Estimate voltage drop for a cable of a given size, or find a recommended cable size that keeps voltage drop within a safe limit for your load, cable length, and material. This is a simplified estimation tool — always confirm final cable sizing against local electrical code, installation standards, and a qualified electrician.
How to Use This Calculator
Choose whether you want a recommended cable size, or the voltage drop for a cable size you already have.
Enter voltage, phase, cable material, and cable length (one-way run length).
For cable size mode, enter your load, power factor, and the maximum voltage drop % you'll allow (3% is a common general guideline). For voltage drop mode, enter the current and cable size directly.
What the Calculator Does
In cable size mode it first works out the load current: I = P ÷ (V × PF) for single phase, or I = P ÷ (√3 × V × PF) for three phase, with P in watts. It then tries each standard size in turn, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185 and 240 mm², and picks the smallest one whose voltage drop stays within the percentage you allow.
The drop uses resistivity values of 0.0175 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminium. The single-phase formula multiplies by 2 because current flows out on the phase wire and back on the neutral; the three-phase formula uses √3 for a balanced load. Only resistance is considered, which is a fair approximation for the smaller sizes used in homes and shops but underestimates the drop on large cables, where reactance starts to matter.
If even 240 mm² cannot meet your limit, the tool still shows 240 mm² along with the actual drop it would give, so check the percentage before relying on the recommendation.
Worked Examples
A 5 kW single-phase load at 230 V and power factor 1, fed by copper cable over a 30 m run, with a 3% limit. Current = 5,000 ÷ 230 = 21.74 A. On 1.5 mm² the drop would be 15.22 V (6.62%), and on 2.5 mm² 9.13 V (3.97%), both over the limit. On 4 mm² it is 2 × 0.0175 × 30 × 21.74 ÷ 4 = 5.71 V, or 2.48%, so the tool recommends 4 mm². With aluminium the same load needs 6 mm², giving 6.13 V (2.67%).
A 10 kW three-phase motor at 415 V with power factor 0.8 on a 50 m copper run: current = 10,000 ÷ (1.732 × 415 × 0.8) = 17.39 A, and 2.5 mm² gives a drop of 10.54 V, or 2.54%. Voltage drop is acceptable here, but you would still need to confirm that 2.5 mm² can carry the motor's current, including its starting current, in the way it is installed.
Voltage Drop per Amp per Metre
Values below are in millivolts for each amp of current and each metre of one-way cable length, using the same resistivity figures as the calculator. Multiply by current and length, then divide by 1,000, to get the drop in volts.
Size (mm²)
Copper, single phase
Aluminium, single phase
Copper, three phase
1.5
23.33
37.60
20.21
2.5
14.00
22.56
12.12
4
8.75
14.10
7.58
6
5.83
9.40
5.05
10
3.50
5.64
3.03
16
2.19
3.52
1.89
25
1.40
2.26
1.21
Manufacturers publish their own mV/A/m figures, which account for conductor temperature and construction and are usually somewhat higher than these simple values.
Before You Buy Cable
Voltage drop is only one check. The cable must also be able to carry the current without overheating. That current rating depends on whether the cable runs in conduit, on a wall, underground or bunched with others, and it is listed in the manufacturer's catalogue. Use whichever size is larger.
Enter the one-way route length, not the total wire bought. The formula already allows for the return conductor.
Use the real supply voltage. Indian low-voltage supply is nominally 230 V single phase and 415 V three phase, but rural lines often run lower, which raises the current and the drop for the same load.
Long runs to pumps, borewells, outbuildings or EV chargers are where voltage drop usually decides the size. Short indoor circuits are more often limited by current rating.
Buy cable marked with the BIS standard mark and get the installation done by a licensed electrician, as required by your state electricity rules.
Frequently Asked Questions
Single-phase: Vd = (2 × ρ × L × I) ÷ A. Three-phase: Vd = (√3 × ρ × L × I) ÷ A, where ρ is the cable material's resistivity, L is length in metres, I is current in amps, and A is the cable cross-section in mm².
Aluminium has higher electrical resistance than copper of the same cross-section, so it causes a larger voltage drop and typically needs a larger cable size for the same current and length.
No. This tool gives a simplified estimate for planning purposes only. Actual cable sizing must also account for ambient temperature, installation method, grouping with other cables, and local electrical code — always confirm with a qualified electrician.
A limit of about 3% for an individual circuit and around 5% from the meter to the farthest point are widely used rules of thumb. Sensitive equipment and motors that start under load benefit from a tighter limit. Your electrician or the applicable code may set a different figure.
Use 1 for purely resistive loads such as heaters, geysers and incandescent lamps. For motors, pumps and compressors check the nameplate; if it is not shown, 0.8 is a cautious assumption. A lower power factor means more current for the same kW, so it can push the recommendation to a larger size.