Cable Sizing Calculator

Sizing a cable properly means checking it against three separate requirements, and the cable has to pass all three — not just the biggest one.

• Current-carrying capacity. Every cable has a base rating for its conductor size, insulation type, and installation method. That base rating then gets derated for ambient temperature, grouping with other loaded circuits, and contact with thermal insulation, so the conductor stays within its rated operating temperature under real site conditions.
• Voltage drop. Even a cable with plenty of current-carrying capacity can fail on a long run, because voltage lost along the conductor scales with distance and current. AS/NZS 3000 caps total voltage drop at 5% from the point of supply to the load.
• Short-circuit and earth-fault withstand. The cable also has to survive the fault current available at that point in the installation for as long as it takes the upstream protection to clear it, without the conductor or insulation being damaged.

This calculator handles the first two automatically from your load, installation method, and run length, and will run the third if you enter a fault level and clearing time. For a deeper look at how the derating factors stack up, see the cable derating calculator. To check a specific run against the 5% limit on its own, use the voltage drop calculator.

The same cable carries different current depending on how it is installed, and it is not a small difference. A conductor clipped direct to a wall in open air sheds heat straight into the surrounding air on all sides. Put that same conductor inside conduit, and the heat has to conduct through the conduit wall first, with far less air movement to carry it away. Bury it in the ground, and the surrounding soil is a worse conductor of heat again, plus the fault current path and drying-out effects around the cable come into play.

Because the conductor has to stay under its maximum operating temperature to protect the insulation, a more restrictive installation method means a lower current rating for the same cable size — or a bigger cable to carry the same current. That is why the installation method you select in the calculator above changes the result even when the load and cable type stay identical. Grouping multiple circuits together compounds it further, since bunched cables all competing to dump heat into the same limited space each run hotter than they would on their own.

Say you are feeding a 15kW three-phase motor at 400V with a 0.85 power factor, running 40m of cable enclosed in conduit on a wall, ambient temperature 35°C, sharing that conduit with two other loaded circuits. Before anything else, work out the full load current the cable actually has to carry:

25.5A is the input current, not the answer — it is only the starting point for the current-carrying capacity check. From there, the calculator applies the ambient temperature derating for 35°C, a grouping derating for three circuits sharing the conduit, and then checks the 40m run for voltage drop at that current. Enter these exact assumptions above and it will return the minimum compliant cable size and confirm whether voltage drop clears the 5% limit — worth doing rather than guessing, since the derating and grouping factors together can shift the result by more than one standard cable size compared with a cable in free air.

The earthing conductor does not need to match the active conductor size one-for-one as cables get bigger. AS/NZS 3000 Table 5.1 sets a minimum earth size relative to the active: for smaller circuits the earth typically matches the active size, but once the active gets large enough the table allows the earth to be a smaller fraction of it. That is because the job of the earth conductor during a fault is short-duration — it has to carry fault current only for as long as it takes the protection to trip, not carry full load current continuously the way the active does.

There is a second check that can push the earth size back up: the adiabatic (I²t) calculation, which looks at the actual prospective fault current and how quickly the upstream protection clears it. On sub-mains protected by slower-clearing devices, the adiabatic result can demand a bigger earth than the Table 5.1 minimum. Always check both — the table minimum and the adiabatic result — and use whichever is larger. If you need to verify that your protection actually clears a fault fast enough for the earth size you have chosen, the earth fault loop calculator checks that side of it.