Bar shows cable area as a share of the containment area; it turns full at 100%. Fill is not the only limit: grouping reduces current ratings, and the design must meet BS 7671.
Formula and breakdown
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This cable tray fill calculator works out how full a cable tray, trunking or compartment is from the outer diameters and numbers of the cables in it, for electricians, designers and estimators in the UK and Europe who need to pick a containment size or check that more cables will fit. It uses a 45% space factor by default, shows the minimum area and width you need, and counts how many more cables the run can take.
Quick answer: A cable tray fill calculator adds each cable’s area, π x outer diameter squared / 4, and divides by the tray’s inside width x usable depth. UK trunking data uses a 45% space factor. Six 14 mm cables plus 24 data cables of 6 mm fill a 150 x 50 mm tray to about 21%.
What Is a Cable Tray Fill Calculator?
A cable tray fill calculator is a tool that compares the total cross-sectional area of a group of cables with the area of the tray or trunking that carries them, and reports the result as a percentage of the space used. In plain words: the area of each round cable equals π times its outer diameter squared divided by 4; add those areas for every cable, divide by the containment area, and compare the answer with the fill limit, often called the space factor.
The circle formula is ordinary geometry, as the Wikipedia article on the area of a circle sets out: area equals π r², which is π d² / 4 for a diameter d. Manufacturers use the same idea. Marshall-Tufflex trunking data lists a “cable factor” for each cable, which for its data cables is simply the area of the circle (a 6.0 mm cable has a factor of 28.3 mm²).
How Do You Use the Cable Tray Fill Calculator?
- Choose whether to enter the inside width and usable depth, or the cross-sectional area from the tray or trunking maker’s data sheet.
- Set the fill limit. The default is the 45% space factor used in UK trunking data; change it if your project specification sets another figure.
- For each cable type, pick a preset or choose “Custom” and type the outer diameter from the cable data sheet.
- Enter how many of each cable will run in the tray. Set unused rows to 0.
- Read the fill percentage, the status against your limit, the minimum area and width, and the number of extra cables that would still fit.
- Open the breakdown to check every cable area before you order containment.
What Is the 45% Space Factor for Trunking?
The 45% space factor means that cables should take up no more than 45% of the inside cross-section of trunking, leaving 55% as air and room to pull cables in. A NAPIT technical article on distribution trunking describes 45% as the figure historically used that should not be exceeded, and Marshall-Tufflex trunking data sheets quote every compartment area both in full and at 45%.
The same 45% figure appears as a rule of thumb for conduit sizes that are missing from the IET On-Site Guide tables, according to the Marshall-Tufflex spacing factor guide. For open cable tray, BS 7671 itself does not give one fill percentage in the sources checked for this page, so the cable tray fill calculator leaves the limit editable: use your project specification or the tray maker’s guidance. In North America, tray fill is set by the NEC, Article 392, which uses different rules.
How Do You Calculate the Area of a Cable?
The area of a round cable is π x outer diameter² / 4, so a 14 mm cable takes 153.9 mm² and a 6 mm data cable 28.3 mm². Always use the outer diameter including the sheath, from the maker’s data sheet, not the conductor size in mm². A 2.5 mm² conductor sits inside a cable that is much larger overall.
The table lists areas for the preset cables in the calculator, with the number of each that fit in a 150 x 50 mm tray at 45% (3,375 mm² usable) and in a trunking compartment whose 45% area is 1,834 mm², the figure Marshall-Tufflex gives for compartment 3 of its MTRS100 trunking. The 14 mm and 21 mm rows are example diameters for multicore power cables; check the real figure for your cable.
| Cable | Outer diameter | Area | Fit in 150 x 50 mm tray at 45% | Fit in 1,834 mm² (45%) |
|---|---|---|---|---|
| 6491X 1.5 mm² single core | 3.19 mm | 8.0 mm² | 421 | 229 |
| 6491X 2.5 mm² single core | 3.89 mm | 11.9 mm² | 283 | 154 |
| 6491X 4.0 mm² single core | 4.60 mm | 16.6 mm² | 203 | 110 |
| Data cable | 6.0 mm | 28.3 mm² | 119 | 64 |
| Data cable | 8.4 mm | 55.4 mm² | 60 | 33 |
| Example multicore power cable | 14 mm | 153.9 mm² | 21 | 11 |
| Example multicore power cable | 21 mm | 346.4 mm² | 9 | 5 |
The single core areas are the cable factors in the Marshall-Tufflex data sheet; the diameters shown are the equivalent circle. Dividing the maker’s 45% compartment area by the cable factor, as its trunking capacity data sheet instructs, gives the same counts as the cable tray fill calculator.
Why Is Width x Depth Not Always the Real Area?
Nominal width x depth overstates the usable area of most trunking, because lips, lid clips, dividers and fixings take up space inside. Marshall-Tufflex, for example, lists MTRS100 compartment 3 at 4,075 mm² and notes that fitting 30 mm deep accessory boxes reduces the area by 2,600 mm². Where the maker publishes a compartment area, choose “Area from the maker’s data sheet” in the cable tray fill calculator and enter it.
For open tray, measure the inside width and use the side height as the usable depth, unless cables are only laid in a single layer. Bends, tees and crosses matter too: the NAPIT article warns that capacity falls by 50% at tees and four-way crosses unless offset fittings are used, so size the run for its tightest fitting.
Is Fill the Only Check When Sizing Cable Tray?
No. Fill only checks physical space; power cables bunched together also need their current-carrying capacity reduced by grouping factors under BS 7671, because they cannot shed heat as easily. Check the conductor size for the grouped run with the voltage drop calculator and a cable sizing method, and for DC strings use the solar cable size calculator.
Containment also has to meet the IP and fixing requirements of BS 7671, carry the weight of the cables between supports, and keep power and data cables segregated where required. Design, installation and certification of fixed wiring in the UK must be carried out by a qualified electrician to BS 7671; in England, Part P of the Building Regulations applies to work in homes, and Wales has its own equivalent. The cable tray fill calculator is a planning aid for the space check only.
Worked Example: Adding Power Cables to a 150 mm Tray
Example: a contractor wants to run eight multicore power cables with an outer diameter of 21 mm and 24 data cables of 6.0 mm on a 150 x 50 mm tray, using a 45% fill limit from the project specification. The 21 mm diameter is an example value; the real figure comes from the cable data sheet.
- Power cables: 8 x π x 21² / 4 = 8 x 346.4 = 2,770.9 mm².
- Data cables: 24 x π x 6² / 4 = 24 x 28.3 = 678.6 mm².
- Total cable area: 3,449.5 mm². Tray area: 150 x 50 = 7,500 mm².
- Fill: 3,449.5 / 7,500 = 46.0%, which is over the 45% limit.
- Minimum area: 3,449.5 / 0.45 = 7,665 mm². At 50 mm depth that needs 153 mm of width, so the next 50 mm step is a 200 mm tray.
The cable tray fill calculator shows the same figures. Moving to a 200 x 50 mm tray drops the fill to about 34.5% and leaves room for growth.
Frequently Asked Questions
How do you calculate cable tray fill?
Work out each cable’s area as π x outer diameter squared / 4, multiply by the number of cables and add the totals. Divide by the tray’s inside width x usable depth and multiply by 100. Compare the percentage with your fill limit, such as the 45% space factor used for trunking in the UK.
What is the maximum fill for cable trunking in the UK?
UK trunking data from manufacturers such as Marshall-Tufflex is based on a 45% space factor, and a NAPIT article describes 45% as the figure historically not to be exceeded. Check the maker’s compartment area, since lips and dividers reduce the usable space below width times depth.
Should I use the conductor size or the outer diameter?
Use the outer diameter over the sheath, from the cable data sheet. The conductor size in mm² is only the copper area. A cable’s overall area is much larger because of insulation, bedding, armour and sheath, and fill depends on the full outside size.
Does cable tray fill affect current rating?
Fill itself does not set the rating, but cables grouped closely on a tray or in trunking cannot lose heat as well, so BS 7671 requires grouping factors that reduce their current-carrying capacity. A run that passes the fill check may still need larger conductors or more spacing.
How many cables fit in 150 x 50 mm trunking?
At a 45% space factor on the full 7,500 mm², about 119 data cables of 6.0 mm or 283 single core 2.5 mm² cables fit. Real trunking has a smaller internal area than width times depth, so use the maker’s published area for an accurate count.
Can the cable tray fill calculator size containment for mixed cables?
Yes. Enter up to four cable types, each with its own outer diameter and quantity. The calculator adds all the areas, reports the fill, the minimum area and width needed, and how many more of the first cable type would still fit within your limit.
Checked October 2026 by the Solaxyra Editorial Team. Sources: Marshall-Tufflex trunking capacity data sheet (MTRS, Sceptre), Marshall-Tufflex: Spacing factors, a brief guide, NAPIT: Design considerations for distribution trunking, Wikipedia: Area of a circle, GOV.UK: Approved Document P.