Conduit fill calculator
Check that a cable bundle fits in a conduit.
Reads AS/NZS 3000:2018. Tables checked 19 Sept 2026.
Try a job 7
Each job fills in example values. Replace them with your values.
- Fill a conduitThe space factor of a 4 mm² single core circuit in a 25 mm conduit.
- Fit a submain in a conduitWhether four 16 mm² singles fit in a 32 mm conduit.
- Fit two flat cables in a conduitWhether two 2.5 mm² twin and earth cables fit in 20 mm.
- Fit PV string cablesWhether six 6 mm² singles fit in a 32 mm conduit.
- Fit a 35 mm² submainWhether four 35 mm² singles fit in a 50 mm conduit.
- Fit a motor circuitWhether four 6 mm² singles fit in a 25 mm conduit.
- Fit cables in an Australian conduitWhether four 4 mm² singles fit in a 25 mm conduit on the Australian sizes.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Country
- New Zealand
- Conduit size
- 32 mm
- Cables
- 2 rows
Conduit fill
97.66%
- Permitted fill
- 40%
- Recommended conduit
- 50mm
- Conduit internal area
- 539.13mm²
- Cable area
- 526.52mm²
This result is a design aid. Verify it against the current standard.
Calculation steps
- Conduit internal area539.13mm²
AS/NZS 2053 / AS/NZS 5000 manufacturer data (derived)
- Cable area526.52mm²
AS/NZS 2053 / AS/NZS 5000 manufacturer data (derived)
- Fill percentResult97.66%
AS/NZS 3000:2018 App C Para C6.2; Tables C10–C12 give a maximum-count guide
- Recommended conduit50mm
About this calculator
How this is calculated
The tool checks that a bundle of cables fits in a conduit. It uses the fill percentage method of Paragraph C6.2 in Appendix C of AS/NZS 3000:2018. Appendix C also holds Tables C10 to C12, which list a maximum count of cables instead.
- Conduit area. The tool reads the internal area of the conduit size you pick. The data comes from the maker and AS/NZS 2053 sizes. The AmpSize hub marks this value as derived.
- Cable area. It reads the overall area of each cable by size and cores. It multiplies by the count and adds the rows.
- Fill percent. It divides the cable area by the conduit area.
- Permitted fill. It applies the space factor of Paragraph C6.2. The factor is 50 % for one cable and 33 % for two. It is 40 % for three or more.
- Result. The tool passes the bundle when the fill is at or below the permitted figure. It gives the smallest conduit that passes.
| Step | Table or clause | What it decides |
|---|---|---|
| Fill percent | AS/NZS 3000 App C Para C6.2 | The space factor for the bundle |
| Maximum count | AS/NZS 3000 Tables C10 to C12 | A guide by cable count |
Worked example
- 4 single-core cables of 6 mm² in a 25 mm conduit. The cables take 81.71 mm² of a 301.72 mm² conduit. The fill is 27.08 % against a 40 % limit. The bundle passes.
Limits
- The tool does not model bends, pull length or the pulling tension. A long or bent run needs a bigger conduit.
- It does not apply a derating for cables in conduit. Use the correction factors calculator for that.
- It reads one conduit. It does not check a trunking or a cable tray.
- The conduit areas are maker data, not a Standard value. Check them against the conduit you buy.
See also: Conduit fill and installation methods, Conduit fill for 4 x 6 mm² in 25 mm and Appendix C conduit space factor.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the conduit fill calculator do?
The tool totals the cross sectional areas, cable by cable, for the whole bundle. It divides that total by the internal area of the conduit to get the fill percent. It compares the fill with the permitted limit and suggests the smallest nominal conduit that fits the bundle.
What fill limit does the tool use?
The tool uses 50 percent for one cable, 33 percent for two cables and 40 percent for three or more. These are the space factors in AS/NZS 3000:2018 Appendix C, Paragraph C6.2. Tables C10 to C12 of the same Appendix give a maximum cable count method instead.
Which inputs matter most?
The nominal conduit size sets the internal area. The conductor size, the core count and the cable count set the cable area. At the same conductor size, a single core cable and a multicore cable have different overall diameters. The core choice therefore changes the result.
What are the common gotchas?
The tool uses the fill percentage method, not the cable count method. Overall diameters are derived from manufacturer data and vary by brand. Bends and draw in force are not modelled. The conduit size and the cable rows sit in the page address, so a link reproduces the bundle.
Worked examples
- AS/NZS 3000 Appendix C: conduit space factorWhat the conduit space factor in AS/NZS 3000 Appendix C governs, which tool reads it, and a worked 50 mm conduit that reaches 52.95 % fill against a 40 % limit.
- Conduit fill: 4 x 6 mm² singles in 25 mmFour 6 mm² single-core cables in 25 mm conduit, with the fill percent, the space factor and the smallest conduit the tool recommends for the bundle.
Background
Clauses and tables this tool reads
- AS/NZS 3000:2018 App C Para C6.2Cables in enclosures
- AS/NZS 2053 heavy-duty conduit (manufacturer data)Conduit internal diameters
- AS/NZS 5000.1 cables, manufacturer dataCable overall diameters
Every clause and table, with how each was verified
Tables last checked on 19 Sept 2026.
Related tools
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- Maximum demand calculatorEstimate maximum demand for domestic or non-domestic loads.AS/NZS 3000 Table C1/C2
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