About the Voltage Drop Calculator
Voltage Drop Calculator works out how much voltage is lost along a run of cable and what cable size keeps the loss within your limit. Choose DC, single-phase AC or three-phase AC, copper or aluminium, and enter the supply voltage, the current, the one-way length in metres or feet and the cable size in mm² or AWG. The tool shows the drop in volts and percent, the voltage left at the load, the heat lost in the cable and the loop resistance. It also finds the smallest standard cable that stays within the percentage you accept, in mm² and in AWG. Power factor and conductor temperature can be adjusted. It is a planning aid: it does not check current ratings, grouping, insulation or local wiring rules, so confirm your design with the regulations that apply where you live or with a qualified electrician.
Updated September 2026
How to use the Voltage Drop Calculator
- 1Choose DC, AC single-phase or AC three-phase, and copper or aluminium.
- 2Enter the supply voltage, the current in amps and the one-way cable length.
- 3Pick the cable size in mm² or AWG.
- 4Set the power factor and temperature if they differ from the defaults, and the highest drop you accept.
- 5Read the drop, the voltage at the load and the smallest cable size that meets your limit.
Why voltage drop matters
A long cable with a heavy load loses a noticeable amount of voltage. On a 12 V system, such as lighting in a caravan, a boat or a solar setup, even a fraction of a volt is a big share of the supply. At 230 V a 3 percent drop is about 7 V, which many devices tolerate, but at the end of a very long run motors may not start properly and lights may look dim.
Voltage drop also wastes energy as heat in the cable. The tool shows this loss in watts, which is useful when deciding if a thicker cable pays for itself.
How to reduce voltage drop
- Use a thicker cable, that is, a larger mm² or a smaller AWG number.
- Shorten the run, for example by placing the supply closer to the load.
- Use a higher voltage: the same power at 48 V draws a quarter of the current at 12 V.
- Use copper instead of aluminium for the same size.
- Keep cables cool, as resistance rises with temperature.
Using the calculator
Enter the real current your load draws. If you only know the power, Watts, Amps and Volts Calculator converts it to amps. The temperature box corrects for cables that run hot: at 70 degrees copper has about 20 percent more resistance than at 20 degrees. For AC circuits, the power factor lowers the drop a little for inductive loads, and a value of 1 gives the worst case.
The smallest size shown is the exact area that meets your percentage, rounded up to the next standard metric size and to the nearest AWG size that is at least as large. Round up further to allow for future loads.
Limits and safety
The results use standard resistivity values and simple formulas, so they are estimates. They ignore cable reactance, which matters for large cables on long AC runs, and installation effects. Always follow the wiring regulations that apply to you, and have mains work done or checked by a qualified electrician.
For related calculations, Ohm's Law Calculator solves voltage, current and resistance, Power Factor Calculator helps with AC loads, Electricity Cost Calculator shows the running cost, and Wire Gauge Calculator converts between AWG and millimetres.
Features
- DC, single-phase and three-phase circuits
- Copper and aluminium conductors
- Metric sizes and AWG from 30 to 0000
- Metres or feet
- Power factor and temperature correction
- Smallest cable for your voltage drop limit
- Heat loss and loop resistance
Frequently asked questions
What is voltage drop?
Every cable has some resistance, so when current flows some of the voltage is used up in the cable itself instead of at the load. The difference between the voltage at the start and at the end is the voltage drop. Too much makes lights dim, motors run hot and devices misbehave.
How do you calculate voltage drop?
For DC and single-phase circuits, multiply 2 × current × length × resistivity, then divide by the cable's cross-section area in mm². For three-phase, replace the 2 with the square root of 3. Copper has a resistivity of about 0.0172 ohm mm² per metre at 20 degrees. The tool does this and also gives the percentage.
What voltage drop is acceptable?
Many wiring rules suggest 3 percent for lighting and other final circuits and up to 5 percent overall from the supply to the load, but limits differ by country and type of circuit. Check your local regulations. Motors and sensitive electronics may need less.
Why is the length counted one way?
You enter the length of the cable run from the supply to the load. The calculator knows that current also travels back, and includes the return conductor in single-phase and DC circuits by using the factor 2. For three-phase it uses the square root of 3.
Is aluminium worse than copper?
Aluminium has about 64 percent more resistivity than copper, so for the same drop it needs a cable with about 1.6 times the area. It is lighter and cheaper for long, large runs, which is why it is used for supply cables, but it needs suitable connectors.
Does this tell me the right cable for a circuit?
Not on its own. A cable must also be big enough to carry the current without overheating, which depends on the cable type, how it is installed and how many cables run together. Use the voltage drop result together with a current rating table and your local rules.