Voltage Drop Calculator
Every conductor has resistance, so current flowing through it loses voltage along the run. Choose DC, single phase or three phase, pick a copper or aluminium wire size (or enter your own resistance per length), and get the volts lost, the percent drop, the voltage left at the load, and an optional estimate of the energy wasted as heat each year.
Formula
Worked example
16 A through 25 m of 2.5 mm2 copper (about 0.0074 Ω/m), single phase: Vd = 2 × 16 × 0.0074 × 25 = about 5.9 V, roughly 2.6% of a 230 V supply, leaving about 224 V at the load.
Why voltage drops along a wire
A conductor is not a perfect path: it has a small resistance that depends on the material, the cross-sectional area, and the length of the run. When current flows through that resistance, Ohm’s law says a voltage develops across it, and that voltage is energy taken from the supply and dissipated as heat in the cable. The longer the cable and the larger the current, the more voltage is lost before power reaches the load. In a DC or single phase circuit the current travels out to the load and back again, so the run length counts twice; a balanced three phase circuit instead uses a factor of the square root of three, which is why this calculator switches the multiplier when you change the system.
Choosing a wire size or entering resistance
You can let the calculator look up the resistance from a standard conductor size, choosing copper or aluminium and a metric (mm2) or North American (AWG and kcmil) gauge, or you can type a resistance per metre straight from a cable datasheet. Copper has a lower resistivity than aluminium, so a copper conductor of the same size drops less voltage, while aluminium is cheaper for large feeders. Resistance per metre falls as the conductor gets thicker, so stepping up one or two sizes is the usual cure for an excessive drop. For alternating current with an inductive load you can also enter a power factor and a reactance per metre; the calculator then uses the effective impedance, resistance times the power factor plus reactance times the sine of the phase angle.
Reading the percentage, power loss and cost
The headline number is the volts lost in the cabling itself; subtract it from the supply voltage to find what actually reaches the load. The percentage compares that loss to the supply voltage, and it is the figure electricians care about because guidance is written in percent: a widely used rule keeps the drop on a final circuit near 3% and the total across feeders and branches near 5%. The same resistance that drops the voltage also burns power as heat at a rate equal to the current squared times the loop resistance. Turn on the cost estimate to convert that wasted power into an annual energy cost from your operating hours and electricity rate, which often justifies fitting a larger cable. This model uses a single resistance figure and ignores temperature rise, so for code compliance always confirm against the manufacturer’s cable tables.
Approximate conductor resistance (per core)
| Conductor size | Copper (Ω/m) | Typical use |
|---|---|---|
| 1.5 mm2 / ~15 AWG | 0.0121 | Lighting circuits |
| 2.5 mm2 / ~13 AWG | 0.0074 | Socket / ring circuits |
| 4.0 mm2 / ~11 AWG | 0.0046 | Cookers, showers |
| 6.0 mm2 / ~9 AWG | 0.0031 | Heavy appliances |
| 10 mm2 / ~7 AWG | 0.0018 | Sub-mains |
| 10 AWG (5.26 mm2) | 0.0033 | US branch circuits |
| 2 AWG (33.6 mm2) | 0.00051 | US feeders |
DC resistance at 20 C; copper unless noted. Use for quick estimates and confirm against cable tables.
Frequently asked questions
What is the difference between single phase and three phase voltage drop?
For DC and single phase the current flows out along one conductor and back along another, so the calculator multiplies the one-way length by two. For a balanced three phase circuit the line to line drop uses a factor of the square root of three (about 1.732) instead, because the return currents in the three lines partly cancel. Select the system at the top and the formula switches automatically.
How much voltage drop is acceptable?
A common guideline keeps the drop on a final branch circuit near 3% of the supply voltage, with no more than about 5% total across feeders and branches combined. Higher drops waste energy as heat and can cause dim lighting, sluggish motors, and unreliable electronics. Always check the standard that applies in your region, such as the NEC or the local wiring rules.
Should I enter a power factor and reactance?
For DC, short runs, or resistive loads you can leave the power factor at 1 and the reactance at 0, and the calculator uses resistance only. For long alternating current runs feeding inductive loads such as motors, enter the load power factor and a per-metre reactance from the cable datasheet; the calculator then uses the effective impedance, which gives a more realistic drop.
How is the energy cost of voltage drop worked out?
The same resistance that drops the voltage also turns power into heat at a rate equal to the current squared times the loop resistance. Turn on the cost estimate, enter the operating hours per year and your electricity rate, and the calculator multiplies the wasted power by both to give an annual cost. A larger cable lowers the resistance and so lowers this ongoing loss.