Cable Length Calculator Australia

How to estimate cable length on a drum

Weighing a partial cable drum gives an immediate, non-destructive estimate of remaining cable length. The calculation subtracts the empty drum weight (tare weight) from the gross weight on the scale, then divides the remaining net cable mass by the conductor mass per metre.

Cable manufacturers in Australia stamp or print the empty drum tare weight on the timber or plastic flange. If the label is unreadable, sparkies also use the electrical resistance method on dead cables: short two cores at the far end, measure loop resistance with an ohm meter, and divide by the AS/NZS 3008 resistance per metre for that conductor size.

Maximum cable run length and voltage drop

Having enough metres on the drum is only half the equation. AS/NZS 3000 Clause 3.6 dictates that total voltage drop from the point of supply to any final sub-circuit point must not exceed 5% (11.5V on a standard 230V single-phase supply, or 20V on a 400V three-phase supply).

Longer cable runs increase total conductor resistance, driving up voltage drop under heavy current loads. Three-phase circuits achieve longer allowable run lengths for the same conductor size because balanced phase currents reduce line-to-line voltage drop per metre compared to single-phase loops.

Worked example — checking cable on a drum

Consider a part-used drum of 6mm² flat TPS copper stored in your service vehicle. The drum flange lists a tare weight of 12 kg. Rolling the drum onto a platform scale gives a total gross weight of 45 kg.

With 141 metres remaining on the drum, you can confirm whether the reel is sufficient for a 90-metre workshop run, while leaving 50 metres reserved for smaller jobs.

Common mistakes with cable length estimation

• Forgetting the tare weight: Dividing gross weight directly by weight per metre without subtracting the drum weight inflates the estimated length by 20% to 40%.
• Using the wrong cable type weight: Flat TPS, circular multi-core, and single-core building wires of the same mm² size have different sheath thicknesses and total weights per metre.
• Ignoring voltage drop on long runs: Having 100 metres of 2.5mm² TPS on a drum does not mean you can run a 20A GPO circuit 100 metres away without exceeding AS/NZS 3000 voltage drop limits. Always verify maximum run distance with our Voltage Drop Calculator.
• Not accounting for waste and terminations: Straight building runs require extra length for wall cavity drops, switchboard entries, joist bends, and termination tails. Add 10% to 15% allowance to raw tape measurements.

Common cable weights per metre reference

Reference values for standard Australian V-90 PVC building cables. Use these baseline figures when manufacturer spec sheets are not available on site:

This calculator provides estimates for reference only. Always verify results against the current edition of the relevant Australian Standard. OzTradie Calc is not a substitute for professional engineering advice.