Free Voltage Drop Calculator Online

Voltage Drop Calculator: How Much Voltage a Wire Loses Over Distance

Every real conductor has some resistance, and pushing current through that resistance over any meaningful length costs you a small amount of voltage before it ever reaches the load. A voltage drop calculator estimates exactly how many volts are lost along a specific run of wire, given the current flowing through it, how far the wire travels, its gauge, and what it's made of. Enter those four things and the tool reports the drop in volts, the drop as a percentage of the source voltage, and the voltage that actually arrives at the far end — all computed instantly in your browser as you adjust any field, with no file to upload and nothing sent to a server.

This calculation matters because a wire that's too thin or a run that's too long doesn't fail outright — it just quietly delivers less voltage than the source is supplying, which can dim lights, make motors run hotter and less efficiently, or trip up sensitive electronics that expect a voltage within a narrow band. Estimating the drop before running a wire lets you catch an undersized gauge on paper instead of discovering it after the fact.

How to Use This Voltage Drop Calculator

The tool is arranged around a small set of inputs that describe the run, plus two toggle groups for phase and material:

1. Choose the circuit's phase — Single Phase or Three Phase — using the toggle at the top. Most household and small-shop wiring is single-phase; three-phase is typical of larger industrial or commercial equipment.
2. Choose the conductor material — Copper or Aluminum — using the second toggle. This changes which resistance figure the calculation pulls from internally.
3. Enter the load Current in amps — the amount of current the circuit will actually carry.
4. Enter the One-Way Length in feet — the distance from the source to the load measured along the actual wire path in a single direction, not there-and-back. The formula itself accounts for the return path on single-phase circuits, so doubling the length yourself would overstate the drop.
5. Enter the Source Voltage in volts — the voltage supplied at the beginning of the run.
6. Pick the Wire Gauge (AWG) from the dropdown, covering the standard sizes from 14 AWG up through 4/0.
7. Three result cards update immediately: Voltage Drop in volts, Drop % as a share of the source voltage, and End Voltage — what actually reaches the load after the drop. A status line beneath the cards flags whether the drop falls within the commonly recommended 3% ceiling or exceeds it.

The Formula Behind the Numbers

For a single-phase circuit, this calculator applies the standard round-trip voltage-drop formula: Vdrop equals 2 times the one-way length in feet, times the current in amps, times the wire's resistance per 1000 feet, divided by 1000. The factor of 2 accounts for the fact that current has to travel out to the load along one conductor and back to the source along another, so the electrical path length is double the physical one-way distance you'd measure with a tape. For a three-phase circuit, the calculator instead multiplies by the square root of 3 (roughly 1.732) in place of the factor of 2, since a balanced three-phase load doesn't incur the same simple round-trip loss as a single-phase circuit — this is the standard simplified three-phase form of the same underlying formula.

The wire's resistance per 1000 feet comes from a standard table of DC resistance values for solid copper wire at 20°C, the same table commonly published for American Wire Gauge sizing. A few anchor points from that table: 14 AWG copper measures 2.525 ohms per 1000 feet, 12 AWG measures 1.588 ohms per 1000 feet, and 10 AWG measures 0.999 ohms per 1000 feet — resistance drops as the gauge number goes down and the wire gets physically thicker. For aluminum conductors, this calculator doesn't maintain a second full table; instead it takes the copper resistance figure for the chosen gauge and multiplies it by 1.6, a commonly used approximation for how much higher aluminum's resistivity runs compared to copper at the same gauge. That means an aluminum run will always show a noticeably larger voltage drop than the identical copper gauge carrying the identical current over the identical distance.

Working through the calculator's own reference case shows how the pieces fit together: 20 amps flowing through 100 feet of 12 AWG copper wire (one-way) from a 120-volt source. The resistance for 12 AWG copper is 1.588 ohms per 1000 feet. Plugging into the single-phase formula: 2 × 100 × 20 × 1.588 ÷ 1000 = 6.35 volts of drop. As a percentage of the 120-volt source, that's 6.35 ÷ 120 × 100 = roughly 5.29%, leaving an end voltage of about 113.65 volts reaching the load. Since 5.29% is above the commonly cited 3% guideline, this particular combination of gauge, current, and distance would typically call for a thicker wire or a shorter run.

AWG size	Copper resistance (Ω/1000 ft)	Aluminum resistance (Ω/1000 ft, ×1.6)
14	2.525	4.040
12	1.588	2.541
10	0.999	1.598
8	0.628	1.005
6	0.395	0.632
4	0.249	0.398
2	0.156	0.250
1/0	0.0983	0.157
4/0	0.049	0.078

Why Voltage Drop Matters in Practice

A handful of practical situations make this figure worth checking before a wire goes in the wall or the conduit:

• Long outdoor or detached-structure runs. Wiring a detached garage, shed, or barn often means a much longer conductor run than typical interior wiring, and the drop that would be negligible over 20 feet can become significant over 200.
• Motor and pump circuits. Motors that receive noticeably less than their rated voltage tend to draw more current to compensate, running hotter and less efficiently, and in some cases failing prematurely.
• LED and low-voltage lighting runs. A string of LED fixtures fed from a single low-voltage transformer can visibly dim toward the far end of a long run if the wire gauge wasn't sized with the distance in mind.
• Choosing between copper and aluminum on a budget. Aluminum conductors are typically cheaper for a given ampacity but carry a noticeably higher resistance per foot, which sometimes means stepping up a gauge size to keep the drop within an acceptable range on a long aluminum run.
• Comparing single-phase and three-phase options for the same load. Three-phase distribution can meaningfully reduce voltage drop compared to single-phase for the same current and distance, which is part of why larger equipment and longer commercial runs often favor it.

A Note on Safety and Real Installations

Everything this calculator produces is an estimate built from a standard resistance table and a simplified formula — it doesn't know the ambient temperature of your actual installation, the conduit fill, the specific alloy of the conductor you're holding, or any of the other variables that affect a real wire's behavior in the field. Treat the output here as a planning aid for comparing gauge and distance options, not as a final engineering answer. Any wiring connected to a building's electrical system must be sized, installed, and verified according to your local electrical code — the National Electrical Code (NEC) in the United States, or your region's equivalent — and the work should be checked by a licensed electrician. This tool is not a substitute for that process, and mains-voltage wiring carries real injury and fire risk if it's undersized or installed incorrectly.

Common Mistakes in Voltage Drop Estimates

The most frequent error is entering the round-trip distance instead of the one-way distance — since the single-phase formula already doubles the one-way length internally, plugging in a distance you've already doubled yourself will overstate the drop by a factor of two. A second common mistake is forgetting that ambient temperature affects real conductor resistance; the standard table used here is specified at 20°C, and a conductor running hotter than that in practice will have a somewhat higher actual resistance than the table value, meaning the true drop in a hot attic or a heavily loaded conduit can run a bit above this estimate. Third, people sometimes compare the percentage drop against the wrong reference voltage — always compute the percentage against the actual source voltage for that specific circuit, not a nominal system voltage that might not match what the source is actually delivering. Finally, remember that the aluminum figures here are derived from a single documented multiplier applied to the copper table rather than a separately verified aluminum table, so for a project where the extra precision genuinely matters, cross-check against a manufacturer's published aluminum conductor data.

Calculating Voltage Drop Without a Tool

The formula is straightforward enough to work by hand with a calculator and a printed AWG resistance table — the arithmetic is just multiplication and one division. Where a dedicated tool saves time is in not having to look up or memorize the resistance-per-1000-feet figure for a dozen different gauges, and in automatically applying the correct factor (2 for single-phase, the square root of 3 for three-phase) without having to remember which applies when. For anyone building this into a larger estimating tool, a spreadsheet, or an internal application rather than checking one run at a time in a browser tab, the identical voltage-drop logic is available as a metered API endpoint — feed it current, length, voltage, gauge, material, and phase, and it hands back the drop, percentage, and end voltage. Request details and pricing are in the API documentation.

Why does one-way length matter instead of round-trip length?

The single-phase formula already multiplies by 2 internally to account for the outbound and return conductors, so the length field expects only the physical one-way distance you'd measure from source to load — doubling it yourself before entering it would double-count the return path.

What's a typical acceptable voltage drop?

A commonly cited target is keeping total voltage drop under roughly 3% of the source voltage for a branch circuit, which is the threshold this calculator flags results against. That figure is a widely used rule of thumb rather than a universal legal limit — always confirm the specific requirement that applies to your project against your local electrical code.

Does this account for conduit fill or multiple conductors bundled together?

No. The resistance table used here reflects a single conductor's DC resistance at 20°C. Bundling multiple current-carrying conductors together in the same conduit affects heat buildup and ampacity ratings, which is a separate consideration from the voltage-drop resistance figure calculated here.

Why is the aluminum drop always higher than copper for the same gauge?

Aluminum has a higher electrical resistivity than copper, so a given gauge of aluminum wire carries more resistance per foot than the same gauge of copper. This calculator applies a 1.6x multiplier to the copper resistance value to approximate that difference, which is why switching material without changing gauge always increases the estimated drop.

Related Electrical Tools

Voltage drop is one piece of a larger wire-sizing decision. The wire size calculator takes the same current, length, voltage, material, and phase inputs used here and works forward to recommend the smallest standard gauge that satisfies both ampacity and a target drop percentage, rather than checking a single gauge you've already chosen. For the underlying voltage-current-resistance relationship this formula builds on, the Ohm's law calculator solves the same three quantities for a single component rather than a length of conductor. And once a circuit's design is settled, the electricity cost calculator can estimate what running the connected load will actually cost over a day, month, or year.