20mm PVC Conduit: The Electrician's Guide to Fittings, Boxes, and Installation

PVC conduit is the standard cable containment for surface-run wiring in commercial and industrial premises, external walls, garages, outbuildings, and any domestic installation where cables are run exposed. It protects cables from mechanical damage, simplifies future re-cabling, and is required in certain locations under BS 7671. This guide covers the full Profix 20mm conduit range at APM — every fitting type, when to use it, and the installation basics that reduce callbacks.

Heavy Duty vs Light Duty: Which Conduit to Specify

The Profix range offers two grades for 20mm conduit:

• ProFix 20mm Heavy Duty PVC Conduit, Black — £2.50 per length — heavy duty (HD) grade is for all external installations, any location with significant mechanical risk, cable routes in garages and outbuildings, and industrial applications. The thicker wall gives greater impact resistance and UV stability.
• Profix 20mm Light Duty PVC Conduit, White — £2.49 per length — light duty (LD) for internal, dry, low-risk surface runs. Domestic surface wiring in inhabited rooms, chasing accessories. White is the finish-friendly option for living spaces.

Rule of thumb: external or mechanical risk = heavy duty black. Internal, dry, domestic = light duty white. When in doubt, specify heavy duty — the price difference is negligible and the performance margin is significant.

The Complete Box Range

Conduit boxes are the junction and termination points in a conduit installation — where runs change direction, branch, terminate at accessories, or provide inspection access. Choosing the right box type eliminates unnecessary bends and keeps installation clean.

• Angle Box, Black — £0.69 — two entries at 90°, on the same face. For 90° horizontal direction changes in a run. The standard corner box.
• Through Box (Straight), White — £0.69 — two entries in line. For inspection access on a long straight run, or for pulling cable through a joint.
• Angle Box, White — £0.81
• 3-Way Box (T-junction) — £1.37 — three entries: two in line and one at 90°. For branching a run to feed an additional circuit or accessory.
• 4-Way Angle Box — £1.37 — four entries in a cross configuration. For complex junction points where three or four runs meet.
• "H" Box (Flat Tee) — £0.96 — two entries on one face and one on the opposite, offset. Used where a vertical drop meets a horizontal run with a branch in the same plane.
• "U" Box (Offset) — £0.96 — two entries on adjacent sides with an offset, for direction changes where a standard angle box won't sit flat against the surface.

Bends and Tees: Inspection Access Points

Standard conduit bends and tees have inspection covers — a removable lid that allows cable to be pulled through after installation, or inspected and replaced later. This is the defining advantage of conduit over non-inspection routes: the cables are always accessible.

• Inspection Bend 20mm, White — £0.77 — 90° bend with removable inspection cover. For changes of direction where you need cable pull access. Use instead of angle box where a low-profile finish is needed.
• Inspection Bend 20mm, Black — £0.77
• Normal (Solid) Bend 20mm — £0.32 — non-inspection, fixed 90° bend. For runs where the cable is pulled through before the bend is fitted, or where access is not required. Cheaper and lower-profile than inspection bends.
• Inspection Tee 20mm, White — £1.08 — T-junction with inspection cover. For branching runs with cable pull access at the junction point.
• Inspection Tee 20mm — £1.10

Couplers, Adaptors, and Saddles

• Conduit Couplers 20mm Black, 10-pack — £1.36 — for joining two conduit lengths end-to-end. Use with solvent cement on external runs; push-fit is adequate internally. Always couple rather than leaving a bare joint — a bare joint collects moisture and allows cable abrasion.
• Conduit Couplers 20mm White, 10-pack — £1.36
• Female Adaptors 20mm Black, 10-pack — £2.72 — for connecting conduit into a box entry that uses a female thread (box-entry connection). One end is plain conduit entry, the other is threaded for box connection.
• Female Adaptors 20mm White, 10-pack — £2.56
• Male Adaptors 20mm, 10-pack — £2.72 — for connecting conduit to a box with a male thread entry. The male thread screws into the box knockout, conduit slides onto the plain end.
• Space Bar Saddles 20mm Black, 10-pack — £2.40 — the standard surface-mounting clip for conduit. The space bar creates a gap between conduit and wall, allowing air circulation and moisture drainage. Required on external installations. Space bar saddles every 400–600mm on horizontal runs, 600–800mm on vertical.
• Space Bar Saddles 20mm White, 10-pack — £2.40
• Round Clip (U-clip) 20mm Black — £0.17 each — the simple alternative to space bar saddles for internal dry runs. Screw fix, lower profile, no air gap. Not suitable for external use.

How to Install 20mm Conduit: Key Rules

Cable fill

20mm conduit has a maximum cable fill of 40% of the cross-sectional area (BS 7671 guidance). In practice: 20mm conduit comfortably takes 4–6 × 1.5mm single-core cables, or 2–3 × 2.5mm singles. Overfilling makes pulling difficult and increases heat build-up. Use 25mm conduit for larger cable groups.

Pulling cable through

Always use a draw wire (fish tape) or pull-through rod. Pull from a box, not from mid-run. For runs over 5m with bends, use a cable lubricant — it reduces friction significantly and prevents insulation damage on tight bends. Inspection bends and boxes are there for a reason: install them at every change of direction and every 10m on long runs.

Bending conduit

Use a bending spring inside the conduit for hand-forming gentle curves — do not apply heat to PVC conduit in the field. For tight 90° bends, use the Profix inspection bend fitting rather than trying to bend the pipe. An improperly bent pipe collapses the bore and makes cable pulling difficult or impossible.

External and damp locations

Use heavy duty conduit and space bar saddles. Seal all box and conduit entries with a suitable sealant on external runs to prevent moisture ingress. Ensure all fittings are UV-stable grade (heavy duty black is; standard white LD may not be for extended external exposure).

Quick Reference: Fitting Selection

All Profix 20mm conduit and fittings are in stock for same-day collection. For larger quantities — first fix on a commercial unit, garage or outbuilding project — call ahead and we'll pull the order.

APM Plumbing & Electrical | 24 Western Avenue, Acton, London W3 7TZ | 020 8702 8080 | apmi.uk