⚡ Voltage Drop Calculator for Single-Phase Wiring
By ToolNimba Editorial Team · Updated 2026-06-20
Single-phase, two-conductor DC or AC resistive estimate. The 3% target follows the common branch-circuit recommendation in the US National Electrical Code (NEC) FPN. Verify against your local code and conductor temperature rating before wiring.
This voltage drop calculator works out how much voltage a single-phase circuit loses along its conductors. Enter the supply voltage, the load current, the one-way run length, and the copper wire size, and the tool returns the drop in volts, the drop as a percent of the supply, and the voltage that actually reaches the load. It uses the standard formula Vd = 2 x L x I x R / 1000 with built-in copper resistance values by AWG, so you can size a wire run before you buy cable.
What is the Voltage Drop Calculator?
Every conductor has resistance, and any current flowing through that resistance produces a voltage drop, just as Ohm's law (V = I x R) predicts. On a long run, that lost voltage can be enough to dim lights, slow motors, or trip undervoltage protection at the far end. The voltage drop calculator estimates the loss so you can choose a wire gauge that keeps the load running at close to full voltage.
For a single-phase circuit the current travels out along one conductor and back along the other, so the full loop is twice the one-way run length. That is why the formula carries a factor of 2: Vd = 2 x L x I x R / 1000, where L is the one-way length, I is the current in amps, and R is the conductor resistance per 1000 units of length (per 1000 ft or per 1000 m). The resistance values built into this tool come from standard copper conductor tables, so picking a wire size from the dropdown fills in R for you. The division by 1000 simply matches R, which is quoted per 1000 units rather than per single foot or meter.
The result matters most as a percentage of the supply voltage. A 4.8 volt drop sounds small, but on a 12 volt system it is 40 percent and unacceptable, while on a 240 volt system it is just 2 percent and fine. The widely used guideline, echoed in the US National Electrical Code (NEC) informational notes, is to keep a branch circuit under 3 percent and the combined feeder plus branch under 5 percent. This calculator flags whether your result lands within 3 percent, between 3 and 5 percent, or above 5 percent.
To cut voltage drop you have three levers: shorten the run, lower the current, or use a thicker (lower-gauge) wire with less resistance. Doubling the run roughly doubles the drop, and dropping from 14 AWG to 10 AWG more than halves the resistance per foot. Note that this tool models a DC or resistive AC load and ignores conductor reactance, which only becomes significant on large feeders carrying high currents over long distances.
When to use it
- Sizing the wire for a long branch circuit, such as a feed to a detached garage, shed, or well pump.
- Checking whether a 12 volt or 24 volt DC run (solar, RV, marine, or LED lighting) will lose too much voltage.
- Confirming a circuit meets the 3 percent branch and 5 percent total voltage drop recommendation before inspection.
- Comparing two wire gauges to decide if upsizing the conductor is worth it for a given run length.
How to use the Voltage Drop Calculator
- Choose feet or meters, then enter the supply voltage and the load current in amps.
- Enter the one-way run length, that is the distance from the source to the load (the tool doubles it for the return path).
- Pick the copper wire size in AWG, or choose manual entry to type a resistance per 1000 units yourself.
- Read the voltage drop in volts, the drop percentage with a pass or fail flag, and the voltage delivered at the load.
Formula & method
where L = one-way run length, I = current (A), R = conductor resistance per 1000 units of length (ohms).
Drop percent = (Vd / Vsupply) x 100. Voltage at load = Vsupply minus Vd. The factor of 2 accounts for both the outgoing and return conductors.
Worked examples
A 120 V branch circuit drawing 15 A runs 100 ft (one-way) on 12 AWG copper. 12 AWG has about 1.98 ohms per 1000 ft.
- Identify the values: L = 100 ft, I = 15 A, R = 1.98 ohm / 1000 ft, V = 120 V
- Apply the formula: Vd = 2 x 100 x 15 x 1.98 / 1000
- Numerator: 2 x 100 x 15 x 1.98 = 5,940
- Divide by 1000: Vd = 5.94 V
- Percent: (5.94 / 120) x 100 = 4.95%
Result: Voltage drop is 5.94 V, or 4.95% of supply. That exceeds the 3% target, so consider upsizing to 10 AWG.
A 240 V circuit drawing 20 A runs 150 ft (one-way) on 10 AWG copper. 10 AWG has about 1.24 ohms per 1000 ft.
- Identify the values: L = 150 ft, I = 20 A, R = 1.24 ohm / 1000 ft, V = 240 V
- Apply the formula: Vd = 2 x 150 x 20 x 1.24 / 1000
- Numerator: 2 x 150 x 20 x 1.24 = 7,440
- Divide by 1000: Vd = 7.44 V
- Percent: (7.44 / 240) x 100 = 3.10%
Result: Voltage drop is 7.44 V, or 3.10% of supply. That is just over 3%, acceptable for a feeder but borderline for a branch circuit.
DC resistance of uncoated copper conductors (approximate, ohms per 1000 ft and per 1000 m)
| Wire size (AWG) | Ohms per 1000 ft | Ohms per 1000 m |
|---|---|---|
| 14 | 3.14 | 10.3 |
| 12 | 1.98 | 6.50 |
| 10 | 1.24 | 4.07 |
| 8 | 0.778 | 2.55 |
| 6 | 0.491 | 1.61 |
| 4 | 0.308 | 1.01 |
| 2 | 0.194 | 0.636 |
| 1/0 | 0.122 | 0.400 |
| 2/0 | 0.0967 | 0.317 |
| 4/0 | 0.0608 | 0.199 |
Typical voltage drop limits and what they mean
| Circuit part | Recommended limit | Notes |
|---|---|---|
| Branch circuit only | 3% or less | NEC informational note target for branch circuits. |
| Feeder plus branch | 5% or less | Combined limit from source to the final load. |
| Sensitive electronics | 2% or less | Tighter target where stable voltage matters. |
| Long low-voltage DC | Project specific | 12 V and 24 V runs need much thicker wire for the same percent. |
Common mistakes to avoid
- Using the round-trip length instead of one-way. The formula already doubles the one-way length with its factor of 2. If you also enter the total loop length, you double-count the wire and overestimate the drop by 100 percent. Enter only the distance from the source to the load.
- Judging by volts instead of percent. A 6 volt drop is trivial on 240 V (2.5 percent) but catastrophic on 12 V (50 percent). Always read the drop as a percent of the supply voltage, not the raw volts, before deciding if a wire is large enough.
- Forgetting that aluminum has higher resistance. The built-in values are for copper. Aluminum conductors of the same gauge have roughly 60 percent more resistance, so they drop more voltage. Use the manual resistance entry with an aluminum value if your cable is aluminum.
- Ignoring temperature. Conductor resistance rises with temperature, so a wire running hot near its ampacity limit drops slightly more voltage than the cool-table value suggests. For tight designs, use the resistance at the conductor operating temperature rather than at room temperature.
Glossary
- Voltage drop
- The loss of voltage along a conductor caused by its resistance carrying current, calculated as the current times the conductor resistance.
- AWG
- American Wire Gauge, a standard sizing scale for wire. A smaller AWG number means a thicker conductor with lower resistance.
- One-way length
- The distance from the power source to the load along a single conductor, before the round-trip factor of 2 is applied.
- Ampacity
- The maximum current a conductor can carry continuously without exceeding its temperature rating. Separate from voltage drop, which can require a larger wire than ampacity alone.
- Single-phase
- A common power configuration using two current-carrying conductors (or a line and neutral), where current flows out on one and returns on the other.
- NEC
- The US National Electrical Code, whose informational notes recommend keeping branch-circuit voltage drop at or below 3 percent.
Frequently asked questions
What is an acceptable voltage drop?
A common guideline is 3 percent or less for a branch circuit and 5 percent or less for the combined feeder plus branch from the source to the load. The 3 percent and 5 percent figures come from informational notes in the US National Electrical Code, which are recommendations rather than hard requirements. Sensitive electronics often aim for 2 percent or less.
Why is there a factor of 2 in the voltage drop formula?
In a single-phase circuit, current flows out to the load on one conductor and returns on the other, so it travels through twice the one-way run length. The factor of 2 in Vd = 2 x L x I x R / 1000 accounts for both the outgoing and return conductors. You only enter the one-way distance and the formula handles the round trip.
How do I reduce voltage drop on a long run?
You have three options: shorten the run, lower the current the load draws, or use a thicker (lower-AWG) wire with less resistance. Upsizing the conductor is the most common fix. For example, moving from 12 AWG to 10 AWG drops the resistance per foot from about 1.98 to 1.24 ohms per 1000 ft, cutting the voltage drop by roughly a third.
Does this calculator work for three-phase circuits?
No, this tool is built for single-phase, two-conductor circuits, including most household branch circuits and DC runs. Three-phase voltage drop uses a different multiplier (the square root of 3, about 1.732, instead of 2) because of how the phases share return current. Use a dedicated three-phase calculator for those systems.
Why does a 12 volt circuit need such thick wire?
Voltage drop matters as a percentage of the supply, and low-voltage systems have very little voltage to spare. A drop of 3.6 volts is only 1.5 percent on 240 V but a full 30 percent on 12 V. That is why solar, RV, marine, and automotive 12 V and 24 V runs often call for much heavier copper than a 120 V circuit carrying the same current.
Is the voltage drop wire size the same as the ampacity wire size?
Not always. Ampacity sets the minimum wire size that can carry the current without overheating, while voltage drop can demand an even larger wire on long runs. You must satisfy both. A short circuit is usually limited by ampacity, but on a long run the voltage drop requirement often forces you to a thicker conductor than ampacity alone would need.