Voltage drop formulas
Single-phase AC or DC: voltage drop = 2 × length × current × resistance ÷ 1000. Three-phase AC: voltage drop = √3 × length × current × resistance ÷ 1000. Length is the one-way distance in feet, current is in amps, and resistance is the conductor's resistance in ohms per 1000 feet (see the table below). Parallel conductors divide the effective resistance by the number of conductors.
Worked example -- 120V circuit, 20A, 100 feet one-way, 12 AWG stranded copper (1.98 Ω per 1000 ft):
- Voltage drop: 2 × 100 × 20 × 1.98 ÷ 1000 = 7.92 V
- Percent drop: 7.92 ÷ 120 = 6.6% (over the 3% branch-circuit guidance)
- Voltage at the load: 120 − 7.92 = 112.08 V
- Switching to 8 AWG copper drops that to 2.59%, at or under 3%.
The same circuit as three-phase AC (208V, 30A, 150ft, 6 AWG copper) uses √3 instead of 2: √3 × 150 × 30 × 0.491 ÷ 1000 = 3.83 V (1.84%). A 12V DC circuit (15A, 25ft one-way, 10 AWG copper) drops 0.93 V (7.75%) -- a much bigger percentage than the same drop would be on a 120V circuit, which is why low-voltage 12V/24V systems (solar, RV, automotive, LED lighting) are especially sensitive to wire size and run length.
What is voltage drop and why it matters
Every real wire has resistance, so some of the source voltage is lost as heat along the way, and the load sees less than the source voltage. A little drop is normal; too much causes dim incandescent or LED lights, motors that run hotter and with less torque, flickering or underperforming electronics, and wasted energy. It's most noticeable on long runs, high-current circuits, and low-voltage systems (12V/24V solar, RV, automotive, landscape lighting), where a couple of volts is a large share of the total.
NEC 3% and 5% voltage drop guidance
The National Electrical Code publishes informational notes (not enforceable requirements in most jurisdictions) recommending no more than 3% drop on a branch circuit and no more than 5% total for a branch circuit and its feeder combined. Some local codes, utilities or engineering specs do make these numbers mandatory, so check what applies where you're installing. The calculator above flags both.
Tips to reduce voltage drop
- Use a bigger wire. Each larger AWG size roughly cuts resistance -- and drop -- by about a third.
- Shorten the run where possible, or route power closer to the load.
- Raise the system voltage (e.g. 24V instead of 12V) -- drop as a percentage falls sharply since the same current causes the same volt-loss, which is now a smaller share of a higher voltage.
- Add parallel conductors to cut the effective resistance (two conductors of the same size roughly halve the drop).
- Reduce the current if the load allows it (a more efficient device, or splitting the load across circuits).
Max one-way distance at 3% drop (120V, 20A, copper)
| Wire size | Max one-way distance |
|---|---|
| 14 AWG | 29 ft |
| 12 AWG | 45 ft |
| 10 AWG | 73 ft |
| 8 AWG | 116 ft |
| 6 AWG | 183 ft |
| 4 AWG | 292 ft |
Computed with the same formula and NEC Chapter 9 Table 8 resistance values as the calculator above, for a 120V, 20A single-phase circuit. Use the calculator for your own voltage and current.
Conductor resistance reference (NEC Chapter 9, Table 8)
DC resistance in ohms per 1000 feet, uncoated copper and aluminum, at 75°C: the values the calculator uses. For 18–8 AWG, NEC Table 8 lists solid and stranded wire separately; these are the stranded values (the usual building wire, and slightly more conservative). Solid wire in those sizes has about 2% less resistance.
| AWG / kcmil | Copper (Ω/1000 ft) | Aluminum (Ω/1000 ft) |
|---|---|---|
| 12 AWG | 1.98 | 3.25 |
| 10 AWG | 1.24 | 2.04 |
| 8 AWG | 0.778 | 1.28 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 2 AWG | 0.194 | 0.319 |
| 1/0 AWG | 0.122 | 0.201 |
| 4/0 AWG | 0.0608 | 0.1 |
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Frequently asked questions
What is voltage drop?
Voltage drop is the loss of voltage along a wire as current flows through its resistance, so the voltage at the load (motor, light, outlet) is lower than at the source. For a 120V circuit with 20A flowing 100 feet one-way on 12 AWG copper wire, the drop is about 7.92 volts, leaving about 112.08 volts at the load -- a 6.6% drop.
What is the voltage drop formula?
For DC and single-phase AC: voltage drop = 2 × length × current × resistance ÷ 1000, where length is the one-way distance in feet and resistance is in ohms per 1000 feet. For three-phase AC: voltage drop = √3 × length × current × resistance ÷ 1000. The "2×" accounts for the round trip (out and back) on a single-phase or DC circuit; three-phase uses √3 instead because of how the phases combine.
How do I calculate voltage drop?
Multiply 2 (or √3 for three-phase) by the one-way length in feet, by the current in amps, by the wire's resistance in ohms per 1000 feet, then divide by 1000. Example: 120V, 20A, 100 feet one-way on 12 AWG stranded copper (1.98 Ω per 1000 ft): 2 × 100 × 20 × 1.98 ÷ 1000 = 7.92 volts, or 6.6% of 120V.
What is an acceptable voltage drop percentage?
There's no single legal limit, but the National Electrical Code (NEC) publishes informational guidance -- not a hard requirement -- recommending no more than 3% drop on a branch circuit and no more than 5% total for a branch circuit plus its feeder combined. Staying under those numbers avoids dim lights, sluggish motors and wasted energy as heat, even though it's not mandatory in most jurisdictions.
Why does voltage drop matter?
Excess voltage drop makes incandescent and LED lights dim, can make motors run hot and lose torque, and wastes energy as heat in the wire itself. It matters most on long runs, high-current circuits, and low-voltage systems like 12V solar, RV or automotive wiring, where even a couple of volts is a large percentage of the total.
How do I reduce voltage drop?
Use a larger (lower-gauge-number) wire, shorten the run, lower the current (a more efficient load), raise the system voltage, or add parallel conductors to cut the effective resistance. For example, switching the 120V/20A/100ft example above from 12 AWG to 8 AWG copper cuts the drop from about 7.92V (6.6%) to about 3.11V (2.59%).
What size wire do I need to avoid excess voltage drop?
For the 120V/20A/100ft one-way example, the smallest standard copper size that keeps drop at or under 3% is 8 AWG (2.59% drop). Use the calculator above with your own voltage, current and length -- longer runs and higher currents need a larger wire.
Does copper or aluminum have less voltage drop?
Copper has lower resistance than aluminum of the same gauge, so it has less voltage drop. For example, 4 AWG aluminum on a 240V/40A/200ft run drops about 8.13V (3.39%), more than the same size in copper. Aluminum wiring is often sized one or two gauges larger than copper to compensate, and always needs connectors and terminations rated for aluminum.
Does this calculator account for AC resistance and reactance?
No -- it uses each conductor's DC resistance (NEC Chapter 9, Table 8), the same table used for most voltage-drop estimates. NEC Table 9 lists additional AC resistance and reactance for large conductors, which this calculator ignores, so results are slightly optimistic (a little low) for AC circuits on big conductors. For a precise design on a large or long three-phase feeder, use Table 9 or consult an electrician.
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