What is the difference between heavy gauge and light gauge (BESA) steel conduit?

Heavy gauge steel conduit (also called Class 3 or heavy wall) is manufactured to BS EN 61386-21 and is the standard for fixed electrical installation work. It has substantial wall thickness that allows it to be threaded and bent without collapsing, and provides the mechanical protection and earth continuity properties required by BS 7671. Light gauge conduit (BESA conduit, named after the British Engineering Standards Association) has thinner walls and is primarily used for surface-mounted flexible conduit drops to light fittings and equipment — it is not suitable for fixed wiring runs where mechanical protection or earth continuity is required.

Can steel conduit be used as the circuit protective conductor?

Yes, provided the conduit system is continuous, all joints are mechanically sound and electrically continuous, and the conduit is bonded and earthed at the origin of the installation. Under BS 7671, the steel conduit may serve as the CPC for circuits installed within it, provided the earth fault loop impedance measured at the furthest point meets the requirements of BS 7671 Table 41.2 for the relevant protective device. In practice, many installers include a separate earth conductor within the conduit as additional assurance, particularly on longer runs where conduit earth impedance may be marginal.

What is the minimum bend radius for conduit?

The minimum internal radius of a conduit bend is specified in BS EN 61386. For heavy gauge steel conduit, the minimum internal bend radius is 2.5× the conduit internal diameter for machine-formed (bender-formed) bends. Hand-formed bends in flexible conduit may have tighter minimum radii depending on the conduit type. Under-radius bends crush or kink the conduit, reduce the internal bore, and make cable drawing extremely difficult or impossible. A good conduit bend should maintain the full internal bore throughout.

How many cables can I install in a conduit?

The number of cables that can be installed in a conduit is limited by the conduit fill ratio. BS 7671 Appendix 4 Table 4J1 specifies the space factor method for determining conduit fill. As a practical guide, the total cross-sectional area of cables (including insulation) should not exceed 40% of the conduit internal cross-sectional area. This ensures cables can be drawn in without damage and allows for future additions. Exceeding this ratio makes cable drawing very difficult and can cause cable sheath damage.

Why does PVC conduit need expansion couplings?

PVC has a relatively high coefficient of thermal expansion — approximately 0.06mm per metre per degree Celsius. On a 10m PVC conduit run between fixed boxes, a temperature change of 30°C will cause the conduit to expand or contract by approximately 18mm. Without expansion couplings (slip couplings), this thermal movement can cause the conduit to buckle, pull out of boxes at joints, or crack fittings. Expansion couplings allow the conduit to slide axially while remaining watertight. They should be fitted at intervals of approximately 6m on straight runs and at every change of temperature zone.

Do I need to earth PVC conduit?

No. PVC conduit is non-conductive and cannot carry an earth path. PVC conduit does not need to be earthed and cannot serve as a circuit protective conductor. Cables installed in PVC conduit must each have their own CPC (the earth core within the cable) as required by BS 7671. PVC conduit boxes and accessories are also non-conductive — metal earthing is not required at these points. This is one of the practical advantages of PVC conduit over steel, as the earthing requirement is simplified.

What is the correct way to fire stop conduit penetrations?

When conduit passes through a fire-rated wall, floor, or ceiling, the annular gap between the conduit and the structure must be sealed with an approved intumescent fire stopping system. The conduit itself does not provide fire separation — the empty bore acts as a path for flame and smoke. Approved systems include intumescent putty, fire-rated mastics, and collar devices that expand in heat to crush and seal the conduit bore. The installed system must be documented on a fire stopping schedule. Fire stopping must be installed in accordance with the manufacturer's instructions and by a competent person.

INSTALLATION GUIDE

Conduit Installation Guide UK: Steel and Plastic Conduit Wiring

The complete guide to conduit installation — heavy gauge steel, light gauge, and PVC conduit; bending; draw wires; thread cutting; expansion couplings; earthing steel conduit as a CPC; and fire stopping at penetrations.

Start 7-day free trial

Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians

14 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.

ShareX in W

1,000+

UK electricians

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer — DP Electrical

Key Takeaways

1Steel conduit (heavy gauge to BS EN 61386-21) provides excellent mechanical protection for cables and can itself serve as the circuit protective conductor when continuously threaded and bonded, but must be correctly earthed at both ends.
2PVC conduit is lighter, cheaper, and easier to cut and join than steel, but cannot serve as a CPC and provides no earthing function. It is suitable for surface wiring in domestic and light commercial applications.
3The minimum bend radius for conduit is specified in BS EN 61386 — typically 2.5× the conduit internal diameter for machine-bent heavy gauge steel conduit. Under-radius bends crush the conduit and make cable drawing impossible.
4Expansion couplings (slip couplings) must be fitted in steel and PVC conduit runs at intervals where the run crosses a structural expansion joint, and in PVC conduit runs exceeding approximately 6m, to accommodate thermal movement.
5All penetrations through fire-rated walls, floors, and ceilings must be fire stopped with an approved intumescent system — conduit alone does not maintain fire compartmentation and can act as a chimney for flame and smoke.

01 · Installation Guide

Types of Conduit Used in UK Electrical Installations

Conduit is a hollow tube used to enclose and protect electrical cables. It provides mechanical protection, allows cables to be drawn in and out without disturbing the building fabric, and — in the case of steel conduit — provides an earth path for the cables within. Conduit is used in domestic, commercial, and industrial installations across the UK.

Heavy gauge steel conduit (Class 3) — the standard for commercial and industrial fixed wiring. Manufactured to BS EN 61386-21. Threaded at joints and into boxes. Can be bent with a conduit bender to form sets, right-angle bends, and offsets. Can serve as the CPC when correctly earthed. Standard sizes: 16mm, 20mm, 25mm, 32mm, 50mm OD.
Light gauge steel conduit (BESA) — thinner wall, lighter weight. Used for flexible conduit drops to luminaires and equipment rather than fixed wiring runs. Not suitable for threading and not recommended for use as a CPC.
PVC conduit (round and oval) — lightweight, non-conductive, corrosion-resistant. Easier to cut and join than steel. Cannot serve as a CPC. Used extensively in domestic surface wiring and commercial skirting and trunking installations. Oval conduit is used for chasing into plaster in domestic first fix.
Flexible conduit — steel wire armoured flexible tube used for final connections to motors, luminaires, and equipment where vibration or movement is present. Must be used with appropriate end fittings for mechanical and earth continuity.

All conduit installations must comply with BS 7671:2018+A4:2026 and BS EN 61386 (conduit systems for cable management).

Free download

Get the BS 7671 A4:2026 Cheat Sheet — free

Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.

Every regulation change summarised
New model forms (EIC + MEIWC)
Free PDF — no subscription

02 · Installation Guide

Heavy Gauge Steel Conduit — Properties and Uses

Heavy gauge steel conduit is the workhorse of commercial and industrial electrical wiring. Its combination of mechanical protection, earthing capability, and long service life makes it the standard specification for industrial premises, schools, hospitals, and commercial buildings where durability is paramount.

Mechanical protection — steel conduit provides the highest level of mechanical protection for cables, suitable for areas subject to impact, vandalism, or accidental damage. It is the preferred system in factories, warehouses, and exposed industrial areas.
Earthing function — a correctly installed and continuously threaded steel conduit system can serve as the circuit protective conductor (CPC/earth) for circuits within, per BS 7671 Regulation 543. All joints must be electrically continuous.
Finishes — bright (uncoated, for internal dry use), black enamel (additional corrosion protection), and hot-dip galvanised (for outdoor, damp, or humid environments). Stainless steel conduit is available for highly corrosive environments.

03 · Installation Guide

PVC Conduit — Round and Oval Types

PVC conduit is widely used in domestic and light commercial electrical installations in the UK. It is cheaper, lighter, and easier to work with than steel conduit, but does not provide earthing capability and has lower mechanical strength.

Round PVC conduit — standard surface-mounted conduit for domestic and commercial skirting wiring, garage wiring, and utility rooms. Solvent-welded fittings for permanent joints. Standard sizes: 16mm, 20mm, 25mm, 32mm OD.
Oval PVC conduit — used for chasing into plaster in domestic first fix wiring. The oval profile is lower profile in a plaster chase than round conduit and is easier to cover with plaster. Available in 16mm × 4mm and 25mm × 16mm nominal sizes.
Expansion couplings required — in PVC conduit runs exceeding approximately 6m, expansion (slip) couplings must be fitted to accommodate thermal movement. Failing to include expansion couplings will result in buckling, cracking, or pull-out at box connections.

04 · Installation Guide

Bending Conduit — Radius Requirements and Technique

Conduit bending is a fundamental skill for any electrician working with steel conduit systems. A good bend maintains the full internal bore of the conduit, allowing cables to be drawn through easily after installation.

Minimum bend radius — for heavy gauge steel conduit, the minimum internal radius is 2.5× the conduit internal diameter (approximately 4× the external diameter). For 20mm OD conduit this gives a minimum internal radius of approximately 40mm. Tighter bends crush the conduit and make cable drawing impossible.
Conduit bender types — hand-operated benders (hickey benders) for 16mm and 20mm conduit; hydraulic or ratchet benders for 25mm and above. Always use a bender shoe matched to the conduit size — an undersized shoe will kink the conduit.
Right-angle bends — a 90° bend with a smooth radius. Mark the starting point of the bend on the conduit, insert into the bender at the mark, and apply steady pressure until the conduit reaches 90°. Verify with a set square. Overbent conduit can be gently straightened but will have internal deformation.
Sets and offsets — used to move the conduit run out of plane to clear obstacles. A set is two bends in opposite directions. An offset is a set where the conduit returns to parallel with the original run. Accurate measurement and marking is essential for neat sets.
PVC conduit bending — PVC conduit can be bent using a spring bender (inserted inside the conduit) or a hot-air gun. Spring benders prevent the PVC from kinking during bending. Heat bending allows gentle curves but requires practice to avoid flattening the conduit.

05 · Installation Guide

Draw Wires and Threading Cables Through Conduit

Draw wires (also called fish wires or pulling lines) are essential for threading cables through conduit runs, particularly in embedded or inaccessible conduit where the cable cannot be pushed through from one end.

Leave a draw wire — always leave a draw wire in the conduit during installation, even if cables are being installed immediately. The draw wire allows future cables to be pulled in without difficulty, particularly in long or bent runs.
Draw wire materials — galvanised steel draw wire (traditional), nylon draw tape (lighter and less likely to kink), and glass-fibre draw rods (for pushing through conduit when a draw wire is not present). Fish tapes are flexible flat steel strips wound on a reel for reaching through long runs.
Cable lubricant — apply cable pulling lubricant (pulling compound) to cables before drawing through long runs. This reduces friction significantly and prevents cable sheath damage on bends. Do not use grease, oil, or washing-up liquid — these can degrade cable insulation.

06 · Installation Guide

Junction Boxes and Conduit Outlet Boxes

Junction boxes (inspection elbows, tees, and boxes) are an essential part of a conduit system, providing access for drawing cables and housing connections.

Inspection boxes — steel conduit boxes with removable covers allow access to the conduit run for cable drawing. BS 7671 requires that all joints and connections be accessible for inspection and maintenance. Inspection boxes should be positioned at bends and at regular intervals on long straight runs.
Maximum run between draw boxes — in practice, the maximum run between access points should not exceed approximately 10m on straight runs, reducing to 5m on runs with multiple bends. Longer uninterrupted runs make cable drawing extremely difficult.
Steel vs plastic boxes — steel (BESA) back boxes for steel conduit runs; plastic back boxes for PVC conduit. The box depth must accommodate the conductors and any connections made within. Select box depth for the number and size of conductors.

Try Elec-Mate free for 7 days

16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.

Start free trial

07 · Installation Guide

Thread Cutting for Steel Conduit

Steel conduit is joined at conduit boxes, couplers, and locknuts using threaded connections. Thread cutting is a necessary skill for steel conduit installation work.

Die stock and dies — steel conduit threads are cut with a die stock holding the correct size die (16mm, 20mm, 25mm, 32mm). The die is run onto the cleaned and de-burred conduit end using cutting oil to lubricate and carry away swarf.
Thread length — cut a minimum of 5 full thread turns for engagement in a standard coupler or box entry. Cut more for junction boxes with locknuts — the thread must engage fully with both the box entry and the locknut.
Earth continuity at threads — for steel conduit to serve as the CPC, threaded connections must be mechanically tight and electrically continuous. Loose or corroded threads create high-resistance earth paths. Check earth continuity throughout the completed installation.

08 · Installation Guide

Expansion Couplings for Long Conduit Runs

Thermal expansion and contraction of conduit runs must be accommodated to prevent buckling, box pull-out, and mechanical damage.

PVC conduit — expansion (slip) couplings required at approximately every 6m on straight runs and wherever the conduit crosses a temperature change zone. PVC has a high coefficient of thermal expansion and will buckle without adequate expansion provisions.
Steel conduit — expansion couplings are required wherever conduit crosses a structural expansion joint in a building, or in long exposed runs subject to large temperature variations (e.g., external conduit between buildings). In indoor commercial installations, steel conduit runs rarely require expansion couplings due to the low coefficient of expansion and controlled internal environment.
Earth bond across expansion coupling — where an expansion coupling is fitted in a steel conduit run used as a CPC, a separate earth bond conductor must be fitted across the expansion coupling to maintain earth continuity. The expansion coupling itself does not maintain a reliable earth path as it is designed to slide.

09 · Installation Guide

Earthing Steel Conduit

Steel conduit must be earthed as a system. Where it serves as the CPC, its earth impedance must be verified to meet BS 7671 requirements for the protective device installed.

System earth connection — the conduit system must be connected to the main earthing terminal at the origin of the installation. The connection point must be mechanically secure and electrically continuous.
Earth continuity testing — after completion, measure earth continuity from the furthest conduit box back to the MET using a low-resistance ohmmeter. The resistance must be low enough that the earth fault loop impedance at the furthest point meets BS 7671 Table 41.2 for the protective device.
Common failure points — loose threaded connections, corroded joints, painted or coated conduit surfaces at connections, and locknuts not fully tightened. Insulating washers accidentally fitted at box entries will break the earth path.

10 · Installation Guide

Fire Stopping at Conduit Penetrations

Any penetration through a fire-rated structure by conduit — or by cables within conduit — must be fire stopped to maintain the fire compartmentation of the building. This is a Building Regulations requirement and a matter of life safety.

Conduit alone is not fire stopping — steel and PVC conduit both have hollow bores that allow flame and combustion gases to pass through a fire-rated structure. The conduit must be sealed with an approved fire stopping product.
Approved products — intumescent putty, fire-rated mastic, intumescent collar devices (for PVC conduit — the collar expands in heat to crush and seal the conduit), and proprietary fire stop block systems. Each product must be installed per the manufacturer's instructions and data sheet.
Documentation — fire stopping must be recorded on a fire stopping schedule, giving the location, product used, installer, and date of installation. This documentation forms part of the building's fire safety file under the Building Regulations.

11 · Installation Guide

For Electricians: Certifying Conduit Installations

Conduit installations must be certified with the appropriate Electrical Installation Certificate, including earth continuity verification for steel conduit systems used as CPCs, and earth fault loop impedance measurements at the furthest points of the installation.

Certify Conduit Installations on Site

Use the Elec-Mate EIC app to complete and issue Electrical Installation Certificates for conduit wiring installations. Record earth continuity, insulation resistance, and earth fault loop impedance results on site — instant PDF export.

Complete conduit installation certificates on your phone

Elec-Mate's EIC and EICR apps help you certify commercial conduit wiring installations on site.

Try it free for 7 days

Frequently Asked Questions About Conduit Installation

What electricians say

Verified reviews from the UK App Store.

One App for Everything!

Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.

Fantastic app for electricians

I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.

Absolutely amazing

I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.

Trusted by electricians across the UK

Real feedback from real sparks

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer

Sole Trader · DP Electrical

“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”

Nathan Perry

Electrician · NP Electrical Services

“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”

Jake Pizey

3rd Year Apprentice · Apprentice

7-Day Free Trial — Cancel Anytime, No Hassle

Complete Conduit Installation Certificates on Your Phone

Join 1,000+ UK electricians using Elec-Mate to certify wiring installations on site. Record earth continuity, loop impedance, and test results — instant PDF export. 7-day free trial.

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer, DP Electrical

From £6.99/mo after trial — less than a coffee a week

or download the app

7 days free, then from £6.99/moCancel in one tap — no calls, no hassleiOS, Android & WebBS 7671 compliant

Conduit Installation Guide UK: Steel and Plastic Conduit Wiring

Key Takeaways

Types of Conduit Used in UK Electrical Installations

Get the BS 7671 A4:2026 Cheat Sheet — free

Heavy Gauge Steel Conduit — Properties and Uses

PVC Conduit — Round and Oval Types

Bending Conduit — Radius Requirements and Technique

Draw Wires and Threading Cables Through Conduit

Junction Boxes and Conduit Outlet Boxes

Try Elec-Mate free for 7 days

Thread Cutting for Steel Conduit

Expansion Couplings for Long Conduit Runs

Earthing Steel Conduit

Fire Stopping at Conduit Penetrations

For Electricians: Certifying Conduit Installations

Certify Conduit Installations on Site

Complete conduit installation certificates on your phone

Frequently Asked Questions About Conduit Installation

What is the difference between heavy gauge and light gauge (BESA) steel conduit?

Can steel conduit be used as the circuit protective conductor?

What is the minimum bend radius for conduit?

How many cables can I install in a conduit?

Why does PVC conduit need expansion couplings?

Do I need to earth PVC conduit?

What is the correct way to fire stop conduit penetrations?

Continue reading

Cable Tray Installation

Wire Cable Basket Installation

Armoured Cable (SWA) Installation

FP200 Gold Cable Guide

EICR Certificate App

What electricians say

One App for Everything!

Fantastic app for electricians

Absolutely amazing

Trusted by electricians across the UK

Complete Conduit Installation Certificates on Your Phone