Voltage Drop & Wire Size Calculator

What voltage drop is, and why long runs get expensive

Voltage drop is the voltage lost pushing current through the resistance of the conductors themselves. The source delivers 240 V, the load at the far end sees 235.3 V, and the missing 4.7 V is real power dissipated as heat in the wire. On a short branch circuit it is a rounding error. On a 150 ft run to a shop, a pump or a subpanel, it is the difference between a tool that starts and one that trips its own overload.

The intuition is Ohm's law applied to the whole loop: voltage lost equals current times resistance, and the resistance of a conductor grows with its length and shrinks with its cross-sectional area. Doubling the run doubles the drop; doubling the area halves it. That is why the fix for a long run is almost always a bigger conductor, not a bigger breaker.

The formula, and where the 2 and the 1.732 come from

Single phase: Vd = 2 × L × I × R ÷ 1,000. The 2 is there because current travels out on one conductor and back on the other, so the loop is twice the one-way length. Enter the one-way run — the number you measured — and the formula accounts for the return trip. This is the single most common error in hand calculations: dropping the 2 and under-reporting the drop by half.

Three phase: Vd = 1.732 × L × I × R ÷ 1,000. The factor is √3, which falls out of the vector relationship between phase voltages. For a balanced three-phase load the neutral carries no current, so the return path does not pay the same penalty as it does in a single-phase circuit.

Divide the result by the system voltage and multiply by 100 for a percentage. L is one-way length in feet, I is the load in amps, and R is the conductor resistance in ohms per 1,000 ft at 75°C from NEC Chapter 9 Table 8. Note what the formula does not include: power factor, reactance, or the temperature of the conductor in service. Table 8 is a dc-resistance table; Chapter 9 Table 9 lists ac effective impedance for anyone doing a rigorous ac calculation. For residential and light-commercial runs under a few hundred feet, Table 8 is what most electricians use, and this calculator follows that convention — stated openly rather than hidden.

NEC Chapter 9 Table 8 — resistance chart

These are the numbers the calculator walks to find your minimum size. Resistance is per 1,000 ft at 75°C for uncoated conductors, so a 100 ft one-way run uses R ÷ 10, and the loop resistance for single phase is twice that.

Two things to read off that chart. First, resistance is almost proportional to 1 ÷ area: going up three sizes (12 to 8 AWG) cuts resistance from 1.93 to 0.764, a factor of 2.5, which is what makes a long run affordable on a small circuit. Second, aluminium sits at about 1.6× the resistance of the same size in copper, so a 6 AWG aluminium conductor performs roughly like a 4 AWG copper one for voltage drop — and roughly 1.5 sizes worse for ampacity.

Ampacity — the check voltage drop does not make

A conductor can be electrically fine for voltage drop and still be illegal, because voltage drop says nothing about whether the wire can carry the current without overheating. That is ampacity, and it comes from NEC Table 310.16. The base table assumes no more than three current-carrying conductors in a raceway, cable or direct burial, at 30°C ambient. Ampacity, not voltage drop, is what decides whether a conductor is legal.

Two rules ride on top of that table. First, 240.4(D) caps the overcurrent device for the small conductors outright: 15 A on 14 AWG copper, 20 A on 12 AWG copper, 30 A on 10 AWG copper, and 15 A and 25 A on 12 and 10 AWG aluminium. Those limits do not bend because the 75°C column looks better. Second, 110.14(C) limits you to the ampacity column the terminations are rated for — for circuits of 100 A or less, that is normally the 60°C column unless every device in the circuit is listed for 75°C. That single sentence is why so many real-world runs land one size larger than a back-of-the-envelope calculation suggests. The calculator has a terminations selector for exactly this reason, and it defaults to 60°C. The same table with the full size range and the 90°C column, plus the ambient and conductor-bundle derating factors, is on the NEC 310.16 wire ampacity chart.

Worked example — 32 A over 150 ft at 240 V

A 7.7 kW load drawing 32 A on a 240 V single-phase circuit, 150 ft from the panel. Start with 6 AWG copper, R = 0.491 Ω per 1,000 ft.

Vd = 2 × 150 × 32 × 0.491 ÷ 1,000 = 4.714 V. Check it by hand in two steps if you want: 2 × 150 = 300 ft of conductor; 300 ÷ 1,000 = 0.3; 0.3 × 0.491 = 0.1473 Ω of loop resistance; 0.1473 × 32 A = 4.714 V. As a percentage, 4.714 ÷ 240 × 100 = 1.96%, comfortably inside the 3% target, and the load still sees 235.3 V.

Ampacity holds too: 6 AWG copper is 55 A at 60°C, so a 40 A breaker is fine. The power burned in that run is 32 A × 4.714 V = 151 W while the load runs — 0.3% or so of the load, but over a working day it is a small heater inside your wall. If the run were 300 ft instead, the drop would double to 9.4 V (3.9%) and 6 AWG would fail the target; the calculator would walk you to 4 AWG, which drops 5.8 V, or 2.4%.

Worked example — why a 100 ft, 20 A circuit at 120 V needs 8 AWG

12 AWG is the default answer for a 20 A circuit, and at 30 ft it is the right one. At 100 ft it is not. Vd = 2 × 100 × 20 × 1.93 ÷ 1,000 =7.72 V, which at 120 V is 6.4% — more than twice the design target, with the load seeing 112.3 V. Step up to 10 AWG: 2 × 100 × 20 × 1.21 ÷ 1,000 = 4.84 V, still 4.0%. Step up to 8 AWG: 3.06 V, or 2.6%, and 8 AWG copper carries 40 A at 60°C so a 20 A breaker is well inside it. That is the answer, and it is the kind of result that surprises people who size by breaker alone.

Now move the same circuit to 240 V and nothing else changes: the drop is still 7.72 V on 12 AWG, but 7.72 ÷ 240 = 3.2% instead of 6.4%. Drop in volts depends on current and resistance; drop in percent depends on the voltage you started with. That is why 240 V tools and 240 V subpanels tolerate distance so much better than 120 V circuits.

Common mistakes

• Using the round trip as the run length. Take the one-way distance and let the 2 (or 1.732) handle the return. Enter 300 ft for a 150 ft run and you will double the answer and buy wire you do not need.
• Using the 60°C ampacity column with 75°C wire. The insulation rating is not the limit — the weakest termination in the circuit is. For 100 A and below that is 60°C unless the terminals are listed otherwise (110.14(C)).
• Assuming 3% is code. It is an informational note in 210.19(A)(1) and 215.2(A)(1), not an enforceable rule, and 90.5(C) says notes are not mandatory. Many AHJs and specs enforce it anyway — this is a design target you should meet, not a citation you can win an argument with.
• Sizing by voltage drop and forgetting the breaker. A 100 ft run may push you to 8 AWG on a 20 A circuit; a 20 A breaker on 14 AWG is illegal no matter how short the run. Run both checks and take the larger conductor.
• Ignoring temperature. Table 8 is stated at 75°C, and a conductor in a hot attic or a loaded conduit runs hotter than the table assumes, so the real drop is higher than calculated.
• Designing exactly on the limit. Landing on 2.98% leaves nothing for a loose terminal, an added load, or a slightly long tail of wire. Give it a size of headroom.
• Forgetting the equipment grounding conductor. Sizing the current-carrying conductors is half the job; the equipment grounding conductor is sized separately per 250.122.

Sizing a feeder out to a subpanel is the same two checks applied to a bigger load — ampacity first, then distance. The worked version, with a 100 A / 240 V distance table and the 83% rule that people wrongly apply to subpanels, is in the what size wire for a 100 amp subpanel guide, and the 200 A case is in the what size wire for a 200 amp service guide.

Voltage drop FAQ

Related calculators and guides

Long runs usually end at something structural. If the circuit feeds a detached shop, the concrete slab guide and the concrete calculator cover the pad, the rebar and footing calculator sizes the reinforcement, and the deck footing calculator handles the posts. For trench drainage alongside the conduit, use the drain pipe slope calculator. If you are working on the building rather than the service, the header size calculator and the stair math guide cover the framing side.

This calculator sizes the conductor for distance. The device that protects it is a separate answer: the breaker size chart covers the standard ratings in NEC Table 240.6(A), the 125% rule for continuous loads, and the breaker and wire for water heaters, dryers, ranges, air conditioners and mini-splits. The conductor's own ampacity, with the ambient and bundle corrections, is on the Table 310.16 ampacity chart.