AS/NZS 3008 Cable Sizing Calculator

The 40 °C trap

This is the one that catches everybody arriving from BS 7671 or the IEC, and it goes the opposite way to how people expect.

AS/NZS 3008 tabulates in-air capacities at 40 °C, not 30 °C. The table already assumes an Australian summer. So a designer who applies a 0.87 derating for a 40 °C ambient — the habit BS 7671 teaches — has derated for a temperature the table had covered, and ends up specifying a cable a size larger than needed.

It also runs the other way. At 30 °C the correction factor is above one: 1.20 for V-75. A cooler-than-reference installation gains capacity, which no amount of BS 7671 instinct prepares you for.

Buried circuits are a third reference again — 25 °C soil, not air. The calculator moves the temperature field to the right base when you change installation method, so you are never quietly applying an air figure to a soil table.

V-75 or X-90

The two everyday insulation families, and the choice is worth more than most people assume. X-90 (XLPE) runs at a 90 °C conductor against V-75's 75 °C, which across the tables here is worth about eighteen per cent of capacity for the same copper.

That is frequently a whole size. A 60 A load in conduit needs 25 mm² in V-75 and 16 mm² in X-90 — try it above. On a long run the copper saved usually pays for the dearer cable several times over, which is why X-90 dominates commercial work here in a way it does not in the UK.

Volt drop does not improve, though. The mV/A/m figures are a property of the conductor, not the insulation, so a size dropped on capacity may be given straight back by volt drop on a long route. The calculator says which of the two governed.

Installation method

Chosen before any factor, and it moves the answer more than the factors do. Australia and New Zealand bury far more cable than the UK does, so two of the five methods are underground.

• Unenclosed, spaced — a clear air gap all round. The best case.
• Unenclosed, on a surface — clipped to a wall, tray or catenary.
• Enclosed in conduit — in air or in a wall. The harshest of the three in-air methods.
• Buried direct — 25 °C soil reference. Often better than conduit in air, because soil carries heat away.
• Underground enclosure — conduit below ground. The air gap inside the duct costs you against direct burial.

A 40 A circuit needs 6 mm² clipped, 6 mm² buried direct, and 10 mm² in conduit either in air or underground.

Volt drop is one budget

AS/NZS 3000 clause 3.6.2 allows 5 % from the point of supply to any point of use — one figure covering the whole path, not BS 7671's separate 3 % for lighting.

That sounds more generous and often is not, because the 5 % has to cover the submain and the final subcircuit together. Size the submain tightly and the final circuit inherits whatever is left.

Capacity tables

What this does not do

• Earth fault loop impedance. A separate check, and one a cable can fail after passing both of these.
• Short-circuit withstand. The adiabatic check on the conductor and the earth.
• Aluminium and single-core in trefoil. Copper multicore only.
• Maximum demand. Ib is what you supply after diversity, not the connected load.

A note on the figures

The method here is the standard's. The capacity tables are representative values following the Table 4 pattern, and my confidence in them is lower than for the NEC and BS 7671 sets on this site — those two are ground I know better. Check the output against a current copy of AS/NZS 3008.1.1 before you rely on it, and treat this as a way to get to the right size quickly rather than as the authority for it.