How do we measure the amount of current a cable can carry over a particular distance?
A current which flows in a conductor will cause a voltage drop over the conductor’s length. This voltage drop is due to the resistance of the conductor.
The Wiring Rules states a maximum limit for voltage drop of 5% for low voltage systems.
Therefore there is a simple calculation which can be done that relates the percentage volt drop, the cable length, and the voltage drop factor for a particular cable.
This calculation and voltage drop factors (in mV/A.m –milliVolt per Amp metre). The formulae for how much current is:
Where:
I = cable current that produces the maximum voltage drop
V = system voltage (ie normally 400V for 3 phase and 230V for single phase)
Vc = mV/A.m volt drop factor for cable from AS/NZS 3008.1.1
L = circuit length
Note: In addition to performing this voltage drop calculation, the continuous current rating of the cable must be checked.