Wire Voltage Drop Calculator
Calculate the estimated voltage drop in copper or aluminium wire from the current, supply voltage, wire size and one-way cable length.
Calculate wire voltage drop
Enter the circuit details below. Cable length means the one-way distance from the supply to the load.
Advanced AC options
Calculation details
- Conductor material
- Copper
- Wire size
- 2.5 mm²
- One-way length
- 20.00 m
- Resistivity at selected temperature
- 0.017241 Ω·mm²/m
- Circuit type
- Single-phase AC
Worked calculation
How to use the wire voltage drop calculator
Choose the circuit type and conductor material, then select the wire size. Enter the one-way cable length, load current and supply voltage.
The calculator estimates the voltage lost in the conductors and shows that loss in volts and as a percentage of the supply voltage. It also estimates the voltage available at the load.
Cable length is entered as the one-way distance. For DC and single-phase circuits, the calculator automatically accounts for the outgoing and return conductors.
Wire voltage drop formula
For a simple DC two-wire circuit:
ΔV = 2 × I × R × L
For single-phase AC:
ΔV = 2 × I × L × (R cos φ + X sin φ)
For a balanced three-phase AC circuit:
ΔV = √3 × I × L × (R cos φ + X sin φ)
where I is current, L is the one-way cable length, R is conductor resistance per unit length, X is reactance per unit length and φ is the phase angle represented by the power factor.
How wire size affects voltage drop
A larger conductor has lower electrical resistance. For the same material, current and cable length, increasing the conductor cross-sectional area therefore reduces voltage drop.
Longer cable runs and higher current increase voltage drop. Aluminium also has higher resistivity than copper, so equivalent aluminium conductors generally require a larger cross-sectional area for similar resistance.
Conductor temperature and resistance
Electrical resistance increases as conductor temperature rises. CalcAtlas starts with resistivity values at 20°C and adjusts them using a temperature coefficient for the selected conductor material.
The temperature option is useful for comparison, but the actual operating temperature of installed cable depends on the cable construction, current, ambient conditions, installation method and other factors.
The copper resistivity value used by this calculator at 20°C is based on the International Annealed Copper Standard value published in the NIST Copper Wire Tables .
Voltage drop percentage
Voltage drop % = (Voltage drop ÷ Supply voltage) × 100
The comparison target in the calculator lets you compare the calculated result with a percentage of your choice. It is not automatically a legal or regulatory limit.
What is an acceptable voltage drop?
Acceptable voltage drop depends on the type of installation, the connected equipment and the electrical standards or regulations that apply in your location.
As an example, guidance based on IEC 60364-5-52 gives typical maximum voltage-drop values of 3% for lighting circuits and 5% for other uses in low-voltage installations supplied directly from a public distribution system. Requirements can differ between countries and installations.
For further information, see the Electrical Installation Guide — Maximum voltage drop limit .
The percentage entered in the CalcAtlas calculator is therefore a comparison target only. It should not be used on its own to determine whether a cable or electrical installation complies with applicable regulations.
Worked example
Consider a 230 V single-phase circuit using 2.5 mm² copper conductors at 20°C. The one-way cable length is 20 m and the load current is 16 A.
Using a copper resistivity of 0.017241 Ω·mm²/m, the resistance of one conductor is approximately 0.1379 Ω. Because the current travels through both outgoing and return conductors, the voltage drop is approximately 4.41 V.
This represents approximately 1.92% of a 230 V supply, leaving an estimated 225.59 V at the load.
Important limitations
This calculator estimates conductor voltage drop. It does not determine whether a conductor has adequate ampacity, insulation rating, fault-current capacity, protective device coordination or compliance with local electrical codes.
Actual cable resistance can differ because of conductor construction, manufacturing tolerances, connections, temperature and installation conditions.
For AC circuits, reactance and power factor can affect voltage drop. The default settings use a power factor of 1.00 and zero reactance. Enter project-specific values where appropriate.
Balanced three-phase calculations assume approximately equal phase currents. Electrical installation and conductor sizing should be checked against the regulations and design requirements applicable to the installation.
Frequently asked questions
What causes voltage drop in a wire?
Electrical conductors have resistance. When current flows through that resistance, part of the supply voltage is lost along the conductor.
Does a larger wire reduce voltage drop?
Yes. Increasing conductor cross-sectional area reduces resistance and therefore reduces voltage drop when the material, current and length remain the same.
Should I enter one-way or total cable length?
Enter the one-way distance from the supply to the load. CalcAtlas automatically applies the appropriate circuit factor for DC two-wire, single-phase and balanced three-phase circuits.
Can I use AWG wire sizes?
Yes. Select AWG under Wire size standard and choose the required gauge. The calculator converts the selected AWG size to its approximate conductor cross-sectional area for the resistance calculation.
Does this calculator select the correct cable size?
No. It calculates voltage drop only. Cable selection must also consider ampacity, insulation, installation method, temperature, protective devices, fault conditions and applicable electrical regulations.