Calculate trunking fill using the 45% maximum fill rule. Check any combination of cables against any trunking size, with compartment mode for segregated installations.
Reviewed by the Andrew Moore, founder of Elec-Mate, — IET Wiring Regulations (BS 7671) specialists and C&G 2382 qualified assessors.
Total cable CSA ≤ 45% of trunking internal CSA
The fundamental trunking fill rule — BS 7671:2018+A4:2026 and IET Guidance Note 1
Calculate the minimum trunking size for your cables — BS 7671 compliant
Fill % = (Total Cable Area ÷ Trunking Area) × 100Trunking fill refers to the proportion of a cable trunking's internal cross-sectional area that is occupied by cables. BS 7671:2018+A4:2026 and the IET Guidance Note 1 state that the total cross-sectional area of all cables installed in trunking must not exceed 45% of the trunking's internal cross-sectional area. This is known as the 45% fill rule, and it applies to all types of cable trunking — PVC, galvanised steel, stainless steel, aluminium, dado, skirting, and mini-trunking.
Why 45% and not more? The 45% limit exists for three reasons. First, heat dissipation: when current flows through cables, the conductors generate heat. The air space inside the trunking acts as a cooling medium, allowing heat to transfer from the cable surface to the trunking walls and then to the surrounding environment. If the trunking is packed too tightly, the temperature inside rises, potentially exceeding the cable insulation's rated temperature and causing degradation or failure.
Second, installation practicality: cables need to be laid into the trunking without excessive bending or forcing. They must be arranged neatly to avoid crossing over each other, which creates pressure points that can damage insulation over time. A 45% fill leaves enough space for cables to be routed neatly with gentle bends at fittings and junctions.
Third, future capacity: good design practice — and many specifications — require spare capacity for additional circuits. Building use changes over time, and new circuits are frequently needed for additional power points, data outlets, or replacement of existing services. A trunking system designed to 45% fill on day one has no spare capacity; designing to 30 to 35% fill on day one leaves room for growth. When planning containment, use the cable sizing calculator to determine the required cable cross-section before checking trunking fill, and the voltage drop calculator to verify the circuit meets BS 7671 limits.
Total cable CSA ≤ 45% of trunking internal CSA
The fundamental trunking fill rule from BS 7671
The most common mistake in trunking fill calculations is confusing the conductor cross-sectional area with the overall cable cross-sectional area. When you say "2.5 mm² cable", the 2.5 mm² refers to the cross-section of the copper conductor only. The actual space the cable occupies in the trunking is much larger because it includes the PVC insulation around each conductor, the sheath around the cable, and — for flat cables — the shape of the overall profile.
For round cables (such as single-core PVC insulated cables drawn into trunking), the cable cross-sectional area for fill purposes is calculated from the overall diameter: CSA = pi x (d/2)². A 2.5 mm² single has an overall diameter of approximately 4.6 mm, giving a CSA of pi x 2.3² = 16.6 mm². This is over six times the conductor cross-section.
For flat cables (such as twin and earth), the cable CSA for fill purposes is the overall width multiplied by the overall height. A 2.5 mm² twin and earth cable is approximately 11.5 mm wide by 6.9 mm high, giving a CSA of 79.4 mm² — over 30 times the conductor cross-section. This is why trunking fill calculations must always use the overall cable dimensions, never the conductor size.
The sizes above are nominal external dimensions. The internal dimensions are slightly smaller due to wall thickness — typically 1 to 2 mm per side for PVC trunking and 1 mm per side for metal trunking. The Elec-Mate calculator uses the actual internal dimensions for accurate fill calculations.
BS 7671 Regulation 528.1 requires that cables of different voltage bands are segregated from each other. BS 7671 classifies circuits into two bands: Band I covers extra-low voltage (ELV) circuits such as data, telecommunications, fire alarm, and emergency lighting control wiring; Band II covers mains voltage circuits operating at 230 V / 400 V. Regulation 528.1 (confirmed in GN3 Section 4.8.5 and OSG Section 7.4.1) prohibits Band I and Band II circuits from sharing the same wiring enclosure unless at least one of three conditions is met:
Compartmentalised trunking provides this segregation by including fixed internal dividers that create separate channels. A two-compartment trunking has one divider creating two channels; a three-compartment version has two dividers. Each compartment must be treated independently for fill calculations — the 45% rule applies to each compartment separately.
For example, a 100 x 50 mm three-compartment trunking might divide into one 50 x 50 mm power compartment and one 50 x 50 mm data compartment (the third compartment is sometimes a narrower ELV channel). The power compartment has an internal CSA of approximately 2,500 mm², giving a 45% usable area of 1,125 mm². The data compartment has the same capacity. You cannot add the two compartments together — each is assessed against its own 45% limit.
When specifying compartmentalised trunking, always check the internal dimensions of each compartment from the manufacturer's data, as the dividers reduce the usable space. A 100 x 50 mm trunking divided into two equal compartments does not provide two 50 x 50 mm channels — the divider itself occupies space, and the two channels may be closer to 48 x 48 mm each.
At EICR inspection stage: the 45% fill rule must be verified per compartment individually. BS 7671 Regulation 622.85 requires the inspector to confirm that cables are adequate for current-carrying capacity in accordance with Section 523 — which includes grouping, ambient temperature, and installation method. An overfilled compartment that compromises thermal dissipation is a recordable defect.
Four ring final circuits use 2.5 mm² T&E flat cable. Each ring has two cables in the trunking run (outgoing and return legs of the ring). Total: 8 cables of 2.5 mm² T&E.
Cable CSA per cable = 11.5 x 6.9 = 79.4 mm²
Total cable CSA = 8 x 79.4 = 635.2 mm²
Trunking 45% capacity = 2,500 x 0.45 = 1,125 mm²
Result: 635.2 ≤ 1,125 — PASS. Fill percentage is 25.4%, leaving good spare capacity for future circuits.
An electrician wants to run one 2.5 mm² T&E (socket circuit) and one 1.5 mm² T&E (lighting circuit) through 25 x 16 mm mini-trunking.
2.5 mm² T&E CSA = 79.4 mm²
1.5 mm² T&E CSA = 61.0 mm²
Total cable CSA = 79.4 + 61.0 = 140.4 mm²
Trunking 45% capacity = 400 x 0.45 = 180 mm²
Result: 140.4 ≤ 180 — PASS. However, the fill is 78% of the usable space (35.1% of total), which is tight. No room for additional cables. Consider upgrading to 38 x 16 mm (45% capacity: 274 mm²) to allow for future expansion.
A trunking run carries 6 cables of 4.0 mm² T&E and 4 cables of 2.5 mm² T&E in 50 x 25 mm trunking.
4.0 mm² T&E CSA: 6 x 95.0 = 570.0 mm²
2.5 mm² T&E CSA: 4 x 79.4 = 317.6 mm²
Total cable CSA = 570.0 + 317.6 = 887.6 mm²
Trunking 45% capacity = 1,250 x 0.45 = 562.5 mm²
Result: 887.6 > 562.5 — FAIL. The 50 x 25 mm trunking is too small. Upgrade to 50 x 50 mm trunking (45% capacity: 1,125 mm²), which gives 887.6 ≤ 1,125 — PASS.
Identify every cable that will be routed through the trunking run. For each circuit, note the cable type (flat twin and earth, single core, multicore armoured, data cable, etc.) and the conductor cross-sectional area. Remember to count every cable individually — if a circuit uses a separate line, neutral, and earth (singles in trunking), count each conductor separately.
For each cable, find the overall diameter (for round cables) or the overall width and height (for flat cables). These values are in the manufacturer's data sheet or the IET tables. Do not use the conductor CSA — the overall cable dimension includes the insulation and sheath, which is much larger. The Elec-Mate calculator has all common UK cable dimensions built in.
For round cables: CSA = pi x (overall diameter / 2) squared. For flat cables: CSA = overall width x overall height. Sum the cross-sectional areas of all cables to get the total cable CSA. For example, 3 cables with CSAs of 79 mm², 50 mm², and 50 mm² give a total of 179 mm².
Look up the internal dimensions of the trunking. For rectangular trunking: internal CSA = internal width x internal height. For the common 100 x 50 mm trunking, the internal CSA is approximately 5000 mm². Calculate 45% of this value: 5000 x 0.45 = 2250 mm². This is the maximum total cable CSA permitted.
If the total cable CSA is less than or equal to 45% of the trunking internal CSA, the cables fit. If not, select a larger trunking size. For the example above (total cable CSA 179 mm²), the minimum trunking internal CSA would be 179 / 0.45 = 398 mm². A 25 x 16 mm trunking (internal CSA 400 mm²) is the minimum, but 38 x 16 mm (608 mm²) would be a better choice to allow for future expansion.
Purpose-built for UK electricians working to BS 7671. Eliminates manual CSA lookups and ensures your trunking is correctly sized every time.
Enter the number and type of cables, and the calculator recommends the minimum trunking size.
Handles any combination of cable types in the same trunking — flat twin and earth, singles, multicore, data cables.
Calculate fill for compartmentalised trunking. Define each compartment separately and check the 45% rule independently.
Shows the fill percentage as a visual gauge. Colour-coded: green for comfortable fill, amber for approaching the 45% limit, red for exceeding it.
Includes all standard mini-trunking sizes. Quickly check whether a single circuit or pair of circuits will fit in the mini-trunking size you plan to use.
All cable dimensions and fill limits verified against BS 7671:2018+A4:2026 and the IET Guidance Note 1.
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