How to use this voltage drop calculator
- Choose the system. Single-phase for most residential circuits (120 V or 240 V), three-phase for commercial feeders and motors, DC for solar, battery and low-voltage runs.
- Enter the source voltage and load current. Use the actual load in amps, not the breaker size. For a continuous load, many designers use 125% of the load current to be conservative.
- Enter the one-way length. Measure from the panel or source to the load along the cable route. The calculator accounts for the return path automatically.
- Pick the conductor material and size. The result updates instantly and shows the smallest size that meets your target, plus the longest run your chosen size can handle.
The voltage drop formula
Voltage drop is Ohm’s law applied to the wire itself. Every conductor has resistance, so current flowing through it loses some voltage as heat before it reaches the load. This calculator uses the resistance method:
Where L is the one-way length in feet, R is the conductor resistance in ohms per 1,000 feet, and I is the current in amps. The 2 accounts for current traveling out and back on a single-phase circuit. On a balanced three-phase circuit the factor is √3 (1.732). With parallel conductor sets, divide by the number of sets.
You may also see the "K-factor" version, VD = 2 × K × I × L ÷ CM, where K is about 12.9 for copper and 21.2 for aluminum and CM is the conductor’s area in circular mils. It gives very similar answers. We use the Table 8 resistance values because they are the published figures for each specific wire size at 75 °C.
Worked example: a 150-foot, 20-amp branch circuit
A 120 V single-phase circuit feeds a 16 A load in a detached garage 150 feet away. Is 12 AWG copper good enough?
Upsizing shows how quickly the drop falls:
| Copper size | R (Ω/kft) | Volts lost | % drop |
|---|---|---|---|
| 12 AWG | 1.93 | 9.26 V | 7.72% |
| 10 AWG | 1.21 | 5.81 V | 4.84% |
| 8 AWG | 0.764 | 3.67 V | 3.06% |
| 6 AWG | 0.491 | 2.36 V | 1.96% |
8 AWG lands just over 3%, so 6 AWG copper is the smallest size that meets the 3% recommendation. If the circuit ran at 240 V instead, the percentage would halve, and 8 AWG would be comfortably within target. That's one reason long runs to outbuildings and well pumps are often run at 240 V.
How much voltage drop is acceptable?
The NEC’s informational notes (in 210.19 for branch circuits and 215.2 for feeders) suggest sizing conductors so the drop is no more than 3% on a branch circuit and no more than 5% total for the feeder and branch circuit combined. These notes are recommendations for efficient operation, not enforceable requirements in most cases.
There are exceptions where the code does set limits, such as sensitive electronic equipment (Article 647) and fire pumps (Article 695), and some jurisdictions and energy codes adopt the 3%/5% figures as rules. Motors, LED drivers and electronics also tend to misbehave when voltage sags, so the recommendation is worth following even where it isn’t mandatory.
Rule of thumb: once a 120 V circuit runs past about 100 feet at a meaningful load, check voltage drop. It often decides the wire size before ampacity does.
Resistance values used (NEC Chapter 9, Table 8)
DC resistance of uncoated stranded conductors at 75 °C, in ohms per 1,000 feet:
| Size | Copper | Aluminum |
|---|---|---|
| 14 AWG | 3.07 | — |
| 12 AWG | 1.93 | 3.18 |
| 10 AWG | 1.21 | 2.00 |
| 8 AWG | 0.764 | 1.26 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 3 AWG | 0.245 | 0.403 |
| 2 AWG | 0.194 | 0.319 |
| 1 AWG | 0.154 | 0.253 |
| 1/0 | 0.122 | 0.201 |
| 2/0 | 0.0967 | 0.159 |
| 3/0 | 0.0766 | 0.126 |
| 4/0 | 0.0608 | 0.100 |
| 250 kcmil | 0.0515 | 0.0847 |
| 300 kcmil | 0.0429 | 0.0707 |
| 350 kcmil | 0.0367 | 0.0605 |
| 500 kcmil | 0.0258 | 0.0424 |
Limits of this method
- It doesn't check ampacity. A wire can pass voltage drop and still be too small for the current. Size for ampacity (NEC 310.16 and the terminal temperature rating) first, then check voltage drop.
- It uses DC resistance. For conductors up to about 1/0 this is very close to the AC result. For large conductors on AC with a low power factor, reactance matters, and NEC Chapter 9 Table 9 (effective impedance) gives a more precise answer.
- It assumes a 75 °C conductor. Lightly loaded conductors run cooler and have slightly less resistance, so the real-world drop is usually a little lower than calculated.
- It assumes a balanced load on three-phase circuits.
Frequently asked questions
Do I use one-way or round-trip length?
Enter the one-way distance from the source to the load. The formula’s factor of 2 (single-phase and DC) or 1.732 (three-phase) accounts for the return path.
Is 3% voltage drop a code requirement?
In most cases, no. The 3% branch and 5% total figures come from informational notes in the NEC, which are recommendations. Some special equipment articles, local amendments and energy codes do make them mandatory.
What size wire do I need for a 100 ft, 20 amp, 120 V circuit?
At a full 20 A, 12 AWG copper loses about 7.7 V (6.4%) over 100 feet, and 10 AWG about 4.8 V (4.0%). You’d need 8 AWG copper (about 2.5%) to stay under 3%. At a typical 16 A continuous load, 10 AWG gives about 3.2%. Enter your actual load above for an exact figure.
Does voltage drop matter for aluminum wire?
Yes, more so. Aluminum has roughly 1.6 times the resistance of copper of the same size, so it needs to be about two sizes larger for the same drop. Select “Aluminum” to use the aluminum resistance values.
How do I reduce voltage drop?
Use a larger conductor, shorten the run, raise the circuit voltage (for example 240 V instead of 120 V), add parallel sets on large feeders, or reduce the load on the circuit.
Last reviewed October 11, 2026. Results are estimates for planning. Always verify against the edition of the NEC adopted in your area, local amendments and the authority having jurisdiction. Found a mistake? Tell us.