Voltage Drop Calculator

Voltage Drop Calculation

Voltage drop is the difference between the voltage at the source and the voltage at the load, and it exists because the conductor itself has resistance. For a DC or single-phase circuit, the current travels out along one conductor and back along another, so the resistance of both lengths counts:

V_drop = 2 × I × R × L

Here I is the load current in amperes, R is the conductor resistance per unit length in ohms per foot or ohms per thousand feet, and L is the one-way length. The factor of two is the round trip. For a balanced three-phase circuit, the return path is shared and the relation becomes V_drop = √3 × I × R × L, again with L measured one way.

Worked example: a 240 V single-phase water heater drawing 30 A, located 200 ft from the panel, wired in 8 AWG copper in PVC conduit. Table 9 of the National Electrical Code lists 8 AWG copper at about 0.778 Ω per 1,000 ft at 75 °C. The drop is 2 × 30 × 0.778 × 0.200 = 9.34 V, or 3.9% of 240 V. Stepping up to 6 AWG at about 0.491 Ω per 1,000 ft gives 5.89 V, or 2.5%.

The percentages are computed against the source voltage, which is the convention used by the NEC's informational notes. If you compute against the voltage at the load instead, the number will be slightly larger, and mixing the two conventions across a design package is a reliable way to produce a report that does not reconcile.

What Voltage Drop Does to a Circuit

A resistive load such as a heater simply produces less heat when the voltage falls, because power varies with the square of voltage at constant resistance. A motor behaves differently: as voltage falls it draws more current to deliver the same shaft power, so the winding temperature rises, insulation ages faster, and starting becomes harder. That is why a long, undersized run to a motor is more damaging than the same percentage drop to a resistive load.

In AC circuits the opposition is impedance rather than pure resistance. Impedance combines resistance with reactance, and for small conductors at 60 Hz the reactance term is usually minor compared with resistance. For large feeders in steel conduit, reactance can be a meaningful fraction of the total, which is why Table 9 publishes both resistance and reactance columns and why a calculator that uses resistance alone can understate the drop on big conductors.

Incandescent lamps are famously sensitive: light output falls roughly with the 3.4th power of voltage, so a 5% drop dims the lamp by about 16% and shifts its color toward the red. Electronic drivers and switch-mode supplies usually hold their output steady by drawing more input current, which moves the problem into the supply and the wiring rather than the lamp. Motors, contactors, and variable-frequency drives may trip on undervoltage if the drop is large enough.

Nothing about this is hypothetical on a long run. A 120 V circuit feeding a 12 A pump 150 ft away through 14 AWG copper has a drop of about 11.3 V, or 9.4%, leaving roughly 108.7 V at the motor. The same run in 10 AWG drops about 4.4 V, or 3.6%. The pump does not stop working at 108.7 V, which is exactly why the problem often goes unnoticed until the motor fails early.

The Four Variables That Set the Drop

• Material. Copper has lower resistivity than aluminum, so an aluminum conductor of the same gauge drops more voltage. Aluminum is still used for large feeders and service conductors because it is lighter and cheaper per amp of capacity, but it is usually sized one or two gauges larger than the copper equivalent.
• Cross-sectional area. Resistance is inversely proportional to area, and in the AWG system three gauge steps roughly double the area. Going from 12 AWG to 9 AWG would halve resistance, which is why the practical jump from 12 to 10 AWG produces about a 37% reduction rather than a round 50%.
• Length. Drop is linear in length, and the round-trip factor means every extra foot of run costs twice. A 200 ft circuit has four times the drop of a 50 ft circuit with the same conductor and load, which is why panel placement is a design decision and not an afterthought.
• Current. Drop is linear in current. Doubling the load doubles the drop, and motor starting current can be five to eight times running current for a few seconds, producing a temporary sag that may trip contactors even when the steady-state drop is acceptable.

These variables multiply, so a modest change in two of them compounds. A 150 ft run at 12 A in 14 AWG is a problem; shortening it to 75 ft and stepping up to 12 AWG brings the drop to about 3.5 V, or 2.9%, with no change to the load. Resistance at the connections adds a fifth, less predictable term: a loose terminal or a corroded splice can contribute more drop than several feet of wire, and it will not appear in any calculation.

Wire Conductivity Comparison

Conductivity is usually quoted relative to copper by volume, using the International Annealed Copper Standard. Silver is marginally better than copper, gold is about 70% as conductive, and aluminum is about 61%. The resistance factor in the table is simply 100 divided by the relative conductivity, so it tells you how much to multiply copper resistance for the same gauge.

The practical consequence is larger than the percentages suggest, because the comparison is at equal gauge rather than equal ampacity. A 100 ft run of 12 AWG copper carrying 20 A at 120 V drops about 7.7 V, or 6.4%. The same run in 12 AWG aluminum drops about 12.7 V, or 10.6%. To match the copper performance, the aluminum conductor needs to be roughly two gauges larger, which is why code tables and equipment listings treat the two materials separately.

Silver and gold are not power conductors. Silver is used where contact resistance matters more than material cost, and gold plating is used to keep low-level signal contacts from oxidizing. Neither belongs in a voltage drop calculation for building wiring, and quoting their conductivity in that context is a distraction rather than a design input.

Small aluminum branch-circuit conductors were installed in North American homes in the late 1960s and early 1970s and developed a reputation for overheated connections. The modern use of aluminum is concentrated in large, correctly terminated feeders and service entrance conductors, often with transition splices to copper at the equipment.

Practical Ways to Reduce Voltage Drop

• Go up a gauge or two. A single AWG step cuts resistance by roughly 20% to 25%; three steps halve it. On long branch circuits, one size larger than the code minimum is often the cheapest fix.
• Shorten the run. Move the panel, relocate the load, or add a subpanel near the point of use. This is the only measure that reduces both voltage drop and copper cost at the same time.
• Use copper for branch circuits. Aluminum is appropriate for large feeders where its weight and cost advantages matter, but copper is easier to terminate and more forgiving in small sizes.
• Avoid daisy-chained cords. Each plug and receptacle pair adds contact resistance. Two 50 ft extension cords have more loss than one 100 ft cord of the same gauge, and the connections are the weak points.
• Size for starting current. A motor that starts across the line draws several times its running current. If the lights dim when the compressor starts, the steady-state calculation is not the whole story.
• Check terminations. Torque terminals to the manufacturer's specification and inspect for corrosion. A high-resistance joint shows up as heat, and heat accelerates the corrosion that caused it.
• Consider a local transformer or boost tap. For a remote load on a long run, a buck-boost transformer or a multi-tap distribution transformer can correct the voltage at the point of use when rewiring is impractical. This treats the symptom, so pair it with an actual load calculation.

None of these measures substitutes for measuring the result. A clamp meter and a voltmeter at the load will tell you what the calculation only estimates, and the difference between the two is often a termination that was never tightened.

What the NEC Actually Requires

The National Electrical Code treats voltage drop differently from ampacity. Ampacity is mandatory: conductors must be sized to carry the load without exceeding their temperature rating, and overcurrent protection must match. Voltage drop appears mainly in informational notes, which are explanatory text rather than enforceable rules in themselves.

The familiar figures come from those notes. Branch circuits are advised not to exceed 3% drop, and the combined feeder plus branch circuit is advised not to exceed 5%, measured at the farthest outlet under load. Article 215 contains a parallel note for feeders at 3%. Many local jurisdictions adopt these figures as mandatory amendments, so the code book alone does not tell you what your inspector will enforce.

Some occupancies have stricter or different limits. Article 647 addresses sensitive electronic equipment in certain commercial and industrial areas, with 1.5% for branch circuits and 2.5% total. Article 695 permits up to 15% drop for fire pump circuits during motor starting, because the alternative, a pump that will not start, is worse than a temporary voltage sag. Article 210.19 also requires branch conductors to have sufficient ampacity for the load, which is a separate test from voltage drop.

Treat the 3% and 5% figures as design targets, not as proof of compliance. A circuit can meet the voltage drop recommendation and still fail an ampacity check, and it can pass both and still have a bad neutral or a loose lug. The calculation narrows the design space; inspection and measurement close it.

What Changes Conductor Resistance

Resistance is a property of the material, the geometry, and the temperature. For a uniform conductor it is R = ρL/A, where ρ is resistivity, L is length, and A is cross-sectional area. Doubling the length doubles resistance; doubling the area halves it. That is the entire reason gauge selection matters so much on long runs.

Temperature is the factor most often left out. Copper's resistance rises about 0.393% per °C near room temperature, so a conductor at 75 °C has roughly 21% more resistance than the same conductor at 20 °C. NEC Table 9 values are published at 75 °C for this reason. A design that uses 20 °C laboratory data will understate the drop in a hot attic or a loaded conduit.

At 60 Hz, skin effect pushes current toward the surface of the conductor, but for sizes below roughly 250 kcmil the effect is small. It becomes relevant for large feeders and for harmonic-rich loads, where higher-frequency components see a higher effective resistance. Proximity effect in tightly bundled conductors adds a smaller increment.

Connections are outside the formula but inside the real circuit. A splice, a breaker terminal, or a receptacle adds contact resistance that depends on torque, plating, oxidation, and spring tension. Aluminum is more sensitive to these details than copper, which is why terminations for aluminum conductors are listed and marked for the purpose and why anti-oxidant compound is often specified.

Typical AWG Wire Sizes

American Wire Gauge runs in the opposite direction from intuition: a smaller number means a larger conductor. Three gauge steps roughly double the cross-sectional area, and six steps roughly double the diameter. The table lists common sizes with copper resistance at 75 °C, rounded for design estimates.

Standard residential assignments follow the overcurrent device more than the voltage drop. Fourteen-gauge copper is the usual minimum for 15 A lighting circuits, twelve-gauge for 20 A receptacle circuits, ten-gauge for a 30 A dryer or water heater, and eight or six-gauge for larger appliances and subfeeders. Those are ampacity minimums; a long run may need a larger conductor to keep the drop within reason, and the two requirements are checked separately.

Above 4/0 the gauge system gives way to kcmil sizes, where the number is the area in thousands of circular mils. A 500 kcmil conductor has about 2.4 times the area of 4/0, which is why large feeders are specified in kcmil rather than in gauge numbers.

Residential, Commercial, and Industrial Differences

Residential circuits are short and lightly loaded, so voltage drop is usually a secondary concern until a run reaches a detached garage, a well pump, or a landscape lighting circuit. When it does matter, the fix is usually a larger branch conductor or a subpanel closer to the load. A 120 V circuit is less forgiving than a 240 V circuit for the same absolute drop, because the percentage is computed against a smaller base.

Commercial buildings have longer runs, higher currents, and three-phase distribution. Feeders from a main switchboard to a remote panel can be hundreds of feet, and the cost of copper makes the sizing decision economic as well as technical. Designers often compare the capital cost of a larger feeder with the capitalized cost of I²R losses over the life of the building, and the answer depends on hours of operation and energy prices.

Industrial installations add motors, drives, and high-current processes. A three-phase motor that sees unbalanced or low voltage draws more current on the affected phases, heats unevenly, and loses torque. Starting current magnifies the drop for a few seconds, which is why fire pump circuits are allowed a larger starting drop than other loads and why soft starters and reduced-voltage starting exist.

The correction strategies differ accordingly. In a house, replacing a branch circuit is a small job. In a commercial building, the design is fixed before the wire is pulled, so the calculation has to be right at the drawing stage, and parallel conductors or a relocated subpanel may be the only practical options afterward. In a plant, a tap-changing transformer or a dedicated feeder to a critical motor may be cheaper than rebuilding the distribution.

Final Thoughts

The calculation is simple enough to do by hand, and doing it by hand once is the best way to build intuition for what the tool is telling you. The two things people get wrong most often are the round-trip factor and the temperature basis of the resistance data. A single-phase circuit needs both conductors counted; a table published at 75 °C should not be mixed with a 20 °C value from a different source.

Use the result as a design check, not a certificate. The estimate assumes uniform conductor temperature, clean terminations, and a steady load. Real installations have warm conduits, derated bundles, harmonics, and motors that start. Measure the voltage at the load after commissioning, and investigate any reading that differs materially from the calculation.

For related work, the Ohm's law calculator handles the basic V, I, R relationships, the electricity calculator covers power and energy, and the resistor calculator is useful for component-level work.