Voltage Drop Calculator
Use this voltage drop calculator to check any copper or aluminum cable run in seconds. Enter the voltage, current and distance, and it shows the drop in volts and percent, the voltage at the load and the smallest wire size that keeps you within your limit.
- Recommends wire size
- AWG, kcmil & mm²
- Max run length
- Free, no sign-up
For three-phase, enter the line-to-line voltage.
Please check the highlighted fields.
Voltage drop
7.90 V
6.58% of 120 V over 100 ft of 12 AWG copper
Over 5%Use 8 AWG or larger to stay within 3%.
Voltage drop by wire size
Every size for your circuit. Select a size to apply it.
| Wire size | Resistance (Ω/1000 ft) | Voltage drop | Drop % | Voltage at load | Status |
|---|
Why this voltage drop calculator is more useful
Most calculators only tell you the drop for one wire. This one also tells you which wire to buy and how far each size can safely run.
Instant wire size advice
It finds the smallest conductor that keeps the drop within your 2%, 3% or 5% target.
Every size in one table
Compare drop, voltage at the load and status for all sizes at once, then tap one to apply it.
AWG and mm² together
Switch between AWG and kcmil or metric mm² cable sizes, and between feet and meters.
Maximum run length
See how far your chosen wire can run before it breaks your voltage drop limit.
Heat loss in watts
Know how much power the cable wastes as heat, which matters for solar and long feeders.
Private and shareable
Runs in your browser. Copy a result for a quote or share a link that reopens your exact inputs.
How to use the voltage drop calculator
It takes about 30 seconds. Results update as you type.
- Choose the circuit typePick DC for batteries and solar, single-phase for most homes, or three-phase for commercial supplies.
- Enter voltage and currentTap a common voltage or type your own. Then enter the load current in amps.
- Add the one-way lengthMeasure from the source to the load. The formula already counts the return conductor.
- Select conductor and sizeChoose copper or aluminum, AWG or mm², and the wire size you plan to use.
- Check the result and upgrade if neededIf the drop is too high, follow the recommendation or pick a larger size from the table.
How to calculate voltage drop
Voltage drop depends on the conductor's resistance, the current and the distance. This calculator uses the standard K-factor method found in electrical design guides. K is the resistance of one circular mil foot of conductor at a typical operating temperature.
Here, I is the current in amps, L is the one-way length in feet and CM is the wire area in circular mils. For metric cables, 1 mm² equals about 1,973.5 circular mils, so the same formula works.
Worked example: 20 A circuit, 100 ft away
- Supply and load
- 120 V, 20 A, single-phase
- Wire
- 12 AWG copper, 6,530 CM
- 2 × 12.9 × 20 × 100 / 6,530
- 7.90 V
- Percent drop
- 6.58%
- Voltage at the load
- 112.10 V
- Upgrade to 8 AWG (16,510 CM)
- 3.13 V, 2.60%
So 12 AWG is too small for this run. However, 8 AWG brings the drop under 3% and keeps motors and electronics working properly.
Maximum wire run for 3% voltage drop at 120 V
One-way distance in feet for copper wire on a 120 V single-phase circuit. For 240 V, you can double these distances.
| Wire size | 15 A | 20 A | 30 A | 50 A |
|---|---|---|---|---|
| 14 AWG | 38 ft | 28 ft | 19 ft | 11 ft |
| 12 AWG | 60 ft | 45 ft | 30 ft | 18 ft |
| 10 AWG | 96 ft | 72 ft | 48 ft | 28 ft |
| 8 AWG | 153 ft | 115 ft | 76 ft | 46 ft |
| 6 AWG | 244 ft | 183 ft | 122 ft | 73 ft |
| 4 AWG | 388 ft | 291 ft | 194 ft | 116 ft |
| 2 AWG | 617 ft | 462 ft | 308 ft | 185 ft |
| 1/0 AWG | 982 ft | 736 ft | 491 ft | 294 ft |
Distances use K = 12.9. Always confirm the wire's ampacity under your local code as well.
What is an acceptable voltage drop?
In the United States, the National Electrical Code recommends a maximum of 3% on a branch circuit and 5% in total for the feeder and branch together. In the UK, BS 7671 allows 3% for lighting and 5% for other circuits supplied from the public network. Many designers aim for 2% on runs that feed sensitive electronics.
A large drop makes motors run hot, lights dim and chargers work slowly. It also wastes energy as heat in the cable. For that reason, solar and battery systems at 12 V or 24 V need special care, because even a small drop in volts is a large percentage. You can estimate the energy cost of that waste with the electricity cost calculator.
Three ways to reduce voltage drop
First, use a larger conductor, since doubling the area roughly halves the drop. Next, shorten the run by moving the panel or load closer. Finally, raise the supply voltage where possible, because the same power at 240 V draws half the current of 120 V. To work out current from power, try the Ohm's law calculator.
Benefits of checking voltage drop before you wire
A quick check saves money and prevents problems that are hard to fix later.
Buy the right cable once. Avoid pulling new wire after the walls are closed.
Protect equipment. Motors, compressors and electronics last longer at the right voltage.
Cut energy waste. Less drop means less power lost as heat in the cable.
Plan solar and EV runs. Size long DC and charger cables with confidence. See also the solar rooftop calculator.
Avoid oversizing. Pick the smallest size that works and save on copper.
Document your design. Copy the result into quotes, permits or job notes.
Voltage drop calculator FAQs
Straight answers to the questions electricians, DIYers and solar installers ask most.
The NEC recommends no more than 3% on a branch circuit and 5% in total for the feeder and branch together. BS 7671 allows 3% for lighting and 5% for other circuits fed from the public supply. Keeping the drop low protects equipment and saves energy.
For DC and single-phase AC, multiply 2 x K x current x one-way length in feet, then divide by the wire area in circular mils. For three-phase circuits, use 1.732 instead of 2. K is 12.9 for copper and 21.2 for aluminum.
At 120 V single-phase with copper wire, 12 AWG drops about 6.6%, which is too much. To stay within 3%, use 8 AWG, which drops about 2.6%. At 240 V, 10 AWG drops about 2.1% and is enough.
At 120 V and 20 A with copper, 12 AWG stays within a 3% drop for about 45 feet one way. At 15 A, the limit rises to about 60 feet. At 240 V, both distances double.
Yes. Aluminum has about 64% more resistance than copper of the same size. As a result, aluminum usually needs a size about two AWG steps larger to give the same voltage drop.
Enter the one-way distance from the source to the load. The formula already doubles it for the return conductor in DC and single-phase circuits.
Every wire has resistance. When current flows through it, part of the voltage turns into heat. Longer runs, higher current and thinner wire all increase the drop.
No. It checks voltage drop only. Your wire must also be rated for the current under your local electrical code, so check the ampacity tables and consult a licensed electrician.
Size your cable right the first time
A few seconds with this voltage drop calculator can save a costly rewire. Check your run, follow the size advice and wire with confidence.
Results use the K-factor method at typical operating temperature and ignore reactance, which is small for most building wiring. This tool does not check ampacity or code compliance. Always follow your local electrical code and use a licensed electrician.