Voltage Drop Calculator
Calculate voltage drop over a run and check it against the 3% guideline.
These figures are for estimation only. All electrical work must be designed and verified by a licensed electrician and comply with the NEC and your local code.
How this calculator works
VD = 2 × K × amps × distance ÷ circular mils
The 2 accounts for current travelling out and back. For three-phase, replace it with 1.732. The NEC recommends keeping branch-circuit drop under 3% and total drop including the feeder under 5%.
Worked example
With load current 20 , one-way distance 100 ft, conductor size 6530 , system voltage 120 , conductor material k 12.9 , this calculator returns 6.58 .
Voltage drop by circuit length
Figures below assume load current 20 × conductor size 6530 × system voltage 120 × conductor material k 12.9 . Change any measurement in the calculator to get your own numbers.
| One-way distance | Voltage drop | Volts dropped | Voltage at the load |
|---|---|---|---|
| 25 ft | 1.65 | 1.98 | 118.0 |
| 50 ft | 3.29 | 3.95 | 116.0 |
| 75 ft | 4.94 | 5.93 | 114.1 |
| 100 ft | 6.58 | 7.90 | 112.1 |
| 150 ft | 9.88 | 11.85 | 108.1 |
| 200 ft | 13.17 | 15.80 | 104.2 |
| 300 ft | 19.75 | 23.71 | 96.29 |
A 20 A load on 12 AWG copper at 120 V. One-way distance — the formula already accounts for the return path.
Voltage drop on common circuits
| Job | Size | Voltage drop |
|---|---|---|
| Kitchen circuit, 14 AWG | 15 × 50 ft × 4110 × 120 × 12.9 | 3.92 |
| Bedroom circuit, 12 AWG | 20 × 75 ft × 6530 × 120 × 12.9 | 4.94 |
| Detached garage feeder | 40 × 150 ft × 16510 × 240 × 12.9 | 3.91 |
| Well pump, long run | 10 × 300 ft × 10380 × 240 × 12.9 | 3.11 |
| EV charger, 50 A | 50 × 60 ft × 26240 × 240 × 12.9 | 1.23 |
| Barn subpanel on aluminium | 60 × 250 ft × 41740 × 240 × 21.2 | 6.35 |
Doubling the voltage quarters the percentage drop for the same power delivered, which is why long runs are almost always done at 240 V.
Wire size in circular mils
| Type | Circular mils | Notes |
|---|---|---|
| 14 AWG | 4,110 | 15 A circuits |
| 12 AWG | 6,530 | 20 A circuits |
| 10 AWG | 10,380 | 30 A circuits |
| 8 AWG | 16,510 | 40 A circuits |
| 6 AWG | 26,240 | 55 A circuits |
| 4 AWG | 41,740 | 70 A feeders |
| 2 AWG | 66,360 | 95 A feeders |
| 1/0 AWG | 105,600 | 125 A service |
Use K = 12.9 for copper and 21.2 for aluminium. Going up one wire size increases circular mils by roughly 60%, which cuts the drop by about the same proportion.
Frequently asked questions
How do I calculate voltage drop?
Multiply 2 by K, by the current, by the one-way distance, then divide by the circular mils of the conductor. K is 12.9 for copper and 21.2 for aluminium.
How far can I run 12 gauge wire?
About 115 feet at 20 A on a 120 V circuit before drop reaches 3%. At 240 V the same conductor reaches roughly 230 feet, because the percentage drop halves.
What size wire do I need for a 100 foot run?
At 20 A and 120 V, 10 AWG copper keeps the drop under 3%; 12 AWG lands at about 3.9%. At 240 V, 12 AWG is comfortably within limits.
Is 3% voltage drop a code requirement?
No. NEC 210.19 and 215.2 recommend 3% on a branch circuit and 5% total including the feeder, but both appear in informational notes rather than enforceable text. Inspectors treat them as good practice.
What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% total including the feeder. These are recommendations in informational notes rather than enforceable requirements, but equipment performance suffers beyond them.
How do I reduce voltage drop?
Increase the conductor size, shorten the run, or raise the system voltage. Going up one wire gauge reduces drop by roughly 20%; doubling the voltage cuts it by three quarters for the same power.
Why is the formula multiplied by 2?
Current travels the full length out to the load and back through the neutral, so the conductor length in the circuit is twice the one-way distance. Three-phase uses 1.732 instead because of the phase relationship.
Does voltage drop matter on short runs?
Rarely. Under about 50 ft on a standard branch circuit the drop is negligible. It becomes the governing factor on long runs to outbuildings, well pumps and detached garages.