Conduit Fill Calculator

Parameters

The form has three groups of inputs: the conduit configuration, one row per group of identical cables, and a set of advanced options behind a collapsible block. This is what each field means, what it is measured in, and where it changes the answer.

Cable rows

Size (square millimetres)

The nominal conductor cross-section, from 1.5 to 300 mm squared. It is used only as the key into the outside diameter table. The fill arithmetic never uses the conductor area itself, only the overall cable diameter.

Gotcha. The multicore diameter table only covers 1.5 to 50 mm squared. Pick a larger size on a multicore row and the calculator cannot find a diameter, drops that row from the total, and issues a warning. The fill percentage it then reports is too low. Read the warnings before you trust the number.

Type (single core or multicore)

Single core is one insulated conductor. Multicore is a sheathed cable such as flat twin and earth. This switches which diameter table is used, and it is the single biggest driver of the result.

Typical values. A 2.5 mm squared single core is 5.2 mm across, giving 21.24 mm squared of area. The same conductor size as multicore is 11.5 mm across, giving 103.87 mm squared, nearly five times as much.

Insulation (PVC or XLPE)

Thermoplastic V-90 versus cross-linked X-90. XLPE entries in the diameter table are 0.3 to 0.5 mm smaller than the PVC entry for the same conductor size.

Effect. Small but not always negligible. Three 2.5 mm squared single cores in 16 mm conduit come out at 43.34 percent in PVC and 38.48 percent in XLPE, which is the difference between a fail and a pass at the 40 percent limit.

Quantity (number of cables in that row)

How many identical cables that row represents, from 1 to 100. Add a row for each different cable in the run.

Gotcha. The total count across all rows also sets the fill limit: 53 percent for one cable, 31 percent for two, and 40 percent for three or more. Going from one cable to two therefore tightens the limit sharply, so a run can fail simply because you added a second cable. Count every conductor that will be in the conduit, including earths, spares, and cables pulled in for future circuits.

Label and measured outside diameter (advanced, per row)

Tick Schedule labels and measured diameters in the advanced block and every cable row gains two extra fields. The label is free text such as Actives, Neutral, or Circuit 3, and it is carried into the breakdown table and the derivation so a mixed schedule reads as a schedule rather than a list of sizes. The measured outside diameter, in millimetres, replaces the table lookup for that row only.

Why it matters. A measured diameter is the only way to get a defensible number out of this tool, because the built-in diameters are indicative. It is also the way to include a cable the tables do not cover, such as a 300 mm squared multicore, instead of having the row dropped with a warning. The breakdown table marks any row using a measured figure.

Conduit configuration

Conduit type (rigid PVC, corrugated PVC, rigid metallic, flexible metallic)

Drives two things. First, the usable internal area: rigid PVC is the base table, corrugated PVC is multiplied by 0.85 because the convolutions eat the bore, flexible metallic by 0.80, and rigid metallic by 1.06 because steel wall is thinner than heavy-duty PVC at the same nominal size. Second, the space factor: corrugated and flexible metallic drop the permitted fill by five percentage points, so a three-cable run is held to 35 percent rather than 40 percent.

Effect. Four 6 mm squared single cores auto-size to 25 mm in rigid PVC and to 32 mm in corrugated. Four 2.5 mm squared multicores auto-size to 50 mm in rigid PVC and to 40 mm in rigid metallic. Same cables, a full size either way.

Gotcha. The area factors and the five point space factor penalty are indicative figures built into the tool, not values transcribed from a standard or a catalogue. They are pending verification. Treat the direction of the adjustment as sound and the exact number as provisional.

Auto-size conduit (checkbox)

On, the calculator walks the standard sizes 16, 20, 25, 32, 40, 50, and 63 mm and returns the first one that keeps the fill inside the limit. Off, it checks the size you nominate and reports pass or fail.

Gotcha. If the cables do not fit in 63 mm, the calculator still reports 63 mm along with a warning to split the run or use a larger enclosure.

Conduit size (millimetres nominal, only when auto-size is off)

One of 16, 20, 25, 32, 40, 50, or 63 mm nominal. The internal areas used are 147, 248, 398, 645, 995, 1524, and 2474 mm squared, which correspond to internal diameters of roughly 13.7, 17.8, 22.5, 28.7, 35.6, 44.1, and 56.1 mm.

Gotcha. Those areas are indicative figures built into the tool, not values transcribed from a manufacturer catalogue. Heavy-duty and medium-duty conduit of the same nominal size have different wall thicknesses and therefore different internal areas. Check the datasheet for the product you are actually installing.

Advanced options

Number of bends (90 degree equivalents, default 0)

How many 90 degree bends sit between the two draw-in points at each end of this pull. Each bend is charged at 6 metres of equivalent straight run. Two 45 degree bends count as one 90.

Effect. Above two bends you get a separate warning, because two is the figure most Australian specifications work to between draw-in boxes.

Run length (metres, default 0)

Straight length between draw-in points. Added to the bend allowance to give an equivalent pull length, which is checked against a baseline of 30 metres scaled down as the conduit fills up: full 30 metres at or below half the space factor, 85 percent of it up to eight tenths, 70 percent up to the limit, and 55 percent beyond it.

Effect. Three 2.5 mm squared single cores in 20 mm conduit sit at 25.69 percent against a 40 percent limit, a fill ratio of 0.64, so the allowance factor is 0.85 and the allowed equivalent length is 25.5 metres. A 20 metre run with two bends comes to 32 metres equivalent, which fails and returns a suggested number of extra draw-in points. Leave both bends and run length at zero and the pull-in check, its two derivation steps, and its warnings are not produced at all.

Gotcha. The 6 metres per bend, the 30 metre baseline, and the fill allowance factors are rule-of-thumb figures for flagging an obviously impractical pull. They are not a pulling tension calculation and they are not from a published standard. AS/NZS 3000 requires draw-in points such that cables can be installed without damage, but does not publish a numeric bend or length limit.

Space factor override (percent, off by default)

Replaces the figure derived from cable count and conduit type with one you nominate, from 1 to 100 percent. Use it to apply a house standard, a client specification, or a self-imposed margin that leaves room for future circuits.

Effect. It moves the compliance limit, the pass or fail verdict, and the auto-sized conduit. Three 2.5 mm squared single cores pass at 25.69 percent in 20 mm against the standard 40 percent limit, and fail the moment you set the override to 20 percent. The results panel shows whether the limit came from cable count, the conduit type adjustment, or your override.

Gotcha. The override is not restricted to stricter values. Set it looser than the derived figure and the calculator will use it, and warn you that it did.

Assumptions and limits

What the tool assumes

• Every cable is treated as a perfect circle and the areas are simply added across the whole schedule. There is no packing geometry and no allowance for the air gaps between round cables.
• The space factors applied are 53 percent for one cable, 31 percent for two, and 40 percent for three or more, less five percentage points for corrugated PVC and flexible metallic conduit, and replaced outright by the override if you set one. AS/NZS 3000:2018 Appendix C is commonly quoted as 50, 33, and 40 percent. The figures in this tool are placeholders and have not been transcribed from the published standard, so a result sitting within a percent or two of the limit should be treated as inconclusive.
• Cable outside diameters are indicative table values, not manufacturer data, unless you enter a measured diameter on that row. Real cables vary by brand, core count, sheath type, and whether the cable is flat or circular.
• Conduit internal areas come from a rigid PVC table scaled by an indicative factor per conduit type. Both the base table and the factors are pending verification against manufacturer catalogues.
• The pull-in assessment is a rule-of-thumb screen, not a pulling tension calculation. It has no model of coefficient of friction, cable weight, lubricant, bend radius, or sidewall bearing pressure, and it treats every bend as equally severe regardless of where it sits along the run.

What this calculator does not do

• No pulling tension calculation. Bend count and run length are screened against an indicative equivalent-length model, but bend radius, cable weight, coefficient of friction, lubricant, and sidewall bearing pressure are not modelled, and the tool never reports a tension in newtons.
• No packing or jam ratio check. The classic case of three cables of similar diameter wedging in a bend is not detected, and passing the fill and pull-in checks does not rule it out.
• No grouping derating. Passing the fill check does not mean the cables still carry their rated current once bunched. Run the cable sizing calculator with the correct grouping factor as well.
• No segregation rules, no fire-rated or hazardous-area enclosure requirements, and no cable tray, ladder, or trunking fill.
• No automatic handling of a cable size that is missing from the diameter table. The row is dropped with a warning, which also lowers the cable count and can move the fill limit to a more generous value, producing a result that looks comfortably compliant when it is not. The fix is to enter a measured outside diameter on that row, which is used in place of the table lookup.

Use the output as a first pass on conduit size. Verify cable diameters against the manufacturer datasheet, verify the conduit internal area against the product catalogue, verify the space factor against the current edition of AS/NZS 3000 and AS 2053, and have the installation signed off by the person responsible for it. Nothing on this page has been validated or certified by a chartered professional engineer.

Worked examples

Three complete runs, using the diameter and area figures the calculator itself uses. Cable area is pi multiplied by the square of half the outside diameter, and fill percentage is total cable area divided by conduit internal area, multiplied by 100.

Conduit Fill Guide for AS 2053 and AS/NZS 3000:2018

Conduit fill calculation determines whether a set of cables can physically fit inside a given conduit size while complying with Australian standards. Getting this right before installation prevents cables being jammed into undersized conduit, which makes pulling difficult, damages insulation, and traps heat that degrades cable life. Every cable run through conduit on an Australian electrical installation needs this check.

This calculator computes the total cross-sectional area of your selected cables, compares it against the internal area of the conduit, and checks compliance with the fill limits set by AS 2053 and AS/NZS 3000:2018. You can either specify a conduit size to check or let the calculator recommend the minimum size that satisfies the standard.

Key concepts

• Fill percentage limits. AS/NZS 3000:2018 Appendix C, Paragraph C6.2 applies a space factor that depends on the number of cables in the enclosure. A single cable may occupy up to 50% of the internal area. Two cables are limited to 33%. Three or more cables are limited to 40%. These limits ensure cables can be pulled through bends without excessive force and that adequate airspace remains for heat dissipation.
• Overall cable diameter, not conductor size. The area used for fill calculations is based on the overall outside diameter of the complete cable, including insulation and sheath. A 2.5 mm squared twin and earth flat cable has an overall cross-sectional area significantly larger than the sum of its bare conductor areas. Always use the manufacturer datasheet for the actual overall dimensions.
• Internal conduit area. Conduit is specified by nominal size (e.g. 25 mm), but the actual internal diameter is smaller due to wall thickness. A 25 mm heavy-duty PVC conduit has an internal diameter of approximately 21.5 mm. Use the internal cross-sectional area from the manufacturer datasheet, not the nominal size.
• Grouping derating interaction. Even when the conduit fill percentage is within limits, installing multiple loaded circuits in a single conduit triggers the grouping derating factor (Cg) from AS/NZS 3008.1.1 Table 22. More circuits sharing a conduit means each cable must carry less current. Always check both conduit fill and cable current rating together.

Common scenarios

• Residential switchboard feeds. A typical house might run 6 to 10 circuits from the meter box to the switchboard through a single conduit. Each circuit uses a 2.5 mm squared twin and earth cable. The installer needs to verify that all cables fit within the 40% fill limit and select the appropriate conduit size, often 32 mm or 40 mm heavy-duty PVC for this number of cables.
• Commercial riser or horizontal trunk. In a multi-storey commercial building, electrical risers carry submain cables between floors. These larger cables (16 mm squared to 95 mm squared) have much bigger overall diameters and can fill conduit quickly. A riser conduit often needs to be 50 mm or larger, and separate conduits may be required for power, data, and fire circuits to meet both fill and segregation requirements.
• Adding circuits to existing conduit. When pulling additional cables into conduit that already has cables installed, the fill calculation must account for all cables, existing and new. If the existing cables were installed years ago without documentation, an on-site measurement of the remaining free space is needed before specifying additional circuits.