Can I use the cable factor method for trunking as well as conduit?

The cable factor method as defined in IET Guidance Note 1 is specifically designed for conduit systems. For trunking (cable ducting), BS 7671 uses the space factor method with a 45% limit for cables laid in trunking. Trunking is different from conduit because cables are typically laid in rather than pulled through, so the pulling friction that the cable factor method accounts for is not relevant. The 45% space factor for trunking is calculated from cable and trunking cross-sectional areas.

What conduit types does BS EN 61386 cover?

BS EN 61386 (the European standard for conduit systems, adopted in the UK) covers rigid PVC conduit, flexible PVC conduit, rigid metallic conduit (steel and aluminium), flexible metallic conduit, and composite conduit systems. Each type has defined dimensions including internal diameter, external diameter, and minimum bending radius. The conduit factor tables in IET Guidance Note 1 provide separate factors for PVC (Table 4D) and metallic (Table 4E) conduit, as their different surface friction characteristics affect cable-pulling difficulty.

Is the cable factor method mandatory or can I always use the 45% rule?

Both methods are permitted under BS 7671. Regulation 522.8.1 provides the 45% space factor as a general requirement, and the cable factor method from IET Guidance Note 1 is an accepted and more detailed design approach. In practice, the cable factor method is recommended for any installation with significant run lengths or multiple bends, as the 45% rule may allow a conduit that is technically within the space factor limit but practically impossible to wire due to pulling friction. Many electrical contractors and design engineers use the cable factor method as standard practice and reserve the 45% rule for quick site estimates.

Conduit Fill Calculator — BS 7671 🇬🇧

United KingdomEdition 2018+A2:2022Free Online Tool

BS 7671:2018+A2:2022 (the IET Wiring Regulations) takes a fundamentally different approach to conduit sizing compared to the simple percentage-based methods used by AS/NZS 3000, IEC 60364, and the NEC. Rather than calculating a fill ratio from cable and conduit cross-sectional areas, BS 7671 uses the cable factor method described in IET Guidance Note 1, which assigns numerical factors to both cables and conduits that account for practical installation difficulty.

Regulation 522.8.1 establishes the general principle: wiring shall be installed in conduit or trunking systems so that no damage is caused to the cables or their enclosure during installation or in service. The 45% space factor applies as a simple alternative for single cables drawn into conduit, but the cable factor method provides a more refined analysis. Each cable type and size has a defined cable factor (from IET OSG Tables 4A–4C), while each conduit has a conduit factor that decreases with run length and the number of bends (Tables 4D–4F).

This calculator implements the full BS 7671 cable factor methodology, automatically adjusting conduit capacity for bends and run length, and provides both the cable factor summation and the space factor percentage for comprehensive compliance checking.

Open Conduit Fill Calculator

How Conduit Fill Works Under BS 7671

Step 1 — Identify Cable Types and Sizes

List every cable that will be drawn into the conduit, including active conductors, neutrals, circuit protective conductors (CPCs), and any control wiring. For each cable, note its type (solid or stranded), insulation (PVC thermoplastic or thermosetting XLPE), and nominal cross-sectional area in mm². These characteristics determine the cable factor from IET On-Site Guide Tables 4A–4C. Table 4A covers PVC singles (flat twin stripped into singles), Table 4B covers thermosetting singles, and Table 4C covers sheathed cables drawn in complete.

Step 2 — Look Up Cable Factors

Each cable has a cable factor that represents its effective space consumption in the conduit. Per IET Guidance Note 1 Table 4A, a 2.5 mm² PVC single-core stranded conductor has a cable factor of 30. A 4 mm² PVC single-core stranded conductor has a cable factor of 43. The cable factor is not directly proportional to conductor area—it accounts for insulation thickness, which forms a proportionally larger share of overall diameter on smaller conductors. Sum all individual cable factors: Cable Factor Sum = Σ f_cable,i.

Step 3 — Determine Conduit Factor

The conduit factor depends on three parameters: conduit nominal size (16mm, 20mm, 25mm, 32mm), conduit type (PVC or metallic per BS EN 61386), and the route difficulty—defined by the straight run length and number of bends. IET On-Site Guide Tables 4D–4F provide conduit factors. For example, from Table 4D (PVC conduit), a 20mm conduit on a 5m straight run with one bend has a conduit factor of 388. The same conduit on a 10m run with two bends drops to a conduit factor of 286. Each additional 90° bend significantly reduces the conduit factor because it increases pulling friction and makes installation more difficult.

Step 4 — Compare Cable Factor Sum Against Conduit Factor

The installation complies if: Cable Factor Sum ≤ Conduit Factor. If the sum exceeds the conduit factor, either increase the conduit size (which increases the conduit factor) or reduce the number of cables. Per Regulation 522.8, the system must allow cables to be drawn in and withdrawn without damage. The cable factor method inherently accounts for this by reducing capacity for longer runs and bends, unlike simple percentage methods that ignore route geometry.

Step 5 — Alternative: 45% Space Factor Check

Regulation 522.8.1 also permits a simplified 45% space factor method for cables drawn into conduit. Calculate as: Space Factor = (Total cable CSA / Conduit internal CSA) × 100%, where cable CSA uses overall cable diameters. The 45% limit is a conservative single threshold—it does not vary by cable count as in AS/NZS 3000 or NEC. The cable factor method is preferred because it accounts for installation difficulty; the 45% method is a quick alternative check.

Step 6 — Verify Minimum Bending Radius

BS EN 61386-1 specifies minimum bending radii for conduit systems. For rigid PVC conduit, the minimum bend radius is 2.5 × the external diameter. Table 4A of BS 7671 requires all bends to meet these minima, and the cable factor method already penalizes bends by reducing the conduit factor. However, bends tighter than the specified minimum are prohibited regardless of the conduit factor calculation.

Key Reference Tables

IET OSG Table 4A — Cable Factors for PVC Singles

Cable factors for PVC-insulated single-core stranded and solid conductors from 1.0 mm² to 16 mm². Stranded conductors have slightly larger cable factors than solid due to their larger overall diameter for the same nominal area.

Look up the cable factor for each PVC single-core conductor that will be drawn into the conduit. Sum all cable factors to get the total cable factor. A 2.5 mm² stranded PVC single has a factor of 30; a 2.5 mm² solid PVC single has a factor of 22.

IET OSG Tables 4D–4F — Conduit Factors

Conduit factors for PVC conduit (Table 4D), metallic conduit (Table 4E), and short straight runs (Table 4F). Factors decrease with increasing run length and number of bends. Tabulated for 16mm, 20mm, 25mm, and 32mm conduit.

Determine the conduit factor based on the conduit size, run length (in metres), and number of 90° bends. The cable factor sum must not exceed this conduit factor. Each bend typically reduces capacity by 15–25%.

BS EN 61386 — Conduit System Dimensions

Defines internal and external diameters, wall thicknesses, and mechanical ratings for conduit systems used in the UK. Covers rigid PVC, flexible PVC, and metallic conduit in standard metric sizes.

Obtain precise internal diameters for the space factor calculation. Required when using the 45% space factor alternative method rather than the cable factor method.

Regulation 522.8 — Space Factor Requirements

Establishes that the space factor for cables drawn into conduit shall not exceed 45%. This is the simplified alternative to the cable factor method. Applies to all conduit types with cables drawn in after conduit installation.

Use as a quick-check method when the cable factor tables are not available or for simple installations. If the 45% space factor is met, the installation is compliant without needing the full cable factor calculation.

IET Guidance Note 1 — Tables 4A–4C (Cable Factors by Type)

Complete cable factor tables covering PVC singles (Table 4A), thermosetting singles (Table 4B), and sheathed cables (Table 4C). Sheathed cables drawn in complete have significantly larger factors than stripped singles.

Select the correct table based on cable type. If drawing sheathed cable into conduit (not recommended but sometimes done), use Table 4C factors which are 2–3× larger than the singles factors, reflecting the much larger overall diameter.

IET OSG Table 4C — Conduit Capacities by Cable Size

Pre-calculated maximum number of cables per conduit size for common cable types and run configurations. Provides a quick reference without manual cable factor summation for standard installations.

Use for rapid estimation of conduit size when all cables are the same type and size. For mixed cable sizes, the manual cable factor summation method is required.

Worked Example — BS 7671 Conduit Fill

Scenario

A lighting circuit requires 4 × 2.5 mm² PVC stranded single-core cables plus 2 × 4 mm² PVC stranded single-core cables to be drawn into a 20mm PVC conduit. The conduit run is 5 metres long with one 90° bend. Verify compliance using the BS 7671 cable factor method.

Identify cable types

All cables are PVC-insulated single-core stranded conductors. Four cables at 2.5 mm² and two cables at 4 mm². Use IET On-Site Guide Table 4A for cable factors.

Cable type: PVC singles, stranded — use Table 4A

Look up individual cable factors

From IET OSG Table 4A: a 2.5 mm² stranded PVC single has a cable factor of 30. A 4 mm² stranded PVC single has a cable factor of 43.

f(2.5 mm²) = 30, f(4 mm²) = 43

Calculate total cable factor sum

Sum all individual cable factors for the six cables being drawn into the conduit.

Cable Factor Sum = (4 × 30) + (2 × 43) = 120 + 86 = 206

Cable Factor Sum = 206

Determine conduit factor

From IET OSG Table 4D (PVC conduit), for a 20mm conduit on a 5m run with one 90° bend, the conduit factor is 388.

Conduit Factor (20mm, 5m, 1 bend) = 388

Compare cable factor sum against conduit factor

The cable factor sum (206) must not exceed the conduit factor (388). Since 206 ≤ 388, the installation complies with the cable factor method.

206 ≤ 388

Cable Factor Sum 206 ≤ Conduit Factor 388 — PASS

Check sensitivity: what if the run were longer with more bends?

For the same 20mm conduit on a 10m run with three 90° bends, Table 4D gives a conduit factor of approximately 167. In this scenario, 206 > 167 and the conduit would be too small. A 25mm conduit would be needed, which at 10m with three bends has a conduit factor of approximately 305 (305 ≥ 206).

At 10m with 3 bends: 20mm FAILS, 25mm conduit required

The six cables (4 × 2.5 mm² + 2 × 4 mm² PVC singles) comply in a 20mm PVC conduit for the 5m run with one bend, with a cable factor sum of 206 against a conduit factor of 388. This leaves 47% headroom for future additional cables. However, if the same run had three bends over 10 metres, the 20mm conduit would fail and a 25mm upgrade would be required. This demonstrates the key advantage of the BS 7671 cable factor method—it dynamically adjusts capacity based on installation difficulty, catching problems that a simple percentage fill method would miss.

Common Mistakes When Using BS 7671

1
Confusing the cable factor method with the percentage fill method. BS 7671 offers two approaches: the cable factor method (IET Guidance Note 1 tables) and the 45% space factor (Regulation 522.8.1). They are not interchangeable in application—the cable factor method uses dimensionless factors that account for pulling difficulty, while the space factor uses actual cross-sectional areas. Mixing table values between the two methods produces meaningless results.
2
Not reducing the conduit factor for bends. Each 90° bend in the conduit run significantly reduces the conduit factor—typically by 15–25%. A conduit that easily accommodates cables on a straight 3m run may be hopelessly inadequate on a 10m run with three bends. Ignoring bends is the most common cause of cables jamming during installation in UK practice.
3
Using wrong cable factor tables for stranded vs solid conductors. IET OSG Table 4A provides separate cable factors for stranded and solid conductors of the same nominal size. A 2.5 mm² stranded conductor has a cable factor of 30, while a solid conductor of the same size has a factor of 22—a 36% difference. Using the wrong row leads to either undersized or oversized conduit.
4
Forgetting that sheathed cables drawn in complete have much larger cable factors. If a sheathed cable (e.g., twin and earth) is drawn into conduit without stripping the sheath, Table 4C cable factors apply—these are 2–3 times larger than the singles factors from Table 4A. The preferred practice is to strip the sheath and draw individual insulated conductors, but if the sheath is retained, the larger factors must be used.
5
Applying the 45% space factor without considering route difficulty. The 45% rule from Regulation 522.8.1 is a static limit that does not account for conduit length or bends. A conduit that passes the 45% check may still be impractical to wire if the run is long with multiple bends. The cable factor method is preferred precisely because it captures this additional complexity.

How Does BS 7671 Compare?

The BS 7671 cable factor method is unique among major international wiring standards. While AS/NZS 3000, IEC 60364, and NEC all use percentage-based fill calculations (comparing cable area to conduit area), BS 7671 uses dimensionless factors that inherently account for conduit run length and bends—the primary determinants of installation difficulty. This makes the BS method more conservative for long, complex runs and more permissive for short, straight runs. The trade-off is complexity: percentage methods require only a calculator, while the cable factor method requires specific IET tables. The 45% space factor alternative in BS 7671 bridges this gap but loses the nuanced route analysis.

Frequently Asked Questions

The cable factor method is the primary conduit sizing technique defined in IET Guidance Note 1 and the IET On-Site Guide. Each cable type and size is assigned a numerical cable factor (from Tables 4A–4C), and each conduit size has a conduit factor (from Tables 4D–4F) that varies with run length and number of bends. The sum of all cable factors must not exceed the conduit factor. This approach is more sophisticated than simple percentage fill because it accounts for the practical difficulty of pulling cables through conduit—longer runs and more bends increase friction, reducing the number of cables that can be practically installed.

Regulation 522.8.1 states that for cables drawn into conduit, the space factor should not exceed 45%. The space factor is calculated as the ratio of total cable cross-sectional area (using overall cable diameters) to the conduit internal cross-sectional area, expressed as a percentage. This is a simplified alternative to the cable factor method and does not account for run length or bends. It provides a quick conservative check but may over-size conduit on short straight runs where the cable factor method would permit a smaller conduit.

Each 90° bend in a conduit run reduces the conduit factor by approximately 15–25%, depending on conduit size and run length. For example, a 20mm PVC conduit on a 5m run has a conduit factor of 388 with one bend but only about 286 with two bends. Three bends on a 10m run can reduce capacity to less than half of a straight run of the same length. This is why the cable factor method is preferred over simple percentage fill—it captures the real-world impact of bends on installation feasibility.

Conduit Fill Calculator for Other Standards

🇦🇺AS/NZS 3000

🌍IEC 60364

🇺🇸NEC/NFPA 70