Voltage Drop Calculator

The formula

Vd = (mV/A/m × I × L) / 1000

The mV/A/m figure is published per cable size and already accounts for the √3 in three-phase systems and for the conductor's reactance as well as its resistance. You do not add a √3 on top — a common double-counting error.

The limits

IS 732 permits a total drop of 3 % for lighting and 5 % for other uses, measured from the origin of the installation to the point of use. That budget is for the whole path, not each cable in it. If a submain already uses 2 %, the final circuit has 3 % left of a 5 % allowance.

The lighting limit is tighter because incandescent and some LED drivers visibly dim; motors tolerate a little more but lose torque with the square of voltage.

What excessive volt drop actually causes

• Motors run hot. Torque falls with the square of voltage. A motor at 90 % voltage produces 81 % torque, so it draws more current to hold the same load, and heats up.
• Contactors chatter. Coils drop out around 80 % of rated voltage. A long run plus a starting dip can cross that line.
• Energy is wasted continuously. The drop is resistive loss, paid for every hour the circuit runs.

Fixing a failing run

In rough order of cost-effectiveness:

1. Move the distribution board closer. Drop is linear in length. Halving the run halves the drop, and it is often cheaper than upsizing a long cable.
2. Go up a cable size. Straightforward, and the calculator above tells you which size clears the limit.
3. Run parallel cables. Two cables halve the drop and share the current. Worth it at large sizes where a single cable becomes hard to bend and terminate.
4. Correct power factor at the load. Less current for the same real power means less drop. Helps the demand charge too.

Starting dip is a separate check

This calculator uses running current. During a direct-on-line start, current is 6–8× and the drop scales with it — a run at 4 % running can dip past 20 % on start. That will not overheat the cable, but it can drop out contactors and dim lighting elsewhere on the board. For a DOL motor on a long run, check the drop at starting current too.

Volt drop adds up along the chain

The limit is not per cable. It is the total from the origin of the installation to the point of use, so a submain that drops 3 % and a final circuit that drops 3 % have each “passed” and together have failed.

On anything with more than one level of distribution, set a budget before you size anything — 2 % for the submain and 3 % for the finals is a common split — and size to the budget rather than to the headline figure.

Where the mV/A/m figure comes from

It is not resistance converted. The tabulated figure already contains three things: conductor resistance at its 70 °C operating temperature, inductive reactance, and the √3 for a three-phase circuit.

Reactance is why you cannot derive it from ohms per kilometre. On small cables it is negligible; above roughly 95 mm² it becomes a real share of the total, and on 300 mm² and larger it dominates — which is why volt drop stops improving much as you keep going up in size. Past that point, more copper buys very little.

When volt drop, not heat, picks the cable

On short runs the current rating decides. Past a crossover length volt drop takes over and the cable is larger than heating alone requires. The crossover arrives sooner than people expect, because drop scales with current while the tabulated rating does not.

Cheaper fixes than copper

Volt drop is a function of current and length, so both are levers before cross-section is:

• Move the board. A distribution board nearer the load turns a long heavily-loaded run into a short one plus a lightly-loaded submain.
• Split the load. Two circuits at half the current each drop half as much over the same distance.
• Correct the power factor. Less current for the same work is less drop, everywhere upstream of the correction.
• Check the tap setting. If the whole site sits low, the transformer tap is the fix, not the final circuits.