Voltage Drop Calculator
Work out how much voltage a wire run loses along the way, in volts and as a percentage, and see at a glance whether a larger conductor would be worth it.
Conductor
Aluminum has roughly 1.6 times the resistance of copper for the same gauge, so it drops more voltage over the same run. Sizes not made in the chosen metal are left out of the list.
Measure from the panel to the load, one way only. The calculation already accounts for the current returning on the neutral or the other conductors — do not double the distance yourself.
Circuit
The current the circuit actually carries, not the breaker size. A 20 A breaker feeding a 7 A load drops voltage as a 7 A circuit.
A percentage limit is much tighter in absolute terms at low voltage: 3% of 120 V is 3.6 V, but 3% of 12 V is only 0.36 V. That is why low-voltage DC runs often need surprisingly heavy wire.
Ordinary 120 V and 240 V branch circuits.
Voltage Drop
dropped across 100 ft of 12 AWG copper, one way
The 3% branch-circuit and 5% combined feeder-plus-branch figures come from informational notes in the NEC. Those notes are advisory recommendations for reasonable efficiency of operation, not enforceable code requirements, so exceeding them is not by itself a violation — though many jurisdictions, specifications and utilities treat them as a target.
How It Works
Voltage drop is Ohm's law applied to the wire itself. The conductor has a small resistance, the load current flows through it, and the product of the two is voltage that never reaches the load.
The multiplier is what handles the return path. On a DC or single-phase circuit the current travels out along one conductor and back along the other, so it passes through twice the one-way length — hence the 2. A three-phase circuit uses √3 (about 1.732) instead, because the three line currents are 120° apart and do not simply add and return. In both cases you enter the one-way distance and the multiplier does the rest.
Resistances are the DC values at 75°C from NEC Chapter 9, Table 8. Real circuits also see reactance, which matters on large conductors and long AC runs, and resistance rises as the conductor heats up — so treat the result as a sound estimate rather than an exact measurement.
Compare Wire Sizes
Voltage drop over a 100 ft one-way run at 20 A, in copper, AC — single phase. Drop is proportional to length, so a 50 ft run is half these figures and a 200 ft run is double.
| Size | Ω / 1000 ft | Drop per 100 ft | % of 120 V |
|---|---|---|---|
| 14 AWG | 3.07 | 12.28 V | 10.23% |
| 12 AWG | 1.93 | 7.72 V | 6.43% |
| 10 AWG | 1.21 | 4.84 V | 4.03% |
| 8 AWG | 0.764 | 3.06 V | 2.55% |
| 6 AWG | 0.491 | 1.96 V | 1.64% |
| 4 AWG | 0.308 | 1.23 V | 1.03% |
| 2 AWG | 0.194 | 0.78 V | 0.65% |
| 1/0 AWG | 0.122 | 0.49 V | 0.41% |
| 2/0 AWG | 0.0967 | 0.39 V | 0.32% |
| 4/0 AWG | 0.0608 | 0.24 V | 0.20% |
A size that keeps voltage drop low is not automatically a legal choice: the conductor still has to be sized for ampacity, overcurrent protection and terminal temperature ratings first. Voltage drop is a reason to go larger than the ampacity minimum, never smaller.
This calculator estimates voltage drop for planning and comparison. It does not apply ambient temperature correction, conduit fill derates, continuous-load factors or local amendments. Have an electrician confirm conductor sizing for any circuit you intend to install.
Frequently Asked Questions
- How do you calculate voltage drop?
- Multiply the conductor resistance by the current and the distance the current travels. For a single-phase circuit that is 2 × (ohms per 1000 ft ÷ 1000) × amps × the one-way length in feet, because the current runs out and back. Three phase uses √3 (about 1.732) in place of the 2. Dividing the result by the system voltage gives the drop as a percentage.
- Do I enter the one-way distance or the total wire length?
- Enter the one-way distance, measured from the panel to the load. The formula already multiplies by 2 for single phase to account for the return path, so entering the round-trip length would double the answer. This is the single most common mistake people make with voltage drop.
- Is the 3% voltage drop limit an actual code requirement?
- No. The 3% branch-circuit figure, and the 5% total for feeder plus branch circuit combined, appear in informational notes in the NEC. Informational notes are explanatory and advisory rather than enforceable requirements, so exceeding 3% is not in itself a code violation. Many designers, specifications, inspectors and utilities still treat it as the target, and some local amendments do make it mandatory.
- What is an acceptable voltage drop?
- Keeping branch circuits at or under 3% and the whole path from service to load at or under 5% is the usual design target. Beyond that, motors run hotter and start harder, incandescent lighting dims noticeably, heating elements deliver less power, and electronics with switching supplies draw more current to compensate. Sensitive equipment often has its own tighter requirement in the manufacturer instructions.
- How do I reduce voltage drop on a long run?
- Increase the conductor size, which is the usual fix and the reason long runs get upsized beyond what ampacity alone would require. You can also shorten the run, raise the system voltage so the same power draws less current, split the load across more than one circuit, or switch from aluminum to copper of the same gauge, which cuts resistance by roughly a third.
- Why does aluminum drop more voltage than copper?
- Aluminum is a poorer conductor, so the same gauge has about 1.6 times the resistance of copper and drops proportionally more voltage. The usual remedy is to go up roughly two AWG sizes: 2 AWG aluminum has resistance close to 4 AWG copper. Aluminum is still common for service entrances and feeders because it is cheaper and lighter for the same current-carrying capacity.
- Should I use the breaker size or the actual load current?
- Use the current the circuit actually carries. Voltage drop is caused by the current flowing at the time, not by the rating of the protective device, so a 20 A circuit feeding a 6 A load drops the voltage of a 6 A circuit. For sizing decisions, use the largest current you expect the circuit to see in normal operation, which for a continuous load is the full load current.
- Does this work for DC circuits?
- Yes. The resistive voltage drop calculation is identical for DC and single-phase AC: current travels out along one conductor and back along the other, so the same factor of two applies. Select DC as the current type. The resistance figures used here are DC resistance, so nothing is approximated for a DC run, whereas AC results ignore the small reactive component.
- How much voltage drop is acceptable on a 12 volt system?
- The same percentages are commonly used, but they are much harsher in absolute terms: 3% of 12 V is 0.36 V against 3.6 V at 120 V. Many solar and automotive guides aim for 3% on critical runs such as charging circuits and accept up to 10% on non-critical loads like interior lighting. Decide on the percentage that suits the load rather than assuming the branch-circuit figure applies.
- Do these figures apply to automotive, marine or solar wiring?
- Only as a rough guide. The ampacity and resistance data here follow the National Electrical Code, which governs premises wiring in buildings. Automotive, marine and RV work follow different standards such as SAE and ABYC E-11, which use their own ampacity tables, temperature assumptions and drop targets, and often allow smaller conductors for short engine-bay runs. Use the standard that applies to your installation.