Voltage drop in a circuit represents the vectorial difference between the supply end and the load end. This calculator uses the accurate voltage drop method described by the Standard AS/NZS 3008.1.
Accurate voltage drop calculations will result in smaller cable sizes.
Note that cable operating temperature is not considered with this calculator – Try Cable Pro Web software for the highest accuracy and smaller cable sizes.
The Wiring Rules AS/NZS 3000:2018 sets the limits for voltage drop within an installation.
Please enter the parameters below and press Calculate.
How to calculate voltage drop
For a worked example of how to calculate voltage drop refer to this article.
We use an accurate method for voltage drop which considers power factor. Note this calculator does not consider cable operating temperature (which affects resistance) and is the most accurate.
For single phase AC the voltage drop is calculated as:
\(\Delta V_{1\phi-ac}=I L\{2(R_c cos \theta + X_c sin \theta)\}\)
For three phase AC (balanced) the voltage drop is calculated as:
\(\Delta V_{3\phi-ac}=I L\{\sqrt{3}(R_c cos \theta + X_c sin \theta)\}\)
Where:
I is load current in Amps
L is length of the cable in metres
Rc is cable resistance in Ohms/m; a function of material, size and temperature of conductors.
Xc is cable reactance in Ohms/m; a function of the conductor shape and cable spacing.
The current ratings, cable resistance and cable reactance values used by this calculator were taken from AS/NZS 3008.1.1:2017 (based on IEC Standards).
The effect of power factor on voltage drop explained: For small cable sizes which have greater resistance than reactance a higher power factor results in a larger voltage drop. However for large cable sizes which have a high reactance compared with resistance the opposite is true.
Allowable voltage drop limits
Refer to the article Voltage Drop Limits for LV Installations which lists all the voltage drop and voltage rise limits from the Standard AS/NZS 3000:2018.
How to convert power or horsepower into current
The equations to convert the units of the electrical load inputs used by the calculator are as follows.
The load conversion from kW to A for single phase is calculated as:
\(I_{(A)}=\frac{1000\times P_{(kW)}}{PF\times V_{(V)}}\)
For three phase is calculated as:
\(I_{(A)}=\frac{1000\times P_{(kW)}}{\sqrt{3}\times PF\times V_{P-P(V)}}\)
Where PF is power factor and \(V_{P-P}\) is phase-to-phase voltage (i.e. 3-phase voltage).
The load conversion from kVA to A for single phase is calculated as:
\(I_{(A)}=\frac{1000\times S_{(kVA)}}{V_{(V)}}\)
For three phase is calculated as:
\(I_{(A)}=\frac{1000\times S_{(kVA)}}{\sqrt{3}\times V_{P-P(V)}}\)
The load conversion from horsepower (hp) to A for single phase is calculated as:
\(I_{(A)}=\frac{746\times hp}{V_{(V)}\times PF}\)
For three phase is calculated as:
\(I_{(A)}=\frac{746\times hp}{\sqrt{3}\times V_{P-P(V)}\times PF}\)
Related Resources:
Cable Size Calculator
This calculator determines minimum cable size using the method described by the Standard AS/NZS 3008.1.1 and uses the accurate voltage drop method.
Earth Cable Size Calculator
Earth cable size can be determined using Table 5.1 in AS/NZS 3000 or by calculation.
