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How this is calculated#

Every cable turns some of the energy it carries into heat. The loss is the current squared times the resistance of the conductors that carry it. A larger cable has less resistance, so it wastes less. This tool puts a yearly figure on that waste for two sizes, so you can see what the size up saves.

The tool reads the a.c. resistance of each size from AS/NZS 3008.1, in ohms per kilometre, for the conductor metal and insulation you pick. It reads the table at the conductor's full-load temperature. On a single‑phase circuit the current flows out and back, so two conductors carry the loss. On a balanced three‑phase circuit the neutral carries no current, so three do.

The loss in watts is the number of loaded conductors times the current squared times the resistance times the length. The tool multiplies it by the running hours a year to get kWh. The tariff turns kWh into dollars, and the emission factor turns kWh into kilograms of CO2. Both are figures you set from your own contract and grid.

The difference between the two sizes is the saving a year. If you enter what the size up costs over the smaller cable, installed, the tool divides that by the saving to give the payback in years.

Enter the rms current over the running hours, not the mean or the peak. Loss grows with the square of the current, so a cable that runs near full load for long hours gains the most from a size up.

Worked examples#

• A 100 A feeder of 60 m in 35 or 50 mm² XLPE saves $375.84 a year with the size up. That is 1,252.8 kWh a year, and the extra $800 pays back in 2.1 years.
• A 200 A feeder of 120 m that never stops, 95 or 120 mm² saves $1,892.16 a year. That is 6,307.2 kWh a year, and the extra cost pays back in 1.3 years.
• A 25 A heat pump circuit of 40 m in 6 or 10 mm² PVC saves $68.40 a year. That is 228 kWh a year, and the extra cost pays back in 2.2 years.
• A 150 A aluminium feeder of 100 m, 95 or 120 mm² saves $870.75 a year. That is 2,902.5 kWh a year, and the extra cost pays back in 1.7 years.

Limits#

The resistance is the table figure at the full-load temperature. A cable at part load runs cooler, so the real loss is a little lower than the tool shows.

The tool does not size the cable. Size it first for current, voltage drop and fault with the cable size tool, then compare the size that passes with the next size up. It does not count the time value of money, a tariff that changes through the day, or harmonic currents, which add loss.

See also: Cable size, Voltage drop and Three phase power.