Tātaitai rahi taura DC

How this is calculated#

The tool tests every standard conductor size in turn. It reports the smallest size that passes the current check and the voltage drop check.

1. Tabulated d.c. current. It reads the d.c. rating of each size for your cables, material, insulation and installation method. AS/NZS 3008.1.1:2025 prints these ratings for 40 °C air and 25 °C soil. The tool skips a size with no printed rating for the method.
2. Correction factors. It multiplies the ambient, grouping, soil and burial depth factors into one correction factor (derating factor in 2017). One circuit alone takes no grouping factor. On a tray, ladder, rack or cleats, pick the cable support.
3. Current check. It multiplies the rating by the correction factor to get Iz. The size passes when Iz is at or above the design current.
4. Voltage drop. It multiplies the d.c. mV/A.m by the current and the route length, one way, and divides by 1000. The mV/A.m covers both conductors of the circuit.
5. Limit. It compares the drop with the limit for the supply type. The limit follows the supply type until you type another one.
6. Result. The candidate table shows each size, its Iz, its drop and the check it fails. The result says which check sets the size.

Tables and clauses without a standard named are in AS/NZS 3008.1.1:2025.

Common circuits, worked with the tool#

Each line states its route. Change one input and the size can change, so run your own circuit before you buy cable. Every line uses copper with PVC insulation and one circuit on the route. Each uses the Australian conditions, except where a line says otherwise.

• Caravan, 30 A at 12 V over 6 m. One 2‑core cable, enclosed in air at 40 °C, against the 10 % ELV limit. It needs 10 mm² with a 0.8 V drop, 6.68 %. The drop sets the size, because 4 mm² carries the current but drops 16.8 %.
• Battery to inverter, 100 A at 48 V over 5 m. Two single‑core cables, unenclosed and touching at 40 °C. It needs 35 mm² with a 0.64 V drop, 1.32 %. The current sets the size.
• PV array feed, 15 A at 600 V over 40 m. One 2‑core XLPE cable, enclosed in air at 40 °C, against the 5 % network limit. It needs 1.5 mm² with a 20.82 V drop, 3.47 %. The drop sets the size, because 1 mm² carries the current but drops 5.41 %.
• NZ battery to inverter, 100 A at 48 V over 5 m. The same route at 30 °C in New Zealand. It needs 25 mm² with a 0.89 V drop, 1.84 %. The current sets the size. The result is derived.

New Zealand#

AS/NZS 3008.1.2:2017 has no d.c. ratings. For New Zealand, the tool uses the AS/NZS 3008.1.1:2025 d.c. ratings. It corrects them from 40 °C to the air temperature you enter, 30 °C by default. A buried run corrects from 25 °C to the soil temperature, 15 °C by default. No NZ table prints these figures, so the result says Derived.

What the tool does not check#

The tool does not check the d.c. protective device. Check its rating against Iz. It does not check the prospective short-circuit current of a battery. Check the cable and the device against the battery maker's fault current.

To size the battery bank itself, use the battery calculator. To check a 4–20 mA loop on a d.c. supply, use the loop power budget. For an a.c. circuit, use the cable size calculator or the voltage drop calculator. To check the drop on a d.c. cable you have already chosen, use the d.c. mode of the voltage drop calculator. It works from the d.c. resistance, so its last digit can differ from this tool.