Every ASP Level 2 contractor needs to verify voltage drop before quoting a job. Get it wrong and your cable run fails compliance — get it right and you save the client money by not over-specifying cable.

The AS/NZS 3008 mV/A/m Method

The standard method for calculating voltage drop in Australian electrical installations uses millivolts per ampere per metre (mV/A/m) values from AS/NZS 3008 tables. The formula is straightforward:

Vd = (mV/A/m × I × L) / 1000

Vd — Voltage drop in volts
mV/A/m — Cable characteristic from AS/NZS 3008 (depends on cable size, material, and installation method)
I — Design current in amperes (typically your service rating: 63A, 100A, etc.)
L — Cable length in metres (one-way run, not return)

Compliance Limits

AS/NZS 3000 Clause 3.6.2 sets the maximum allowable voltage drop:

5% total from the point of supply to the furthest point of the installation
1.5% for consumer mains (the section from the meter to the main switchboard)

For a standard 230V single-phase supply, 5% equals 11.5V — any more and you're non-compliant.

Practical Example

Say you're quoting a 63A single-phase underground connection with a 35-metre cable run using 16mm² Cu XLPE:

mV/A/m for 16mm² Cu XLPE = 2.80 (from AS/NZS 3008 Table 40)
I = 63A
L = 35m

Vd = (2.80 × 63 × 35) / 1000 = 6.17V = 2.68% ✓ Compliant

When Cable Length Makes or Breaks the Quote

The longer the run, the bigger the cable you need. This directly affects your quote — going from 16mm² to 25mm² cable can add hundreds to the materials cost. That's why accurate distance measurement matters more than most contractors realise.

Eyeballing a cable run on Google Maps and rounding to the nearest 10 metres might seem close enough, but on a borderline voltage drop calculation, those extra metres are the difference between a compliant 16mm² cable and having to upgrade to 25mm².

Automate It

MAINS calculates voltage drop automatically as you draw your cable route on satellite imagery. It recommends the smallest compliant cable size and shows a pass/fail result against AS/NZS 3008 limits — before you generate the quote.