What this calculator does
Voltage drop is the most common reason an installation that is correct on paper still fails in service: lights dim, motors overheat and electronics reset. This calculator applies the standard drop equation to a one-way conductor resistance, so you can see the voltage actually delivered to the far end and judge whether it is inside your design limit.
How to use it
- Enter the supply voltage and the design load current for the circuit.
- Enter the one-way length of the conductor run — measure the route, not the plan distance, and do not add the return leg twice.
- Pick the conductor resistance from the table, which lists standard copper sizes at 75 °C.
- Choose single-phase, three-phase or DC, then set the acceptable drop limit (3 % for a branch circuit, 5 % for the total run are the usual conventions).
Governing formula
ΔV = k · I · R · L Where:
-
k= 2 for DC/single-phase (out and back), √3 for three-phase [–] -
I= Load current [A] -
R= Conductor resistance [Ω/km] -
L= One-way run length [m]
Derived Equations:
R_1way = R × L / 1000 ΔV% = ΔV / V × 100 V_load = V − ΔV Worked example — 32 A single-phase run of 45 m on 10 mm² copper
The figures below were produced by running this page's own engine with the example inputs (Supply voltage (V), Load current (A), One-way conductor length (m), Conductor resistance, Circuit type, Acceptable voltage drop (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Voltage drop | 5.27 V |
|---|---|
| Voltage drop | 2.29 % |
| Voltage at the load | 224.73 V |
| Conductor resistance (one way) | 0.08 Ω |
| Assessment | Drop 2.3% is within the 3% target. |
- The 10 mm² row gives 1.83 Ω/km at 75 °C.
- Single-phase current travels out and back, so k = 2: the loop resistance is 2 × 1.83 × 45 / 1000 = 0.1647 Ω.
- Compare the resulting percentage against the 3 % branch-circuit limit.
Assumptions & limitations
Engineering calculation. Conductor resistance and voltage drop are calculated directly from circuit length and load current. This is not an ampacity, protection or earthing study.
- Only the resistive component of the drop is calculated; inductance and transient effects are ignored.
- Resistance must match the installed conductor size, material and temperature — the table values are copper at 75 °C.
- No ampacity, thermal protection coordination or earth-fault loop impedance is checked.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
Schedule mode — conductor options
Choosing a conductor is a comparison, not a single answer. Enter the options you are weighing at the same duty and read their drops side by side; the sheet reports the best of them and does not recommend one.
The schedule totals are the sum of every item's own engine result, so they match running this calculator once per item. An item the engine rejects is reported in place and contributes nothing to the totals.
- Options are compared only at the single duty entered above. A conductor sized for one load is not thereby sized for another.
- The cheapest-to-install option is not assessed: this sheet compares electrical drop only, not cost, voltage drop at motor start, or mechanical suitability.
References & standards
- IEC 60364-5-52 — selection and erection of electrical equipment (voltage drop recommendations).
- BS 7671 — current-carrying capacity and voltage drop of installed conductors.
- NEC Chapter 9 Table 8 — AC resistance and reactance of insulated conductors.