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Cable size calculator
Smallest cable that passes current capacity, voltage drop and fault loop.
Cable size
- Smallest cable that passes
- 4 mm²
- Current capacity
- 28 A (breaker 20 A)
- Voltage drop
- 2.87% of 4% available
- Fault loop
- 48% of maximum length
Method
- Tries each size from the smallest up and picks the first that passes all three checks:
- 1. Current capacity for "enclosed (conduit, wall cavity)" ≥ breaker rating
- 2. Voltage drop at the breaker rating ≤ 4%
- 3. Length ≤ 90% of the fault-loop maximum length
- Ruled out 2.5mm²: voltage drop 4.62% > 4% budget
Reference
- AS/NZS 3008.1.1 current-carrying capacity and mV/A·m tables (Cu, V-90, multicore)
- AS/NZS 3000 clause 3.6 (voltage drop) and Appendix B, Table B1 method
Notes
- Indicative values. Verify against AS/NZS 3008.1.1 for the actual installation.
- No grouping or ambient-temperature derating applied.
Verify against the current standard before relying on this.
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