10.3 Conduit Systems: Steel & PVC

10.3 Conduit Systems: Steel & PVC

A conduit is a tube or piping system designed to enclose and protect insulated electrical cables. Unlike sheathed wiring systems (such as 6242Y) which are fixed directly to building structures, conduit forms a hollow, continuous, rewireable enclosure. Operatives first erect the complete conduit containment run and subsequently draw in unsheathed single-core thermoplastic cables (BS 6004 / 6491X).

Conduit systems provide outstanding mechanical protection, permit simple circuit rewiring and additions without structural damage, and can be sealed against the ingress of dust, moisture, and corrosive vapours.

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Classifications of Conduit Systems

Under BS EN 61386, conduit systems are classified by their construction material, mechanical impact resistance, and corrosion protection.

1. Heavy Gauge Rigid Welded Steel Conduit

Steel conduit provides the highest mechanical protection against heavy impact, crushing, vandalism, and fire:

• Class 2 (Black Enamel): Coated internally and externally with a baked black enamel paint. Offers light corrosion protection. Strictly limited to clean, dry indoor commercial and industrial buildings (such as commercial office ceiling voids and dry workshops).
• Class 4 (Hot-Dip Galvanised): Steel conduit dipped in molten zinc, providing a thick, metallurgical zinc alloy coating inside and out. Offers heavy corrosion resistance. Specified for outdoor installations, damp basements, car parks, agricultural buildings, chemical works, and heavy industrial plant.

2. Rigid High-Impact PVC Conduit

Manufactured from unplasticized polyvinyl chloride (uPVC):

• Class 1 (Light Gauge): Thin wall, rarely used in commercial building services.
• Class 2 (Heavy Gauge High-Impact): Thick wall, highly resistant to impact, acids, alkalis, and moisture. Non-conductive (eliminating touch-voltage hazards on containment) and non-corroding. Common across commercial offices, schools, hospitals, food processing facilities, and agricultural livestock units.

3. Flexible Conduit Systems

• Flexible Metallic Conduit (BS 731): Constructed from interlocked helically wound galvanised steel strips, often encased in a smooth PVC outer jacket. Used to connect fixed rigid conduit systems to vibrating equipment, electric motors, HVAC fan coils, and luminaires in suspended ceilings.
• Flexible PVC Conduit: Corrugated plastic tubing used for light-duty routing and domestic appliance connections.

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Rigid Steel Conduit: Workshop Tools & Fabrication

Working with rigid steel conduit requires precision measurement, cutting, deburring, threading, and forming.

  ┌─────────────────────────────────────────────────────────────────┐
  │               Steel Conduit Fabrication Workflow                │
  │  1. Measure & Mark   ──► 2. Cut with 32 tpi Hacksaw             │
  │  3. Ream Internal Burrs ──► 4. Cut M20/M25 Threads with Dies    │
  │  5. Form 90° Bend / Offset on Bender ──► 6. Erect & Lock Joints │
  └─────────────────────────────────────────────────────────────────┘

Essential Fabrication Tools

1. Conduit Vice & Stand: A heavy-duty tripod stand fitted with a pipe vice (chain vice or hinged-jaw vice) to clamp the conduit securely during cutting and threading without crushing the cylindrical wall.
2. Hacksaw (32 tpi Blade): Operatives must fit a High-Speed Steel (HSS) blade with 32 teeth per inch (tpi). Standard 18 or 24 tpi blades have tooth pitches wider than the conduit wall thickness (approx. 1.2–1.6 mm), causing teeth to snag, strip, and snap.
3. Pipe Reamer / Half-Round File: Mandatory immediately after sawing. Cutting steel conduit creates razor-sharp internal burrs. An operative must insert a fluted pipe reamer or half-round engineer's file into the conduit bore to remove all burrs and produce a smooth internal chamfer.
4. Stocks and Dies: Metric conduit threads have a standardized pitch of 1.5 mm (distinguishing them from standard metric bolt threads): standard sizes are M20 × 1.5 and M25 × 1.5. Two-part split dies held in a diestock cut clean external threads. The operative must lubricate the die with tallow, cutting compound, or thread-cutting oil, reversing the diestock every half-turn to break the metal swarf.
5. Hand Conduit Bender: A sturdy mechanical tripod bender incorporating a grooved cast-iron former wheel matched to the conduit outside diameter (20 mm or 25 mm) and a grooved roller. The former supports the inner bend radius while the roller supports the outer conduit wall, preventing kinking, flattening, or wall wrinkling.

Bending Geometry and Offset Calculations

• 90-Degree Bends: Formed by pulling the bending handle steadily until the conduit reaches a right angle, allowing a slight overbend to compensate for metal spring-back.
• Sets (Offsets): Two equal and opposite angle bends used to navigate structural projections (such as steel RSJs, brick offsets, or stepping conduit from a flat wall into an accessory knock-out).

                   Conduit Set (Offset) Geometry:
                   ┌─────────────────────────────┐ Upper Plane
                  /  Angle θ
                 / 
                / Distance between marks = H × cosec(θ)
               / Angle θ
  ────────────┘ Lower Plane
  ◄─────────── Offset Height (H) ─────────────►

The distance between the two bend center marks along the conduit is calculated using trigonometry:

Distance Between Marks = Offset Height (H) × cosec(θ)

Example: An electrician needs to form an offset of 75 mm around a structural brick pillar using 30° bends. The distance between bend marks is 75 mm × 2.0 = 150 mm.

• Bridge Saddles: A double offset formed in a single length to allow the conduit to jump over an intersecting pipe or conduit while returning to the same mounting plane.

Conduit Fittings & The Running Coupler

Steel conduit terminates into circular malleable cast-iron conduit boxes (BS 4568): Terminal (1-way), Through (2-way inline), Angle (2-way 90°), Tee (3-way), and Intersection / Cross (4-way).

When two fixed lengths of rigid conduit meet between anchored boxes and cannot be rotated, they are joined using a running coupler:

1. A long thread (approx. 40–50 mm) is cut onto one conduit end.
2. A hexagon locknut followed by a standard conduit coupler is screwed fully onto the long thread.
3. The opposing conduit length (with standard thread length) is abutted tightly against it.
4. The coupler is unscrewed from the long thread and wound halfway onto the opposing conduit until tight.
5. The hexagon locknut is tightened firmly against the back of the coupler, locking the joint rigid and establishing earth continuity.

Steel Conduit as a Circuit Protective Conductor (CPC)

Under BS 7671 Regulation 543.2.1, heavy gauge welded steel conduit can act as the sole CPC for installed circuits, provided:

• All threaded joints are screwed fully home into fittings (metal-to-metal contact with threads free of paint, enamel, and grease).
• Connections to sheet steel distribution boards and accessory back boxes utilize brass smooth-bore male bushes and serrated shakeproof star washers.
• Expansion joints or flexible connections are bridged by external copper bonding links.
• An earth fault loop impedance test (R1 + R2) confirms an exceptionally low resistance path back to the MET.

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Rigid PVC Conduit Installation & Thermal Expansion

High-impact PVC conduit is lightweight, cuts rapidly, and requires no thread cutting. However, its thermal and physical behavior differs markedly from steel.

Cutting and Jointing

• Cutting: Cut cleanly with a fine-toothed hacksaw or ratcheting PVC pipe cutters. Cut ends must be deburred and lightly chamfered.
• Jointing: Joined into moulded PVC socket fittings using solvent cement. Solvent cement chemically softens and fuses the PVC surfaces together, creating a permanent, pressure-tight joint within minutes.

Cold Bending with an Internal Bending Spring

Rigid PVC conduit up to 25 mm diameter is bent cold using a coiled high-tensile steel internal bending spring:

1. Select the correct spring gauge (20 mm or 25 mm, matched to the conduit size).
2. Attach a strong draw wire to the spring eyelet.
3. Insert the spring into the conduit bore until it centers over the proposed bend line.
4. Place the conduit across the knee and bend slowly and smoothly past the desired angle (approximately 10°–15° extra) to compensate for natural plastic spring-back.
5. To withdraw the spring, twist the draw wire anticlockwise while pulling. Twisting anticlockwise slightly coils and reduces the spring diameter, freeing it from the conduit walls.

Thermal Expansion: The 5x Rule

PVC expands approximately five times more than steel. The coefficient of linear thermal expansion for PVC is 0.08 mm/m/°C (compared to 0.012 mm/m/°C for steel).

A 10-meter run of PVC conduit subject to a 25°C seasonal temperature rise will expand by: ΔL = 10 m × 25°C × 0.08 mm/m/°C = 20 mm

If PVC conduit is glued solidly between fixed boxes without expansion provision, thermal expansion will cause the conduit to bow outwards, shear saddles off walls, and crack fittings.

Mandatory Expansion Solutions:

• On straight runs exceeding 6 meters or runs subject to large temperature fluctuations, install expansion couplers.
• An expansion coupler features an internal rubber O-ring or silicone grease seal. The conduit is inserted into the coupler, pushed home, and then withdrawn by 5 mm to 10 mm to leave an expansion gap.
• Saddle fixings within 1 meter of an expansion joint must be loose-fitting (expansion saddles) allowing free longitudinal sliding.

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Conduit Sizing: The Unit Factor Method (On-Site Guide)

Cables generate heat under load and require adequate air space for heat dissipation. Furthermore, pulling too many cables into a conduit damages conductor insulation due to friction and sidewall bearing pressure.

To ensure conduit is not overfilled, the IET On-Site Guide (Appendix E) provides standardized Cable Factor and Conduit Factor tables.

Sizing Rule

The fundamental rule for conduit sizing is: Sum of (Number of Cables × Cable Factor) ≤ Conduit Factor

Conduit factors decrease significantly as the run length increases and as bends are introduced into the run.

Typical Cable Factors (Single-Core Stranded 6491X in Short Straight Runs)

• 1.5 mm² Stranded Conductor: Cable Factor = 31
• 2.5 mm² Stranded Conductor: Cable Factor = 43
• 4.0 mm² Stranded Conductor: Cable Factor = 58
• 6.0 mm² Stranded Conductor: Cable Factor = 88

Typical Conduit Factors for Straight Runs up to 3 Metres

• 20 mm Diameter Conduit: Conduit Factor = 460
• 25 mm Diameter Conduit: Conduit Factor = 800
• 32 mm Diameter Conduit: Conduit Factor = 1400

Worked Example: Can six 2.5 mm² cables and three 1.5 mm² cables be drawn into a straight 3-meter length of 20 mm conduit?

• Total Cable Factor: (6 × 43) + (3 × 31) = 258 + 93 = 351
• 20 mm Conduit Factor: 460
• Verification: 351 ≤ 460. Yes, compliant.