Containment

Conduit & Trunking Fill Calculator

Add up your cables and get the smallest conduit or trunking that stays within the space factor, with the fill percentage for the size it suggests.

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Smallest that fits -
Total cable area
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Fill at that size
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Opens a printable report; save as PDF from the print dialog.

The space factor method

The quick way to size containment is the space factor: the combined cross-sectional area of the cables should not exceed a set fraction of the inside of the conduit or trunking.

Σ cable area ≤ internal area × space factor

The usual maxima are about 40% for conduit and 45% for trunking. Each cable is counted by its overall diameter, insulation included, not the conductor size. This tool sums the cables you enter and returns the smallest standard size that stays within the factor.

Assumed cable diameters

The presets use typical overall diameters for single-core 70°C thermoplastic (6491X) conductors: 1.0 mm² at 2.9 mm through to 25 mm² at 9.7 mm. Real diameters vary a little by manufacturer, so check the data sheet for a tight design.

When to use the unit method instead

The space factor is a first pass. For conduit runs with bends, or longer than about three metres, the IET On-Site Guide sets out a unit or factor method using cable factors and conduit factors that allow for the run length and the number of bends. Use that for anything you are going to certify.

Common questions

Why count the overall diameter, not the conductor size?
Because it is the finished cable, insulation and all, that has to physically fit. A 2.5 mm² conductor sits inside a cable nearer 3.8 mm overall.
What space factor should I use?
Around 45% for trunking and 40% for conduit are the common figures. You can change the value in the tool if your own standard differs.
Does this replace the On-Site Guide tables?
No. It is an indicative check. For runs with bends the unit method in the On-Site Guide is the recognised approach and should be used for the final design.
For guidance only. This is an indicative space factor check using typical cable diameters. Confirm the actual cable dimensions and use the IET On-Site Guide unit method, and BS 7671, for the final containment design.

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