Imperial cable size converter
Find the device rating for an old imperial cable.
Reads AS/NZS 3000:2018. Tables checked 26 Sept 2026.
Try a job 3
Each job fills in example values. Replace them with your values.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Imperial cable size
- 7/.029
- Installation method
- Unenclosed
- Insulation
- V75
Maximum circuit-breaker rating
32A
- Existing fuse or plug-in circuit-breaker
- 20A
- Calculated area
- 2.98mm²
- Nearest metric size
- 2.5mm²
This result is a design aid. Verify it against the current standard.
Calculation steps
- Wires in the conductor7
- Wire diameter, inches0.03
- Calculated conductor area2.98mm²
- Nearest metric size2.5mm²
- Maximum circuit-breaker ratingResult32A
AS/NZS 3000:2018 Table I1
- Maximum existing rewireable fuse or plug-in circuit-breaker rating20A
AS/NZS 3000:2018 Table I1
About this calculator
How this is calculated
The tool finds the largest fuse or circuit-breaker for an old imperial-size cable. You use it when you alter or repair existing wiring. It reads AS/NZS 3000:2018 Appendix I, Table I1.
- Wires. You pick the size as the wire count and the wire diameter in inches, such as 7/.029.
- Calculated area. The tool works out the conductor area from the wire count and the diameter. This is a calculation. It is not a table value.
- Nearest metric size. It gives the nearest metric size for that area.
- Method and insulation. You pick how the cable is installed, unenclosed or partially surrounded, and the insulation, V75 or V60.
- Circuit-breaker rating. The tool reads the maximum circuit-breaker rating from Table I1.
- Existing device. It also reads the maximum rating for an existing rewireable fuse or plug-in circuit-breaker.
| Step | Table or clause | What it decides |
|---|---|---|
| Maximum rating | AS/NZS 3000 Table I1 | The largest breaker for the size, method and insulation |
| Existing device | Table I1 and Cl 2.5.2 | The largest fuse or plug-in breaker already in place |
Worked example
A 7/.029 conductor, unenclosed, with V75 insulation. The calculated area is 2.98 mm². The nearest metric size is 2.5 mm². The maximum circuit-breaker rating is 32 A. The maximum existing fuse or plug-in breaker is 20 A.
Limits
- Check the scope of Appendix I. It is written for existing imperial conductors.
- The second rating is for a device already in place. Read Clause 2.5.2 before you plan a new device.
- The two notes under Table I1 set conditions. Read them on the page.
- Do not use the area for conductors in parallel. Read Paragraph I3 and Table I2.
- The tool does not check voltage drop or the condition of the old insulation.
See also: Cable size calculator, What size breaker for 4 mm² cable and Checking an existing circuit.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the imperial cable tool do?
The tool reads AS/NZS 3000:2018 Table I1. Pick the imperial size, the insulation and the installation method. AmpSize then reports the largest circuit-breaker the table prints for that conductor. It also reports the largest existing fuse or existing plug-in circuit-breaker. Appendix I gives the scope.
Can I fit a rewireable fuse to an old circuit?
No. AmpSize treats the second column of Table I1 as an existing device only. Clause 2.5.2 governs that point, and Note 2 under the table points at it. Use that rating where a rewireable fuse or a plug-in circuit-breaker is already in place on an alteration or a repair.
How is the metric equivalent worked out?
AmpSize calculates the conductor area from the wire count and the wire diameter, then reports the nearest standard metric size. That area is arithmetic, not a value read from the standard. AmpSize does not encode Table I2, the table for cables connected in parallel. Read it in the standard for that case.
Do the Table I1 ratings apply to every installation?
No. Note 1 under Table I1 sets the conditions the ratings hold under. Read it on the page. Then check the route, the grouping and the voltage drop before you rely on a value.
Worked examples
Clauses and tables this tool reads
Every clause and table, with how each was verified
Tables last checked on 26 Sept 2026.
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