Voltage Drop calculator ⚡

A Standard 20-Amp Circuit Fails at 100 Feet

Run 12 AWG copper 100 feet to a 20 A load on a 120 V branch circuit — the most ordinary wiring job imaginable — and you lose 7.72 volts. That is 6.43%, past the 5% the National Electrical Code recommends for a branch circuit, on a run most people would never think twice about.

Nothing is faulty. The conductor is correctly sized for ampacity: 12 AWG is rated for 20 A all day. Ampacity asks whether the wire will overheat. Voltage drop asks whether enough voltage survives the journey. They are different questions, and only one of them is enforced by the breaker.

The Voltage Drop Formula

Everything reduces to Ohm's law — V = IR. Current through resistance produces a voltage difference. The only complication is that current travels out and back, and three-phase circuits share return paths.

Single-phase and DC voltage drop formula

The 2 accounts for the round trip: current flows out on the hot conductor and back on the neutral, so it crosses the length twice. DC uses the identical formula for the same reason.

Working the opening example — 12 AWG copper (1.93 Ω per 1000 ft), 100 feet, 20 A:

VD = 2 × 100 × 1.93 × 20 ÷ 1000 = 7.72 V, or 6.43% of 120 V.

Three-phase voltage drop formula

The √3 (1.732) replaces the 2. Three-phase conductors do not each need a dedicated return — the three currents are 120° apart and sum to zero in a balanced system, so the neutral carries nothing. The √3 converts the line-to-neutral drop into the line-to-line figure you actually measure.

A 4/0 AWG copper feeder (0.0608 Ω per 1000 ft), 250 feet, 200 A on a 480 V three-phase service:

VD = 1.732 × 250 × 0.0608 × 200 ÷ 1000 = 5.265 V, or 1.10% — comfortably inside the 3% feeder recommendation.

Why 2 and √3, and never the other way round

This is the single most common error in voltage drop work. Using 2 on a three-phase circuit overstates the drop by 15%; using √3 on a single-phase circuit understates it by the same margin — and understating is the dangerous direction, because it lets an undersized conductor pass a calculation it should have failed.

The Metric Voltage Drop Formula (mm²)

Outside North America conductors are specified by cross-sectional area in mm², and the formula works from resistivity rather than a table lookup:

A 10 mm² copper circuit, 50 m, 32 A on 230 V single-phase:
VD = 2 × 0.0175 × 50 × 32 ÷ 10 = 5.60 V, or 2.43%.

A 16 mm² copper feeder, 80 m, 40 A on 400 V three-phase:
VD = 1.732 × 0.0175 × 80 × 40 ÷ 16 = 6.06 V, or 1.52%.

Swap that copper for aluminium and the same run drops 9.70 V (2.42%) — because aluminium's resistivity is 1.6× copper's. Aluminium is not disqualified by this; it is cheaper and lighter, and the standard remedy is simply to go up roughly two AWG sizes. But you cannot substitute like for like. Our AWG to mm² Converter handles the cross-system translation if your drawings mix both.

Voltage Drop Chart by Wire Size

Rather than recalculating from scratch, this chart gives volts dropped per 100 feet per ampere for copper. Multiply by your current and by (length ÷ 100). Resistance values are NEC Chapter 9, Table 8, uncoated copper.

For aluminium, multiply the copper figure by 1.6. These are 75°C values; conductors running hot drift higher, because copper's resistance rises roughly 0.4% per °C.

NEC Voltage Drop Limits: the 3% and 5% Rule

Here is the detail that catches people out: the NEC does not mandate a voltage drop limit. The figures everyone quotes live in Informational Notes — NEC 210.19(A) for branch circuits and NEC 215.2(A) for feeders — and informational notes are advisory, not enforceable.

So a feeder that uses its full 3% leaves only 2% for everything downstream. Budget the whole path — service conductor, feeder, then branch — not each leg in isolation, and record the figures on the panel schedule so the next person doing a load calculation inherits your assumptions rather than guessing them. An inspector cannot fail you on an informational note, but they can and will fail the ampacity and circuit breaker sizing that sits alongside it.

Local amendments frequently do make these mandatory, and utility or PV interconnection rules often impose stricter figures. Treat 3%/5% as the floor of good practice rather than a ceiling you are entitled to reach.

Conductor resistance itself comes from NEC Chapter 9, Table 8 — the table our chart above is built from. Table 9 covers AC impedance, which matters for the reason below.

Acceptable Voltage Drop Outside the United States

The physics is universal; the limits are not.

• IEC 60364 and BS 7671 (UK) work to 3% for lighting and 5% for other uses, measured from the origin of the installation. See the IEC 60364-5-52 wiring systems reference for the underlying selection rules.
• AS/NZS 3008 (Australia / New Zealand) allows 5% total from point of supply to any load.
• CSA C22.1 (Canada) mirrors the NEC's 3% / 5% split.
• PEC (Philippines) follows the NEC structure closely.

Because most of the world runs 230 V single-phase and 400 V three-phase rather than 120/240 V, the same percentage buys a great deal more absolute voltage — a 5% allowance is 11.5 V at 230 V but only 6 V at 120 V. Higher distribution voltage is precisely why long runs are easier outside North America.

What Causes Voltage Drop

Four variables, and only four:

1. Length. Drop is directly proportional to one-way run length. Double the distance, double the drop.
2. Current. Also directly proportional. A circuit fine at 10 A may fail at 20 A on the same conductor.
3. Conductor size. Inversely proportional to cross-sectional area — bigger conductor, less resistance.
4. Material. Aluminium carries 1.6× copper's resistivity for the same area.

Everything else is a second-order effect on those four: temperature raises resistance, poor terminations add localised resistance, and corrosion at a lug can produce more drop than the entire cable run.

What Excessive Voltage Drop Actually Does

Voltage drop is rarely reported as "voltage drop". It arrives disguised as something else — a brownout that only happens when the compressor kicks in, a breaker that trips on hot afternoons, conductor overheating nobody can source.

Motors suffer most. Torque falls with the square of voltage, so a 10% drop costs roughly 19% of available torque. A motor that starts fine unloaded may stall under load, and the resulting locked rotor condition draws inrush current of six to eight times full-load amps — which itself deepens the drop, a feedback loop that ends in tripped protection or cooked windings. Motor starting on a long run is the classic case: an HVAC compressor, an elevator, a submersible pump at the bottom of a borehole.

Lighting flicker on alternating current circuits usually traces to a shared conductor with an intermittent large load. LED drivers hold output steady until their input falls below a threshold, then drop out abruptly — which is why LED flicker often appears suddenly rather than gradually.

Heat is wasted money. Every volt lost becomes heat in the conductor. Losing 5% of a 10 kilowatt load means 500 watts heating a cable run rather than doing work — a permanent efficiency loss on the bill, and in a data centre an added cooling burden. Note that this is real power lost to resistance, distinct from apparent power, which concerns how much current the supply must carry.

Low-voltage DC systems are the worst affected, because the same absolute drop is a far larger percentage. A solar array feeding a battery bank, a PV string at 24 V, or a generator on a temporary site can lose an unacceptable share over a few metres. The percentage is what matters, not the volts — which is also why a 208 V commercial circuit tolerates a longer run than a 120 V one, and why utilities use a step-down transformer close to the load rather than distributing at utilisation voltage.

End-of-circuit voltage is the number worth checking. On a buried cable in a trench, or a temporary extension cord on a site, measure at the far end under load rather than trusting the panel reading. Systems at both 50 Hz and 60 Hz behave identically for resistance purposes; frequency only matters once reactance enters.

Temperature, Terminations and What the Formula Ignores

The formula assumes an ideal conductor at a fixed temperature. Reality is less tidy.

Resistance rises with heat. Copper has a temperature coefficient of about 0.393% per °C, so a conductor at its 90°C rating carries noticeably more resistance than the same conductor cold. Tables are published at 60°C, 75°C and 90°C for this reason, and the ampacity derating you apply for ambient temperature or conductor bundling makes the situation worse, not better — a derated conductor runs hotter, and a hotter conductor drops more voltage.

Stranded and solid conductors of the same gauge differ slightly. Stranded has marginally higher resistance for equal nominal area because the individual strands spiral, adding length. NEC Chapter 9, Table 8 lists both, along with area in circular mils and resistance in ohms per 1000 feet; convert to ohms per kilometre by multiplying by 3.281 when working to metric drawings.

Terminations are the silent contributor. A loose connection or an oxidised lug adds resistance at a single point, and unlike cable resistance it concentrates heat in one spot. Aluminum conductors are particularly sensitive here, since the oxide layer forms quickly and the metal creeps under clamping pressure — which is why aluminium terminations need the correct antioxidant compound and torque specification.

Direct current circuits escape the reactance issues below but are otherwise identical, and PVC conduit avoids the magnetic losses that steel introduces.

Why a Voltage Drop Calculator Can Under-Report AC Circuits

Most online calculators — including several ranking above this page — use DC resistance for AC circuits. For small conductors that is fine. For large ones it is not.

AC current in a large conductor concentrates toward the outer surface (skin effect), so the effective resistance exceeds the DC value. Separately, the magnetic field around each conductor induces reactance, and running conductors through steel conduit raises that reactance further through eddy-current losses — PVC and aluminium conduit do not.

Below about 1/0 AWG the difference is negligible. At 250 kcmil and above in steel conduit, impedance can exceed DC resistance by 15–20%, and a calculation based on resistance alone will under-report the drop. That is what NEC Chapter 9, Table 9 exists for.

Power factor matters here too. The impedance seen by the circuit depends on it, which is why a heavily inductive load — motors, welding sets — can show more drop than a resistive load drawing identical current. Our Power Factor Correction Calculator and the kW to kVA Calculator cover that relationship in detail. And when the drop you need is across a discrete component rather than a cable run, the Resistor Voltage Drop Calculator handles series chains, dividers and LED resistors.

How to Reduce Voltage Drop

In rough order of cost-effectiveness:

• Increase conductor size. The direct fix. Going up two AWG sizes roughly halves the drop.
• Shorten the run. Relocating a sub-panel closer to the load often beats upsizing an entire feeder.
• Raise the system voltage. Running a load at 240 V instead of 120 V quarters the percentage drop for the same power, since current halves and drop is proportional to current. This is why long agricultural and industrial runs use higher voltages.
• Use parallel conductors. Two conductors in parallel halve the effective resistance. NEC 310.10(G) permits paralleling only at 1/0 AWG and larger, and paralleled sets must be identical in length, material and termination — our 10 mm² example above, which dropped 5.6 V, would fall to 2.8 V on two conductors.
• Switch aluminium to copper, or upsize the aluminium by about two sizes.
• Fix the terminations. Before re-pulling cable, check lugs and connections — a single loose or corroded terminal can account for more drop than the conductor itself.

Once the conductor is chosen, confirm it against ampacity and overcurrent protection with our Cable Size Calculator and Wire Gauge Calculator, and verify the breaker's interrupting rating with the Short Circuit Current Calculator.

Frequently Asked Questions

What is voltage drop?

Voltage drop is the reduction in voltage between the source and the load, caused by the resistance of the conductors carrying the current. Every conductor has resistance, and by Ohm's law any current through resistance produces a voltage difference. The energy lost becomes heat in the cable. It is measured either in volts or, more usefully, as a percentage of the supply voltage — a 6 V drop on a 120 V circuit is 5%, while the same 6 V on a 400 V system is only 1.5%.

How do you calculate voltage drop?

For a single-phase or DC circuit, VD = 2 × L × R × I ÷ 1000, where L is the one-way length in feet, R is resistance in ohms per 1000 feet, and I is current in amperes. For a balanced three-phase circuit, replace the 2 with 1.732 (√3). In metric terms, VD = 2 × ρ × L × I ÷ A for single-phase, using resistivity ρ of 0.0175 for copper or 0.028 for aluminium, length in metres and area in mm².

What is an acceptable voltage drop?

The National Electrical Code recommends a maximum of 3% on a branch circuit or feeder, and 5% for the two combined. IEC and BS 7671 allow 3% for lighting and 5% for other loads, while AS/NZS 3008 permits 5% overall. These are recommendations rather than hard limits in the NEC — they appear in informational notes — but local codes often make them enforceable, and sensitive electronics may need tighter figures than any code requires.

What causes voltage drop in a wire?

Four factors: conductor length, current, cross-sectional area and material. Drop rises directly with length and current, falls as the conductor gets larger, and is about 1.6 times worse in aluminium than copper for the same area. Temperature contributes as well, since copper's resistance climbs roughly 0.4% per degree Celsius, and poor terminations add localised resistance that can exceed the whole cable run.

Does wire length affect voltage drop?

Yes, directly and proportionally. Doubling the one-way length doubles the drop, everything else being equal. This is why the formula uses one-way length multiplied by 2 for single-phase — the current has to travel out and back. Shortening a run, often by relocating a sub-panel closer to the load, is frequently cheaper than upsizing the conductor over the full original distance.

Is 3% voltage drop acceptable?

Yes, 3% is the figure the NEC recommends as a maximum for either a branch circuit or a feeder individually, so a circuit at exactly 3% is compliant with the recommendation. The caution is the combined limit: if a feeder already consumes 3%, only 2% remains for the branch circuit downstream before the 5% total is breached. Budget the whole path from service to load rather than each segment separately.

How can I reduce voltage drop?

Increase conductor size, shorten the run, raise the system voltage, or use parallel conductors. Upsizing two AWG sizes roughly halves the drop. Moving from 120 V to 240 V for the same power quarters the percentage drop, because current halves and drop scales with current. Paralleling is permitted from 1/0 AWG upward under NEC 310.10(G), provided the sets are identical. Always check terminations first — a corroded lug can cause more drop than the cable.

What is the difference between resistance and impedance?

Resistance opposes current in any circuit and is what DC calculations use. Impedance is the total opposition in an AC circuit, combining resistance with reactance from the conductor's magnetic field. For small conductors the two are nearly identical. For large ones, skin effect and reactance make impedance meaningfully higher — up to 15–20% in steel conduit at 250 kcmil and above. NEC Chapter 9 Table 8 lists resistance; Table 9 lists AC impedance.

Why does conduit material affect AC voltage drop?

Steel is ferromagnetic, so the alternating magnetic field around the conductors induces eddy currents and hysteresis losses in the conduit itself, raising the circuit's effective impedance. PVC and aluminium conduit are non-ferrous and produce no such effect. NEC Chapter 9 Table 9 gives separate impedance columns for steel, aluminium and PVC raceways for exactly this reason — the difference is small on branch circuits but significant on large feeders.

How do parallel conductors reduce voltage drop?

Running two identical conductors per phase halves the effective resistance, which halves the voltage drop — the same result as doubling the cross-sectional area, but far easier to pull and terminate than one very large cable. NEC 310.10(G) allows paralleling only at 1/0 AWG and larger, and every parallel set must match in length, material, insulation type and termination method, otherwise current divides unevenly and one conductor overheats.

What power factor should I use for voltage drop calculations?

If you are working from resistance alone, power factor does not enter the calculation. It matters when using impedance, because the effective impedance depends on the phase relationship between voltage and current. NEC Chapter 9 Table 9 is tabulated at 0.85 power factor, which suits most mixed commercial loads. For a largely resistive load use a figure closer to 1.0; for motor-dominated installations 0.8 is the usual assumption.