What is the difference between BS EN 61386 conduit classification codes?

BS EN 61386 uses a four-digit classification system. The first digit (1–5) indicates mechanical compression resistance: 1 = very light, 2 = light, 3 = medium, 4 = heavy, 5 = very heavy. The second digit covers the impact resistance (1–5 similarly graded). The third digit covers resistance to heat and cold, and the fourth covers electrical properties including continuity. A standard heavy-duty steel conduit might be classified 4445, indicating very heavy compression, very high impact resistance, wide operating temperature range, and continuous protective conductor capability. When specifying conduit for a project, the classification must match the environmental and mechanical demands of the installation.

How do I calculate how many cables I can fit in a conduit?

BS 7671 Appendix 5 provides cable factor tables for conduit and trunking fill. Each cable size has a factor value, and each conduit size has a maximum total factor. For example, a 20mm conduit has a factor of 460 for straight runs but only 294 for runs with one bend. A 2.5mm² single core PVC cable has a factor of around 30 to 35 depending on the conductor type. Divide the conduit factor by the cable factor to find the maximum number of cables. Always allow spare capacity — aim for no more than 45% of conduit space to be occupied, leaving room for future additions and heat dissipation.

Can I use PVC conduit outdoors?

PVC (polyvinyl chloride) conduit can be used outdoors provided it is correctly rated for UV exposure, temperature variation, and mechanical impact. Standard PVC conduit is generally not suitable for direct burial without protection. For outdoor above-ground runs, use conduit rated to at least IP44 and consider UV-stabilised grades for long-term exposure. For underground runs, use heavy-duty orange ducting conforming to BS EN 61386-24 for buried conduit, installed at a minimum depth of 600mm under garden areas and 750mm under roads or driveways per BS 7671 Regulation 522.8.10.

Does steel conduit need to be earthed?

Yes. When steel conduit is used as a circuit protective conductor (CPC), it must provide a continuous metallic path throughout the entire run. All joints must be screwed (not push-fit), all boxes must be metal, and continuity must be verified by testing. The cross-sectional area of steel conduit as a CPC is assessed against BS 7671 Table 54.7 — a 20mm conduit provides approximately 35mm² effective CPC cross-section for steel. Where continuity cannot be guaranteed (e.g. flexible conduit sections), a separate CPC must be run inside. Even where a separate CPC is provided, exposed metalwork should still be bonded.

How do I bend conduit without collapsing it?

For steel conduit, a proper pipe bending machine (hickey bender for 20mm and 25mm, or a hydraulic bender for larger sizes) is essential. Fill the conduit with dry sand and cap both ends before bending by hand if no bender is available — this prevents the pipe from collapsing. The minimum bending radius for steel conduit is typically six times the outside diameter. For PVC conduit, use a spring bender inserted inside the conduit to support the walls, or gently heat the conduit with a hot air gun to soften it before bending. Never use a naked flame. Allow PVC to cool in the bent position before removing the spring.

What fire stopping is required where conduit passes through walls?

Where conduit penetrates a fire-resisting wall, floor, or ceiling, the penetration must be fire-stopped to maintain the fire resistance of the element. This is required under Building Regulations Approved Document B (England) and the equivalent Scottish and Welsh regulations. Suitable methods include intumescent putty pads, intumescent collars (for PVC conduit), or mineral wool with intumescent mastic. The fire stopping system must be rated to match the fire resistance of the element — typically 30, 60, or 90 minutes. Steel conduit is less problematic than PVC as it does not melt, but any gaps around the conduit must still be filled. Product data sheets should be retained as evidence for building control.

What trunking fill percentage is recommended under BS 7671?

BS 7671 Appendix 5 specifies cable factors for trunking fill. The space factor — the ratio of the sum of the effective cross-sectional areas of all cables to the internal cross-sectional area of the trunking — should not exceed 45% for new installations. This 45% limit allows adequate space for heat dissipation, for cables to lie without damage, and for future additions. Where multi-core cables of different sizes are mixed, calculate the total cable factor using the individual cable factors from the appendix tables and compare against the trunking factor for the selected size. Mini trunking (e.g. 16×16mm, 25×16mm) has published factors in manufacturers’ data and Appendix 5.

INSTALLATION GUIDE

Electrical Installation Conduit — Complete UK Guide

Steel vs PVC conduit, BS EN 61386 classification, trunking fill calculations per BS 7671 Appendix 5, earthing requirements, IP ratings, and fire stopping — everything electricians need for compliant conduit installations.

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10 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Conduit is selected by material (steel or PVC), type (rigid or flexible), and installation environment. BS EN 61386 governs the performance classification of conduit systems using a four-digit code covering mechanical, thermal, electromagnetic and electrical properties.
2Fill calculations for conduit and trunking must comply with BS 7671:2018+A4:2026 Appendix 5 and the associated cable factor tables. Overfilling conduit creates heat build-up and makes future cable changes impractical.
3Steel conduit used as a circuit protective conductor (CPC) must be continuous throughout, with screwed fittings only — no push-fit joints are permissible. Cross-sectional area must be verified against BS 7671 Table 54.7.
4Outdoor and underground conduit installations require appropriate IP-rated fittings and boxes. Buried conduit must meet BS 7671 Regulation 522.8.10 regarding depth and mechanical protection.
5Fire stopping around conduit penetrations through fire-resisting structures is a legal requirement under Building Regulations Approved Document B and must be carried out with intumescent materials rated to match the element being penetrated.

01 · Installation Guide

Steel vs Plastic Conduit — Choosing the Right System

Conduit is one of the most widely used cable management systems in UK electrical installations, providing mechanical protection, a tidy finish, and — in the case of steel conduit — a circuit protective conductor path. The choice between steel and PVC conduit depends on the environment, mechanical demands, and whether the conduit is to serve as the CPC.

Steel Conduit

Material: Hot-dip galvanised or stove-enamelled mild steel. Standard sizes: 16mm, 20mm, 25mm, 32mm, 38mm, 50mm outside diameter. Conforms to BS EN 61386-21 (rigid steel conduit).
Advantages: Superior mechanical protection, fire resistance, can serve as CPC, EMC shielding for sensitive circuits, suitable for hazardous areas (with appropriate fittings), long service life.
Disadvantages: Heavier, more time-consuming to install, requires screwed fittings, susceptible to corrosion in wet environments unless stainless steel or galvanised grade is specified.
Typical applications: Industrial premises, plant rooms, exposed runs in commercial buildings, areas subject to mechanical impact.

PVC (Rigid Plastic) Conduit

Material: Unplasticised PVC (UPVC). Standard sizes: 16mm, 20mm, 25mm, 32mm outside diameter. Conforms to BS EN 61386-22 (rigid non-metallic conduit).
Advantages: Lightweight, corrosion-resistant, faster to install, lower material cost, push-fit or solvent-welded fittings available, no metallic continuity issues.
Disadvantages: Cannot serve as CPC (separate CPC required), less impact-resistant than steel, becomes brittle at low temperatures, melts in fire, requires separate bonding to exposed metalwork.
Typical applications: Domestic surface wiring, office installations, concealed runs in dry environments, underground ducts (heavy-duty grade).

Performance classification under BS EN 61386 uses a four-digit code. For most commercial and industrial applications, specify conduit classified to at least medium mechanical protection (third digit 3 or above). Always check the manufacturer data sheet against the installation environment requirements.

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02 · Installation Guide

Trunking Fill Calculations (BS 7671 Appendix 5)

Overloading conduit or trunking with too many cables is one of the most common installation defects identified on EICRs. The fill limits in BS 7671 Appendix 5 exist to ensure cables can dissipate heat safely and can be withdrawn and replaced without damage.

How to Calculate Conduit Fill

Step 1: Identify the cable type and conductor size for each cable to be installed. Look up the cable factor from BS 7671 Appendix 5 Table 5A (single-core PVC), Table 5B (multi-core PVC), or equivalent.
Step 2: Add up the cable factors for all cables to be installed in the conduit. This gives the total cable factor.
Step 3: Compare the total cable factor against the conduit factor from Table 5C. Use the straight run column for runs without bends, or the bend column if one or more bends are present. Select the conduit size whose factor exceeds the total cable factor.
Example: Six 2.5mm\u00b2 single-core PVC cables (factor 30 each) in a straight run = total factor 180. A 20mm conduit has a straight-run factor of 460. A 16mm conduit has a factor of 290. Either would accommodate the cables, but 20mm is preferred to allow future capacity.

Derating for grouping: Where cables are grouped together, the current-carrying capacity of each cable must be derated using BS 7671 Table 4C1 (for cables in conduit) or Table 4B1 (for trunking). This is separate from the fill calculation — both must be applied.
Trunking space factor: For cable trunking, BS 7671 Appendix 5 specifies that the ratio of the sum of cable cross-sectional areas to the internal trunking cross-sectional area (the space factor) should not exceed 45%. This applies to all new installations.

Always document your fill calculations as part of the installation records. If completing an , note the conduit sizes and cable quantities in the circuit schedule.

03 · Installation Guide

Bending and Cutting Techniques

Neat, correctly made bends are a mark of professional conduit installation. Poorly formed bends reduce the conduit internal cross-section, making cable pulling difficult and risking cable damage. The following techniques apply to both steel and PVC conduit.

Steel Conduit Bending

Use a purpose-made conduit bender (hickey bender) for 20mm and 25mm. For 32mm and above, a hydraulic bender is recommended. The minimum bending radius is 2.5 times the outside diameter for metal conduit per BS EN 61386.
Mark set distances accurately with a chinagraph pencil or felt tip. For a right-angle bend, measure from the end of the conduit to the centre of the bend, subtract the take-up (typically 100mm for 20mm conduit), and mark the bending point.
Cut steel conduit with a pipe slice or junior hacksaw. Deburr all cut ends with a conduit reamer before drawing cables — sharp edges will damage cable insulation. Bush all conduit entries to boxes with PVC or rubber bushes.

PVC Conduit Bending

Insert a correctly sized conduit spring inside the conduit before bending. For cold bending, lever the conduit gently over your knee — do not rush or the conduit will kink. Remove the spring by attaching a length of cord before insertion and pulling it free after bending.
For warm bending, apply a hot air gun evenly along the bend area (approximately 100mm length for 20mm conduit). Rotate the conduit to heat evenly. When pliable, bend over a former and hold until cool. Never use a naked flame.
Cut PVC conduit with a junior hacksaw or pipe cutter. Deburr with a file or deburring tool. Use solvent cement or push-fit couplers for jointing. Ensure push-fit joints are fully engaged — a partial joint will leak in wet environments and is mechanically weak.

04 · Installation Guide

Earthing of Steel Conduit

Steel conduit can be used as the circuit protective conductor (CPC) for circuits it contains, provided specific requirements under BS 7671:2018+A4:2026 Regulation 543 are met. This is a legitimate and widely used practice in industrial and commercial installations, but requires careful attention to jointing and continuity.

Screwed fittings only: Every joint in a conduit system used as a CPC must be made with screwed couplers and screwed-entry boxes. Push-fit or clip-together fittings do not provide reliable electrical continuity and must not be used where the conduit is the CPC.
Flexible conduit sections: Flexible conduit (also known as Anaconda or SWA flexible) does not provide reliable CPC continuity. A separate insulated CPC must be run inside flexible sections and connected at both ends.
Continuity testing: After installation, verify end-to-end continuity of the conduit system as a CPC. The resistance measured should be consistent with the cross-sectional area of the conduit and the length of run. Test at each circuit termination point.
Cross-sectional area check: Verify that the steel conduit provides sufficient CPC cross-sectional area per BS 7671 Table 54.7 for the phase conductor size. A 20mm steel conduit provides approximately 35mm\u00b2 equivalent CPC cross-section.

Where conduit runs are long or the fault current could be high, consider installing a separate insulated CPC inside the conduit in addition to relying on the steel for continuity. Record the CPC type used on the circuit schedule of the .

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05 · Installation Guide

IP Ratings and Environmental Protection

Ingress Protection (IP) ratings define the degree of protection provided by conduit fittings and boxes against solid particles and liquids. The rating is defined in BS EN 60529 and consists of two digits — the first for solid particle protection (0–6), the second for liquid ingress protection (0–9).

IP2X: Protection against fingers and solid objects over 12mm. Minimum requirement for accessible indoor wiring accessories.
IP44: Dust protection (solid objects over 1mm) and splash protection from any direction. Minimum for outdoor above-ground conduit fittings and boxes in exposed locations.
IP65: Complete dust exclusion and protection against water jets from any direction. Required for conduit boxes in car washes, external walls exposed to direct rain, and food production areas.
IP67/IP68: Complete dust exclusion and temporary/permanent submersion protection. Required for underground junction boxes and conduit systems installed below ground level.

BS 7671 Regulation 522.3 requires that the installation method used provides adequate protection for the environment. Select conduit, fittings and boxes with IP ratings appropriate to the worst conditions they will encounter during the lifetime of the installation, not just during commissioning.

06 · Installation Guide

Fire Stopping Requirements

Every penetration through a fire-resisting wall, floor, or ceiling must be fire-stopped to maintain the element's fire resistance rating. This is a legal requirement under Building Regulations Approved Document B (England), Section 7 of the Technical Handbooks (Scotland), and Part B of the Building Regulations (Wales and Northern Ireland).

Steel conduit penetrations: Steel conduit does not melt or combust, so the main concern is any annular gap around the conduit. Pack with mineral wool and seal with intumescent mastic rated to match the wall's fire resistance (30, 60, or 90 minutes). Some manufacturers provide tested steel conduit sealing systems.
PVC conduit penetrations: PVC melts in fire, leaving an open hole. Intumescent collars must be fitted around PVC conduit at every penetration through a fire-resisting element. The collar expands on heating, sealing the gap left by the melting conduit. Use products with third-party test evidence to the required fire resistance period.
Record keeping: Retain product data sheets, installation instructions, and photographs as evidence of compliance. Building control may request this evidence during inspections, and it is invaluable if a subsequent EICR or fire risk assessment raises questions.

07 · Installation Guide

For Electricians — Documenting Conduit Installations

Conduit installations require an Electrical Installation Certificate (EIC) for new circuits and an EICR observation where defects are identified. Elec-Mate provides mobile-optimised certificate tools to make paperwork fast and compliant on-site.

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— document overfilled conduit, missing earth continuity, or absent fire stopping as C2 or C3 observations with full supporting evidence.
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Electrical Conduit Installation — Frequently Asked Questions

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Electrical Installation Conduit — Complete UK Guide

Key Takeaways

Steel vs Plastic Conduit — Choosing the Right System

Steel Conduit

PVC (Rigid Plastic) Conduit

Get the BS 7671 A4:2026 Cheat Sheet — free

Trunking Fill Calculations (BS 7671 Appendix 5)

How to Calculate Conduit Fill

Bending and Cutting Techniques

Steel Conduit Bending

PVC Conduit Bending

Earthing of Steel Conduit

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IP Ratings and Environmental Protection

Fire Stopping Requirements

For Electricians — Documenting Conduit Installations

Electrical Conduit Installation — Frequently Asked Questions

What is the difference between BS EN 61386 conduit classification codes?

How do I calculate how many cables I can fit in a conduit?

Can I use PVC conduit outdoors?

Does steel conduit need to be earthed?

How do I bend conduit without collapsing it?

What fire stopping is required where conduit passes through walls?

What trunking fill percentage is recommended under BS 7671?

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