Installation design
Voltage drop using mV/A/m
- where
- voltage drop (V)
- tabulated circuit voltage-drop coefficient (mV/A/m)
- current (A)
- one-way length (m)
Explain this equation
Gives the voltage drop along a circuit using the mV/A/m value from the cable tables. It is the standard quick check on final circuits and submains.
m is the tabulated voltage-drop figure in millivolts per amp per metre for the cable, installation method and circuit type. I_b is the design current in amperes. L is the one-way length of the run in metres. ΔU is the drop in volts.
Voltage drop is current times impedance, and impedance grows with length. The tabulated figure already combines the conductor resistance and reactance per metre for the whole circuit, including the return conductor. The 1000 converts millivolts to volts.
18 mV/A/m at 10 A over 20 m gives 3.6 V, about 1.6% of 230 V. Double the length or the current and the drop doubles.
Watch out: use the one-way length; the table already covers go and return, so don't double it. Single-phase and three-phase figures differ.
A circuit voltage-drop table can already include the outgoing and return conductor effects. Multiply its applicable mV/A/m figure by current and one-way length.
Convert millivolts to volts by dividing by 1000. Compare against the allocated project limit after including upstream circuits.
Enter your values to see the result.
Common mistake
Doubling route length when the tabulated figure already covers the circuit.
Limits
Use the applicable published circuit mV/A/m value, temperature, phase arrangement and power-factor treatment. The coefficient must already include circuit conductor effects; enter one-way length.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.