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Voltage Drop Calculator
Use this free voltage drop calculator to find out how much voltage is lost along a wire run before it reaches the load. Enter the supply voltage, the current, the one-way length of the run, the wire size, and the wire material, and the calculator shows the volts lost and the drop as a percentage of the supply voltage.
Common wire sizes
| Wire size | Circular mils | Typical use |
|---|---|---|
| 14 AWG | 4,110 | Lighting circuits, 15 A |
| 12 AWG | 6,530 | Outlets and small appliances, 20 A |
| 10 AWG | 10,380 | Water heaters, air conditioners, 30 A |
| 8 AWG | 16,510 | Ranges and sub-panels, 40 A |
| 6 AWG | 26,240 | Large feeders and EV chargers, 55 A |
How it works
Voltage drop is the voltage lost as electricity travels through a wire. No wire is a perfect conductor: every foot of copper or aluminum has a tiny amount of resistance, and as current flows through that resistance, some electrical energy turns into heat and the voltage at the far end falls below the voltage at the source. For a short run inside a house the loss is small and easy to ignore, but for a long run out to a shed, a well pump, or a garden light, the loss can be large enough to make motors run hot, lights dim, and equipment fail early.
The calculator uses the standard single-phase voltage drop formula, which is widely used by electricians for planning residential and light commercial circuits:
In this formula, Vd is the voltage drop in volts, K is a constant that depends on the wire material, I is the current in amps, L is the one-way length of the run in feet, and CM is the cross-sectional area of the wire in circular mils. The factor of 2 is there because the current travels out along one conductor and back along the other, so the electricity actually covers twice the one-way length. K is 12.9 for copper and 21.2 for aluminum, which reflects the fact that aluminum resists current more than copper does.
Wire size is measured in AWG, or American Wire Gauge, where a smaller number means a thicker wire. A 6 AWG wire has about six times the cross-sectional area of a 14 AWG wire, so it drops far less voltage over the same distance. That is why the fix for excessive voltage drop is almost always a thicker wire: moving from 14 AWG to 12 AWG, or from 12 AWG to 10 AWG, cuts the drop sharply without changing anything else in the circuit.
The result is shown two ways. The voltage drop in volts tells you how much is lost, and the percentage tells you how serious that loss is. The National Electrical Code recommends keeping voltage drop to 3 percent or less on branch circuits, and 5 percent or less for the feeder and branch circuit combined. A 120-volt circuit at the 3 percent limit loses 3.6 volts, leaving 116.4 volts at the load. This calculator flags any result above 3 percent so you can see at a glance whether the run is within the guideline.
Three things drive voltage drop more than anything else: distance, current, and wire size. Doubling the length doubles the drop, doubling the current doubles the drop, and halving the wire area doubles the drop. So a long run carrying a heavy load is the worst combination — for example, a 150-foot run to a 20-amp load is far more demanding than a 30-foot run to a 10-amp load, even on the same wire. When you cannot shorten the run, your options are a thicker wire, a lower current, or a higher supply voltage.
Keep in mind that this calculator is for single-phase circuits with copper or aluminum conductors at normal operating temperatures, and its results are estimates for planning. Real installations also involve temperature, conduit fill, connections, and the type of load, and local electrical codes may set their own limits. Always confirm the final wire size with a licensed electrician before wiring a circuit.
Worked example
A homeowner is running power 100 feet from the panel to a workshop outlet. The circuit is 120 volts, the tools on it draw 15 amps, and the wire is 12 AWG copper.
Step 1: Write down the values — V = 120 V, I = 15 A, L = 100 ft, K = 12.9 for copper, CM = 6,530 for 12 AWG.
Step 2: Apply the formula — Vd = 2 × 12.9 × 15 × 100 ÷ 6,530.
Step 3: Work it out — 2 × 12.9 × 15 × 100 = 38,700, and 38,700 ÷ 6,530 = 5.93 volts.
Step 4: Find the percentage — 5.93 ÷ 120 × 100 = 4.94%.
The drop is 5.93 volts, or 4.94 percent, which is above the 3 percent guideline for branch circuits — the load would see only about 114.07 volts. Stepping up to 10 AWG copper (10,380 circular mils) cuts the drop to 3.73 volts, or 3.11 percent, and 8 AWG copper brings it to 2.34 volts, or 1.95 percent, comfortably inside the guideline. That is exactly the kind of trade-off this calculator is built to show before any wire is bought.
FAQ
What is a good voltage drop percentage?
The National Electrical Code recommends keeping voltage drop to 3% or less on branch circuits, and 5% or less for the combined feeder and branch circuit. A drop above 3% on a branch circuit usually means you need a thicker wire or a shorter run.
How do you calculate voltage drop?
For a single-phase circuit, voltage drop = 2 × K × current × one-way length ÷ circular mils. K is 12.9 for copper and 21.2 for aluminum. The factor of 2 accounts for the out-and-back path of the current.
Why does voltage drop increase with distance?
Every foot of wire has a small amount of resistance, so a longer run adds more resistance and loses more voltage as heat. Doubling the length doubles the voltage drop, which is why long runs to sheds, wells, and garden lights often need thicker wire.
Does aluminum wire drop more voltage than copper?
Yes. Aluminum has a higher resistivity, so its K value is 21.2 compared with 12.9 for copper. The same size aluminum wire drops about 64% more voltage than copper, so aluminum runs usually need a larger wire size for the same result.