Voltage Drop Calculator: How Much Voltage Will I Lose?
Enter the voltage, load, run length and wire size to get the voltage drop in volts and percent, and the smallest wire that keeps the drop within your target.
Safety: this tool gives estimates only. Electrical and gas work can injure or kill and usually needs a permit. Hire a licensed electrician or contractor, and follow your local code.
Every wire has some resistance, so some voltage is lost along the way. On a short circuit it doesn’t matter. On a long run to a detached garage, a well pump or an EV charger, it can mean dim lights, motors that run hot and struggle to start, and wasted energy. This calculator works out the drop from the physics of the wire and finds the smallest size that keeps it within your target.
How it works
diameter (in) = 0.005 × 92^((36 − AWG) ÷ 39) 1/0 = 0, 2/0 = −1 …
area (cmil) = (1,000 × diameter)²
ohms per ft = K ÷ area K = 12.9 copper, 21.2 aluminum (75 °C)
drop (V) = 2 × length × amps × ohms per ft (√3 for three phase)
drop (%) = drop ÷ circuit voltage × 100Wire area comes from the American Wire Gauge formula, which sets each size’s diameter. That gives the published areas: 6,530 circular mils for 12 AWG and 10,380 for 10 AWG.
Resistance uses the resistivity of copper and aluminum conductors at 75 °C, a typical operating temperature for loaded building wire, so the drop isn’t underestimated.
Single phase counts both the hot and the return conductor, so the factor is 2 times the one-way length. Three-phase circuits use √3 instead.
Load current should be what the equipment actually draws, from its nameplate, not the breaker size. Continuous loads like EV chargers draw close to 80% of the breaker rating.
What it doesn’t do: ampacity, temperature correction, conduit fill and the breaker rules in the electrical code. Those can require a larger wire than voltage drop does, never a smaller one.
Worked example
A homeowner is running a 240-volt circuit 150 feet to a detached garage workshop. It will carry about 24 amps on a 30-amp breaker, and they’re starting with 10 AWG copper.
- 10 AWG copper: 12.9 ÷ 10,380 cmil = 1.24 ohms per 1,000 ft
- Drop: 2 × 150 × 24 × 0.00124 = 8.95 V, which is 3.73% of 240 V and over the 3% target
- 10 AWG reaches 3% at about 121 feet. At 150 feet the calculator moves up to 8 AWG copper, which drops 5.63 V, or 2.34%
- 6 AWG aluminum would drop 5.81 V (2.42%) and often costs less than 8 AWG copper on a run this long
The electrician still has to confirm that the chosen wire, insulation and breaker meet the code. The drop check only tells you the run won’t starve the tools at the far end.
Voltage drop by wire size: reference table
Resistance and the longest one-way run that stays within a 3% drop for common copper sizes and loads.
| Copper wire | Ohms per 1,000 ft | 10 A at 120 V (ft) | 15 A at 120 V (ft) | 20 A at 120 V (ft) | 30 A at 120 V (ft) | 30 A at 240 V (ft) |
|---|---|---|---|---|---|---|
| 14 AWG | 3.141 | 57 | 38 | 29 | 19 | 38 |
| 12 AWG | 1.976 | 91 | 61 | 46 | 30 | 61 |
| 10 AWG | 1.242 | 145 | 97 | 72 | 48 | 97 |
| 8 AWG | 0.781 | 230 | 154 | 115 | 77 | 154 |
| 6 AWG | 0.491 | 366 | 244 | 183 | 122 | 244 |
| 4 AWG | 0.309 | 582 | 388 | 291 | 194 | 388 |
| 3 AWG | 0.245 | 734 | 490 | 367 | 245 | 490 |
Tips and common mistakes
Use one-way length. Measure from the panel to the load along the route the cable will take, including going up and over. The formula doubles it for the return path.
Plan for the starting current of motors. Pumps and compressors briefly draw several times their running current when they start. A run that’s fine at running current can still dim the lights or stall a motor at startup.
Upsize the whole run, not part of it. Voltage drop depends on the total resistance, so a few feet of small wire at each end add up. Use the same size throughout and splice only where the code allows.
Consider 240 volts for distant loads. If the equipment can run on 240, it draws half the current and the drop falls by half in volts and three-quarters in percent.
Leave electrical work to licensed electricians. New circuits, subpanels and outbuilding feeders need a permit, correct grounding and inspection. A mistake can cause shock or fire.
Frequently asked questions
What is an acceptable voltage drop?
An informational note in the National Electrical Code suggests sizing conductors so the drop is no more than 3% on a branch circuit and 5% in total from the service to the farthest outlet. Many electrical codes treat that as advice rather than a requirement, but motors, lights and electronics all work better within it.
How do I calculate voltage drop?
For a single-phase circuit, multiply 2 by the one-way length, the current and the wire's resistance per foot. For 12 AWG copper at about 1.98 ohms per 1,000 feet, a 16-amp load 100 feet away drops 2 × 100 × 16 × 0.00198 = 6.3 volts, which is 5.3% of 120 volts.
Does a 240 V circuit have less voltage drop?
The volts lost are the same for the same current and wire, but they're a smaller share of 240 than of 120. And because the same power at 240 volts draws half the current, the drop in volts halves too. That's why long runs to sheds, wells and EV chargers are usually 240 volts.
Is aluminum wire OK for long runs?
Aluminum has about 64% more resistance than copper of the same gauge, so it needs to be about two sizes larger for the same drop. It's common and code-approved for feeders and service conductors when installed with rated lugs and connectors, and it costs much less than copper at large sizes.
Does this calculator tell me what wire size to use for my breaker?
No. It checks voltage drop only. Ampacity, the current a wire can carry safely, depends on its insulation, temperature rating, installation and the breaker, and those rules are in the electrical code. Use the larger of the code size and the voltage drop size.