Flexible steel conduit: uses, coated vs uncoated, and how to choose the size

Short answer: Flexible steel conduit (also called flexible metal conduit or spiral steel conduit) is wound from galvanised steel strip and bends by hand. It protects cables against impact, crushing, abrasion and rodents. There are two main types: uncoated (bare galvanised) conduit for dry indoor areas, switchboards and machine connections, and PVC-coated conduit with a black sheath that also keeps out water, dust and oil (IP65) for outdoor and damp locations. Choose the size from the outer diameter of the cables it will carry; the cables should fill no more than about 40% of the conduit's inner cross-section. The ends are connected with conduit glands of the same size.

This guide covers the conduit that protects the cable. For which cable a circuit needs, see our cable size calculation guide.

What is flexible steel conduit?

It is made from thin galvanised steel strip wound into an interlocking helix. That gives it two properties: it resists crushing like rigid conduit, yet bends by hand like a hose. It runs around corners, machine frames and tight spaces without separate elbows.

People search for the same product under different names: flexible metal conduit, steel spiral conduit, spiral cable protector, cable protection hose. In Europe, flexible conduit systems are covered by the IEC/EN 61386 series of standards.

Not the same as a spiral cable wrap

A "spiral cable wrap" or "cable tidy" is something else. It is a plastic coil that bundles loose cables under a desk, behind a TV or around a computer. It tidies things up but does not protect against crushing, water or rodents, and it is not a substitute for conduit in an electrical installation. The steel conduit in this guide is installation material.

Where is it used?

• Machine and motor connections: protects cables running to vibrating or moving equipment. Rigid conduit can crack under vibration; spiral conduit flexes.
• Switchboard outlets: short exposed cable sections from a board to a machine, a light or another box. For board types see our consumer unit guide.
• Lofts and ceiling voids: against rodents gnawing cables and cables being crushed during roof work.
• Outdoors: short runs along a wall, floodlight, wall light and sign connections. Use coated (IP65) conduit here.
• EV charger supply: the last exposed section of the cable between the board and a wall-mounted charger, especially where cars could knock it in a garage or car park.
• Light fitting connections: the short cable section from a junction box to a vapour-proof fitting, floodlight or garden light.
• Car parks, building sites, workshops and solar installations: anywhere cables risk being crushed or hit.

Uncoated vs coated (IP65) conduit

Both share the same skeleton: a galvanised steel spiral. The difference is the outer layer.

• Uncoated (bare galvanised) conduit: silver, bare steel. It protects against impact and crushing, but water and dust can get in between the windings. The uncoated Braytron conduit we stock is IP54. Suitable for dry indoor areas: in and around switchboards, machine connections, ceiling voids, lofts. It is lighter.
• Coated (PVC-sheathed) conduit: the steel spiral is covered with a black sheath. The steel gives mechanical protection, the sheath keeps out water, moisture, dust and oil. The coated Braytron conduit we stock is IP65. Choose it outdoors, in gardens, on facades, in car parks, damp basements, oily machine environments and for EV charger connections.

Simple rule: if water or moisture is possible, use coated; in dry indoor areas where only mechanical protection is needed, use uncoated. For what IP codes mean see IP ratings explained.

Note: to keep IP65, the glands at the ends and the box they enter must have at least the same rating. Sealed conduit connected to a leaky entry is not a sealed system.

What is conduit with a draw wire for?

Conduit with a draw wire has a thin pulling wire running its full length, fitted at the factory. Lay the conduit first, then tie the cable end firmly to the wire and pull it from the other end to draw the cable through.

On long, winding runs, in lofts or where cables will be pulled in later, you do not need a separate fish tape. Several cables can be pulled together. On short, straight sections the cable pushes through easily, so plain conduit is enough. The draw-wire model we stock has a coated (IP65) body.

How to choose the size

Size the conduit by the cable's outer diameter, not its cross-section in mm². Even the same 3×2.5 mm² cable can be thicker or thinner depending on manufacturer and insulation.

1. Find the outer diameter of each cable from its data or measure it with calipers.
2. Single cable: the conduit's inner diameter should be comfortably larger than the cable.
3. Several cables: add up the cable areas (π × diameter² / 4 each). The total should be no more than about 40% of the conduit's inner cross-section. This is the commonly used fill limit for three or more cables; the cables pull easily and can shed heat.
4. If in doubt, go one size up. A cable squeezed into conduit is hard to pull and its insulation gets damaged.

Note: the size in a spiral conduit's name is the nominal size. The inner diameter is smaller by the thickness of the windings and coating, and the sheath also increases the outer diameter. Choose glands and clips by the conduit size, and check cable fit against the inner diameter.

Rough guide:

• A single thin cable (sensor, low voltage, small lighting circuit): 9–14 mm.
• One or a few power cables (sockets, lighting, floodlight): 16–21 mm.
• A heavy supply cable, an EV charger circuit or many cables: 26 mm and up.

The final choice always depends on the real outer diameter of the cable.

Glands and fittings

Never leave the end of the conduit open. Where it enters a junction box, board, motor or light fitting, use a conduit gland. It grips the conduit, stops it pulling out and closes the entry.

• The gland must match the conduit size. Different sizes do not fit each other.
• The gland thread must match the entry hole of the box or device.
• Outdoors and with coated conduit, use a sealed gland with at least the same IP rating as the conduit.
• Cut the conduit with a hacksaw along the spiral and file off the burr; a sharp edge cuts the cable insulation.
• On longer runs, fix the conduit to the wall with clips at intervals; sagging conduit looks poor and puts strain on the cable.

We do not sell conduit glands; buy them separately in the size of your conduit.

Important note on earthing

Steel conduit is metal, but it is not a protective (earth) conductor. IEC 60364-5-54 does not allow flexible metal conduit to be used as a protective conductor. The cable must always contain its own earth core. For earthing basics see earth wire and earthed sockets.

Compared with plastic and rigid conduit

• Plastic flexible conduit (PVC): light, cheap and easy to bend. Used in plaster and inside walls where mechanical risk is low. Much weaker than steel against crushing and rodents.
• Rigid PVC conduit: for straight runs and a neat look on walls. Bends need elbows and fittings. Not suitable for vibrating machine connections.
• Rigid steel conduit: the highest mechanical protection, for industrial sites and areas very exposed to impact. Needs bending and threading, so installation is laborious.
• Flexible steel conduit: sits between the two. Much tougher than plastic, and no elbows like rigid conduit. Ideal for short, winding and vibrating sections.

A common layout: long straight runs in rigid conduit or trunking, and the last flexible section to the machine, light fitting or device in steel spiral conduit.

Outdoor and EV charger use

For garden and facade lighting, coated conduit protects the exposed part of a floodlight or wall-light connection from sun, water and knocks. See facade lighting and how to light a garden.

An EV charger supply cable is thick, so you may need 26 mm or more; measure the real cable diameter before choosing. In garages and car parks, conduit adds protection on low sections a car could hit. For how a charger circuit is planned see home charger installation cost.

Spiral conduit is not designed for long buried runs; underground, use suitable buried cable and ducting made for that job.

Spiral conduit in old installations

Older homes often have exposed surface cables or unprotected cables in the loft. If the cable is old, assess the cable itself first; conduit does not make a worn cable safe. See home rewiring.

Pulling cables and connecting boxes and boards is dangerous; the work must be done by a qualified electrician, and insulation and earth tests carried out afterwards.

Flexible steel conduit in our shop

We stock three types of Braytron flexible steel conduit. All have a galvanised steel body, an operating temperature of −25 / +90 °C and are sold by the reel: 50 m reels for 9–21 mm, 25 m reels for 26–50 mm.

• Braytron uncoated steel conduit: galvanised, IP54; 9, 11, 14, 16, 18, 21, 26, 32, 37, 42 and 50 mm.
• Braytron coated steel conduit: black sheath, IP65; the same 11 sizes from 9 to 50 mm.
• Braytron coated steel conduit with draw wire: IP65; 10 sizes from 11 to 50 mm.

• → Flexible steel conduit category
• → Braytron uncoated steel conduit
• → Braytron coated steel conduit, IP65
• → Braytron coated conduit with draw wire