Voltage Drop Calculator
Check how much voltage your circuit loses over distance — and whether it passes the NEC's 3% recommendation.
Last updated: August 2026
The Voltage Drop Formula Explained
For a single-phase circuit, voltage drop is calculated as VD = (2 × K × I × L) ÷ CM. Each term has a physical meaning. The 2 accounts for the round trip — current flows out on the hot conductor and back on the neutral, so a 100-foot run is really 200 feet of wire. K is the resistivity constant: 12.9 for copper and 21.2 for aluminum, expressed in ohms per circular mil-foot. I is the load current in amps, L is the one-way length in feet, and CM is the conductor's cross-sectional area in circular mils — 6,530 for 12 AWG, 26,240 for 6 AWG, and so on up to 211,600 for 4/0.
Take a 20-amp load on 12 AWG copper, 150 feet out: VD = (2 × 12.9 × 20 × 150) ÷ 6,530 = 11.9 volts. On a 120V circuit that's a 9.9% drop — the load only sees about 108 volts, and anything with a motor on that circuit is going to have a hard life. Divide the volts dropped by the circuit voltage to get the percentage, which is the number the NEC recommendations are written against.
Maximum Run Length by Wire Gauge
A useful way to internalize voltage drop is to flip the formula around and ask: how far can each gauge go at its full rated load before hitting 3% on a 120V circuit? The answer surprises most people — it's much shorter than you'd guess.
| Copper Gauge | At Rated Load | Max One-Way Run (3%, 120V) |
|---|---|---|
| 14 AWG | 15A | ~38 ft |
| 12 AWG | 20A | ~45 ft |
| 10 AWG | 30A | ~48 ft |
| 8 AWG | 50A | ~46 ft |
| 6 AWG | 65A | ~56 ft |
| 4 AWG | 85A | ~68 ft |
| 2 AWG | 115A | ~80 ft |
| 1/0 AWG | 150A | ~98 ft |
| 2/0 AWG | 175A | ~106 ft |
| 4/0 AWG | 230A | ~128 ft |
Yes — 12 AWG copper pushed to its full 20 amps stays under 3% for only about 45 feet on a 120V circuit. That doesn't make longer 12 AWG runs illegal or dangerous; it means that at maximum load the far end of the circuit runs noticeably low. In practice most circuits rarely see their full rated load, and a receptacle circuit drawing 6 amps can run three times as far at the same drop percentage. But for circuits that do run at or near capacity — EV chargers, well pumps, space heaters, shop tools — these distances are the honest planning numbers. Doubling the voltage to 240V doubles every distance in the table for the same load.
Why 3% and 5%?
The NEC's 3% branch-circuit and 5% total (feeder plus branch) figures are recommendations in informational notes, not hard code requirements in most residential situations — but they're the standard of good practice, and some jurisdictions and specific applications (like fire pumps and sensitive equipment) do make them mandatory. The reasoning is about equipment performance: most appliances and motors are designed to tolerate about a 5% deviation from nameplate voltage. Budgeting 2% for the feeder and 3% for the branch circuit keeps the outlet voltage inside that envelope even when everything is running at once.
On a 120V circuit, 3% is 3.6 volts, putting the load at 116.4V. On a 240V circuit, 3% is 7.2 volts. Note that the same load at 240V suffers half the percentage drop of a 120V circuit through identical wire — one of several reasons heavy loads are run at 240V.
Symptoms of Excessive Voltage Drop
- Dimming lights when an appliance, compressor, or power tool kicks on — the classic sign, especially at the far end of long circuits.
- Motors running hot or failing early. Low voltage makes motors draw extra current to deliver the same power, which overheats windings. Well pumps, AC compressors, and refrigerators on long undersized runs die young.
- Warm cords, plugs, or receptacles under sustained load — energy being dropped as heat along the path instead of delivered to the load.
- Sluggish equipment: slow-starting compressors, underperforming heaters, EV chargers delivering fewer miles per hour than rated, breakers that nuisance-trip on motor start.
- Electronics misbehaving — resets, flicker, or shutdowns on circuits shared with heavy cycling loads.
How to Fix Excessive Voltage Drop
There are only a few levers, and they all fall out of the formula. The most common fix is to upsize the conductor: each step up the AWG ladder increases circular mils by roughly 26%, so going up two sizes cuts the drop nearly in half. When you're pulling a new long run anyway, the incremental cost of one size larger is usually small compared to trenching or fishing the cable a second time.
Second, shorten the run — routing directly instead of following walls, or relocating a sub panel closer to the loads it serves, attacks the L term directly. Third, raise the voltage: running 240V instead of 120V to a detached building halves the percentage drop for the same power delivered, which is why detached garages and shops are almost always fed at 240V. Finally, for loads that can't be moved or rewired economically, reduce the current — a smaller-draw EV charging rate or staggering heavy loads keeps the working drop within limits. If you're not sure which fix applies, our wire size calculator does the upsizing math for you automatically.
⚡ Reminder: verify all wire sizing against your local electrical code and have your plans reviewed by a licensed electrician before doing any electrical work.
Frequently Asked Questions
The NEC recommends no more than 3% on a branch circuit and 5% total from the service to the load (feeder plus branch combined). On 120V that means keeping the drop under 3.6 volts; on 240V, under 7.2 volts. These are recommendations rather than hard requirements in most residential cases, but they're the accepted standard for equipment to perform properly.
About 45 feet one-way at a full 20-amp load on 120V before exceeding 3% drop. That's shorter than most people expect. At lighter, more realistic loads the distance stretches proportionally — at 10 amps, 12 AWG stays under 3% out to about 90 feet, and at 240V the distances double.
It matters, but you get twice the headroom: the same load through the same wire loses the same number of volts, which is half the percentage of a 240V circuit versus 120V. That's why long runs to detached garages, sub panels, and EV chargers are fed at 240V — and why a run that fails the 3% check at 120V often passes comfortably at 240V.
Yes — aluminum's K constant is 21.2 versus copper's 12.9, so the same gauge of aluminum drops about 64% more voltage than copper. That's why aluminum conductors are sized roughly two gauges larger for the same job. Sized correctly, an aluminum feeder performs just as well and usually costs far less.