility or AHJ
  • GI series (Light/Medium/Heavy) and end finish stated per spec line
  • Fittings matched to the duct system on the same order
  • Detectable tape specified above non-metallic ducts
  • Certification and test documentation requirements confirmed in writing

Send your cable schedule for a duct quotation

Sanifix stocks HDPE corrugated cable pipes, HDPE corrugated cable fittings, Telekom ducting (HDPE PN10 cable pipe), G.I. pipes and G.I. fittings, and warning tape. Send your trench schedule and cable sizes and we will quote pipe, fittings, and tape as one package, matched to your cable schedule.