فحص الدائرة القائمة

How this is calculated#

The tool takes a circuit that is already installed and runs the five checks of the cable size calculator at its size. It uses the same code, so the two tools agree on every size.

1. Current capacity. It reads the tabulated rating of the installed size for your material, insulation, core count and installation method. It multiplies the rating by the correction factors for the route to get Iz. Iz must be at or above the design current Ib.
2. Voltage drop. It works out the drop for the run length, the power factor and the conductor. It compares the drop with the limit you set.
3. Breaker rating. The breaker rating In must sit between the two: Ib at or below In, and In at or below Iz. The tool gives both margins.
4. Loop impedance. It adds the active and earth conductor resistance to the external impedance Ze. The total Zs must stay under the maximum for the breaker curve and rating.
5. Fault withstand. If you enter a prospective fault current, the conductor must withstand it for the clearing time.
6. Result. The circuit passes when every check it runs passes. The tool gives the smallest standard size that passes with the same breaker, and the largest breaker of the same curve that the cable allows.

Tables without a standard named are in the part of AS/NZS 3008.1 you pick. AS/NZS 3008.1.1:2025 numbers them by section.

Common circuits, worked with the tool#

Each line states its circuit. Every line uses multicore copper cable with PVC insulation and one circuit on the route, enclosed in air at 30 °C. Every breaker is a C curve and Ze is 0.35 Ω, on the New Zealand tables.

• Socket‑outlets, 2.5 mm² on a 20 A breaker over 30 m. It passes every check. The drop is 4.25 %, close to the 5 % limit.
• The same cable on a 32 A breaker with a 25 A load. It fails the current, drop and breaker checks. 6 mm² is the smallest size that passes.
• 4 mm² on a 32 A breaker over 20 m. It fails the breaker check, because Iz is 29 A. The largest breaker the cable allows is 25 A, and 6 mm² is the smallest size that passes on 32 A.
• Three‑phase submain, 16 mm² on a 63 A breaker over 80 m. It fails on loop impedance: Zs is 0.76 Ω against a maximum of 0.49 Ω. With this breaker, 70 mm² is the smallest size that passes.

New Zealand and Australia#

The Standard select picks the part of AS/NZS 3008.1 the tool reads. For Australia, the Edition select picks 2025 or the superseded 2017 edition. The breaker and Zs rules come from AS/NZS 3000, which is the same in both countries.

What the tool does not check#

The tool works from the inputs, not from a test. Measure Ze and Zs on site and compare the reading with the maximum it gives. It does not check RCD rules or discrimination between breakers.

To size a new circuit, use the cable size calculator. To read the factors for a route on their own, use the correction factors calculator.