How the copper V-75 cable sizing tool works
Start with cable construction and the actual installation arrangement. This guide explains the references and assumptions used by the free copper V-75 screening tool. It does not replace a complete installation assessment.
If the design current is not yet known, start with the maximum demand guide . For an existing cable where you only need a drop check, use the voltage drop calculator .
Find a table reference used by the tool
Search the 2025 references used by the tool. A reference link opens the calculator; it does not select or validate an installation for you.
AS/NZS 3008.1.1:2025 reference finder
Find references used by the copper V-75 screening model. Consult your licensed standard for the full table and applicability.
| 2025 Standard | Description & Scope | Conductor & Temp | Action |
|---|---|---|---|
| Table 3.15 | Two loaded cores, thermoplastic multicore Single-phase copper V-75; air 40 °C, ground 25 °C. | Copper V-75 model | Open screening tool |
| Table 3.18 | Three loaded cores, thermoplastic multicore Balanced three-phase copper V-75; choose the actual installation column. | Copper V-75 model | Open screening tool |
| Table 3.33 | Circuits bunched in air or wiring enclosures Row 2 for the bunched wall/conduit arrangement used by this tool. | Copper V-75 model | Open screening tool |
| Table 3.35 | Multicore cables on trays and supports Rows 7 and 10 for one horizontal perforated tray. Spacing does not always remove correction. | Copper V-75 model | Open screening tool |
| Table 3.44 | Ambient air temperature The 75 °C conductor row uses 40 °C air as its reference. | Copper V-75 model | Open screening tool |
| Table 3.45 | Ambient soil temperature The 75 °C conductor row uses 25 °C soil as its reference. | Copper V-75 model | Open screening tool |
| Table 3.46 | Buried-direct depth Depth correction depends on conductor size; base depth is 0.5 m. | Copper V-75 model | Open screening tool |
| Table 3.47 | Underground conduit depth Use the multicore column for the supported underground conduit arrangement. | Copper V-75 model | Open screening tool |
| Table 3.48 | Soil thermal resistivity Base resistivity is 1.2 K·m/W; direct burial and underground conduit use different columns. | Copper V-75 model | Open screening tool |
| Table 5.1 | Adiabatic material constant K Copper V-75: 75 °C initial and 160 °C final gives K = 111.2 for sizes up to 300 mm². | Copper V-75 model | Open screening tool |
| Table 5.2 | Insulation short-circuit temperature limits Verify the complete cable construction, restraint and clearing time. The model supports durations up to 5 s. | Copper V-75 model | Open screening tool |
Choose the applicable installation
Section 3 covers current-carrying capacity and correction factors; Section 4 covers voltage drop and rise; Section 5 covers short-circuit performance. The tool supports copper V-75 multicore cable with two loaded cores for single phase or three for balanced three phase.
Thermal insulation changes the ampacity column. Clause 3.4.3 distinguishes partial surround from complete surround over more than 400 mm along the cable. A universal insulation multiplier cannot describe every arrangement.
The tray option assumes one horizontal perforated row of equally loaded multicore cables. One cable diameter of spacing does not remove correction for every circuit count. Underground grouping, parallel current sharing, aluminium and XLPE sizing need data outside this model.
Harmonic neutral current requires a separate assessment. The entered harmonic allowance increases design current; the calculator does not derive the allowance from harmonic content or calculate the neutral current.
Three checks in this screening tool
The model evaluates the following limits. Protective-device suitability, earth fault loop impedance and the complete installation require additional checks:
The derated capacity \(I_z\) of the cable must equal or exceed the circuit design current \(I_b\) for the selected assumptions; the protective device is not selected here: I_z = I_r × k_amb × k_group × k_depth × k_soil / Kh ≥ I_b
Under AS/NZS 3000 Clause 3.6.2, total voltage drop between the point of supply and final terminal must not exceed 5% (or 7% where a dedicated substation supplies the site). Allocate a submain limit that leaves sufficient allowance for downstream circuits.
Under AS/NZS 3008.1.1:2025 Clause 5.3, conductors must withstand maximum prospective short-circuit fault current without insulation degradation before the protective device clears: S_min = (I_fault × √t) / k
Check a copper V-75 circuit
Enter the installation details to compare cable sizes against the modelled limits. The result shows the correction factors and which check governs the selection.