Voltage Drop Calculator

Voltage drop is the reduction in electrical potential between a source and a load caused by current flowing through conductor resistance and, in AC circuits, reactance. It is usually reported in volts and as a percentage of source voltage. Excessive drop can reduce load voltage, but the acceptable value depends on the equipment, circuit purpose, design target, and governing installation requirements.

Voltage drop occurs because conductors are not ideal. Their resistance converts some electrical energy into heat, and AC circuits can also have inductive reactance. The amount of drop depends on current, conductor length, size, material, temperature, circuit configuration, power factor, and installation-dependent impedance. A longer or smaller conductor generally produces more resistance and therefore more drop.

Start with the source voltage, current, one-way cable length, conductor impedance, and circuit type. DC two-wire calculations commonly use Vd = 2ILR. Single-phase AC uses approximately 2IL(R cosφ + X sinφ), while three-phase AC uses approximately √3 IL(R cosφ + X sinφ). Then calculate percentage as Vd/Vsource × 100 and load voltage as Vsource − Vd.

The formula depends on circuit type. For a known total circuit resistance, Vd = IR. A two-wire DC circuit can use Vd = 2ILR when R is resistance per unit length and L is one-way length. AC calculations can include both resistance and reactance through R cosφ + X sinφ, with a factor of 2 for single phase or √3 for three phase.

For a two-wire DC circuit, calculate the conductor resistance per unit length, correct it for temperature if needed, multiply by twice the one-way length, divide by the number of parallel conductors, and multiply by current. The result is Vd = 2ILR/n. Then calculate voltage-drop percentage and subtract the drop from source voltage to estimate the load voltage.

A common engineering approximation is Vd ≈ 2IL(R cosφ + X sinφ), where L is one-way length, R and X are per-length conductor resistance and reactance, PF = cosφ, and sinφ is derived from PF. The factor of 2 represents the outgoing and returning conductor path. Installation-specific impedance data should be used when higher accuracy is required.

For a balanced three-phase circuit using one-way conductor length, a common approximation is Vd ≈ √3 IL(R cosφ + X sinφ). Unlike a two-wire DC or single-phase loop, the formula uses the √3 line-to-line relationship rather than simply doubling the one-way length. Use conductor resistance and reactance appropriate to the actual cable arrangement and operating temperature.

Divide the calculated voltage drop by source voltage and multiply by 100: Vd% = (Vd/Vsource) × 100. For example, the same 5 V drop represents a larger percentage on a 120 V circuit than on a 480 V circuit. Percentage is useful for comparing circuits, but any target should be treated as a design criterion rather than a universal legal threshold.

There is no single acceptable voltage drop for every installation. The allowable value depends on equipment sensitivity, starting conditions, load type, conductor economics, source regulation, and the governing code or project criteria. Common design targets such as 3% or 5% are widely discussed, but users should verify the specific requirements and recommendations that apply to the actual installation.

Not as a universal rule for every circuit. Common design guidance often references values such as about 3% for branch circuits, but the specific application, code edition, equipment, and project criteria matter. This calculator labels the maximum voltage-drop percentage as a user-selected target and does not present 3% as a mandatory requirement for every electrical installation.

Five percent is sometimes used as a total feeder-plus-branch design guideline, but whether it is acceptable depends on the circuit, connected equipment, starting conditions, local code, and project requirements. A motor or sensitive electronic load may need a tighter limit. Treat the target in this calculator as an engineering design input rather than an automatic code-compliance verdict.

Larger conductor cross-sectional area reduces resistance per unit length, which generally lowers voltage drop for the same material, current, temperature, and length. This is why upsizing conductors can improve load voltage. However, wire size must also satisfy ampacity, insulation temperature, termination, short-circuit, installation, and protection requirements; voltage-drop performance alone cannot establish the final conductor size.

Yes. With other factors fixed, conductor resistance increases in proportion to length, so voltage drop also increases. For two-wire DC and single-phase circuits, a one-way cable run affects both outgoing and return paths. Three-phase formulas normally use one-way length with a √3 factor. Always check whether a calculator expects one-way length or total circuit length before comparing results.

Yes. Voltage drop is directly proportional to current in the common linear approximations. Doubling current doubles the calculated drop when resistance, reactance, power factor, and length remain unchanged. Real systems can be more complex because conductor temperature rises with load, motor power factor can change, and source voltage may sag, so field behavior may not remain perfectly proportional.

Source voltage does not directly change conductor resistance, but it changes the percentage represented by a given voltage drop. A 6 V drop is 5% of 120 V but only 1.25% of 480 V. Voltage can also change load current for some equipment, so the operating current used in the calculation should correspond to the actual electrical condition.

At the same cross-sectional area and temperature, copper has lower electrical resistivity than aluminum, so it generally has lower resistance and voltage drop for the same current and length. Aluminum can still be an appropriate conductor when sized and installed correctly. Material choice also affects ampacity, terminations, weight, cost, and applicable installation requirements.

Metal conductor resistance increases as temperature rises. A common first-order correction is RT = Rref[1 + α(T − Tref)]. This calculator uses approximate temperature coefficients of 0.00393/°C for copper and 0.00403/°C for aluminum with a 20°C reference. Actual cable resistance depends on conductor construction and should be checked against reliable product or standard data.

Yes. For ordinary metallic conductors, resistance changes with temperature. Warmer copper or aluminum has higher resistance and therefore usually produces greater voltage drop at the same current. The temperature coefficient approach is an engineering approximation; conductor stranding, alloy, actual operating temperature, skin/proximity effects, and manufacturer data can change the effective resistance used in an AC circuit.

Resistance R represents the in-phase component of conductor impedance and dissipates real power. Reactance X represents the quadrature component associated with electric or magnetic fields. AC voltage-drop approximations combine them using power factor through R cosφ + X sinφ. DC circuits normally use resistance only because steady-state DC reactance is not treated in the same way.

Yes. In an AC circuit, the approximate voltage-drop expression includes R cosφ + X sinφ. Changing power factor changes the weighting of resistance and reactance. This calculator uses positive sinφ for lagging loads and a negative reactive sign for leading loads. Actual results can also depend on conductor arrangement and the impedance data appropriate to the installation.

The √3 factor arises from the vector relationship among the three phase voltages in a balanced three-phase system. For line-to-line voltage drop, a one-way conductor length is used with the √3 multiplier rather than a simple two-wire loop factor. The formula assumes a balanced system and consistent line quantities; severe unbalance requires a more detailed phase-by-phase analysis.

For DC and single-phase two-wire circuits, this page asks for one-way cable run length and applies the return-path factor internally. For balanced three-phase calculations, one-way conductor length is used with the √3 relationship. If you enter total loop length into a formula already containing the factor of 2, you can double-count the path and overstate voltage drop.

Subtract voltage drop from source voltage: Vload = Vsource − Vdrop. The result is an approximate load-terminal voltage under the entered current and conductor conditions. If the calculated drop equals or exceeds source voltage, the assumptions are not physically suitable for a normal operating circuit and the input values should be checked carefully.

If total circuit resistance is already known, use Ohm's law directly: Vd = I × Rtotal. Then Vd% = Vd/Vsource × 100 and Vload = Vsource − Vd. This mode is useful when resistance comes from a manufacturer, field measurement, or a more detailed cable model rather than from the calculator's material-and-area estimate. Use project-specific conductor and equipment data whenever the result supports installation, equipment selection, or code-sensitive decisions.

Select 12 AWG, choose copper or aluminum, enter the one-way length, current, source voltage, conductor temperature, and circuit type, then calculate. The page derives approximate conductor area geometrically and estimates resistance from material resistivity, or you can enter a project-specific resistance per length. For AC work, also enter reactance and power factor when those effects are relevant.

Select 10 AWG and provide the same circuit data: source voltage, current, one-way run, material, temperature, and circuit configuration. Larger 10 AWG area normally gives lower resistance than 12 or 14 AWG of the same material. Final conductor selection still needs ampacity and installation checks because meeting a voltage-drop target is not equivalent to meeting code-required conductor capacity.

Select 14 AWG, the conductor material, source voltage, current, and cable length. The calculator estimates material resistance from the conductor's cross-sectional area and adjusts it for the entered temperature. For AC circuits, power factor and reactance also matter. A 14 AWG result should never be used as an ampacity approval; only voltage-drop performance is being evaluated.

Enter 100 ft as the one-way cable run, not the total loop length, unless you use the known-total-resistance mode. Select the circuit type, conductor, current, voltage, and other AC inputs as applicable. The calculator applies the two-wire factor for DC/single phase or the √3 factor for three phase and then reports volts, percent, and load voltage.

Enter 200 ft as the one-way cable run and specify the circuit type. Longer conductors increase impedance and therefore voltage drop. For single-phase AC and DC, the calculator accounts for the outgoing and return paths. For balanced three-phase AC, it uses one-way length with the √3 relationship. Verify the resulting load voltage against equipment and project requirements.

Set the source voltage, allowable user-selected drop percentage, current, conductor, material, temperature, circuit type, and AC power factor/reactance where applicable. The calculator computes Vdrop,max = Vsource × target% and rearranges the selected voltage-drop equation to solve the maximum one-way length. Ampacity, installation, and code requirements remain separate checks. Use project-specific conductor and equipment data whenever the result supports installation, equipment selection, or code-sensitive decisions.

Choose Minimum Conductor Size, then enter circuit type, voltage, current, one-way length, material, temperature, target percentage, and AC impedance inputs where applicable. The calculator evaluates each available size in the selected AWG, kcmil, or metric family and returns the smallest listed size that meets the voltage-drop target. It does not evaluate conductor ampacity.

Yes. Excessive voltage drop can reduce motor terminal voltage, lower starting torque, increase current in some operating conditions, and contribute to overheating or nuisance trips. Motor starting may create a much larger temporary current than normal running current, so a steady-state voltage-drop calculation can underestimate the starting voltage sag unless starting current is entered separately.

Yes. If a lighting circuit experiences significant voltage drop, the luminaires may receive less voltage than expected and can appear dimmer or behave incorrectly. The effect depends on lamp and driver technology. LED drivers, electronic ballasts, and controls can respond differently than resistive lamps, so equipment voltage tolerance should be checked rather than relying on brightness alone.

Yes. Low load voltage can affect motors, electronics, heaters, controls, contactors, relays, and other equipment. Symptoms can include reduced torque, nuisance shutdowns, poor regulation, overheating, or improper operation. The acceptable voltage at the equipment terminals depends on the product rating and operating condition, so voltage-drop calculations should be compared with manufacturer requirements.

Yes. Motor starting current can be several times the normal running current, which can create a temporary voltage drop much larger than the steady-state value. If starting performance is important, use an appropriate starting-current estimate and source impedance rather than relying only on full-load current. This calculator is primarily a conductor voltage-drop tool and does not model all upstream source impedance.

The terms are often used informally to describe the same decrease in voltage between source and load. In engineering calculations, voltage drop usually refers to the potential difference caused by circuit impedance under load. The associated energy is not literally destroyed; conductor resistance dissipates real power as heat, while reactive effects exchange energy within the AC system.

Frequency can affect conductor reactance, skin effect, and proximity effect, so accurate AC impedance can vary with frequency. This calculator lets the user enter reactance appropriate to the circuit and treats frequency as context. If a project requires frequency-specific cable impedance, use manufacturer or standard data for that frequency rather than assuming a universal reactance value.

Yes. Cable arrangement, spacing, raceway, conduit, magnetic materials, parallel conductors, bonding, and nearby conductors can affect AC impedance and thermal conditions. This calculator can use user-supplied R and X values to reflect better project data. A simple material-resistivity model is useful for preliminary estimates but cannot represent every installation-dependent AC effect.

Yes. Select Aluminum as the conductor material. The calculator uses a different approximate 20°C resistivity and temperature coefficient than copper. Because aluminum has higher resistivity, a larger area is generally required to achieve the same resistance. Final aluminum conductor design also needs approved terminations, ampacity, installation practices, and the applicable electrical code.

Yes. Select Copper and the required conductor size. The page estimates resistance from copper resistivity, cross-sectional area, parallel conductors, and temperature correction, or accepts manual resistance per length. Copper's lower resistivity generally reduces drop for a given area compared with aluminum. Final sizing must still satisfy ampacity and installation requirements separately.

Yes. Enter 120 V as the source voltage and select DC or Single-Phase AC as appropriate. Because a given voltage drop represents a larger percentage on a lower-voltage circuit, 120 V installations can be sensitive to conductor length and current. Use the actual load current and conductor data rather than applying a generic percentage without checking equipment requirements.

Yes. For a typical 240 V single-phase circuit, select Single-Phase AC, enter 240 V source voltage, one-way length, current, conductor data, PF, and reactance if relevant. For a 240 V DC system, select DC instead. The circuit type determines whether the calculator uses a two-wire DC/AC relationship or another formula.

Yes. Select Three-Phase AC, enter 480 V line-to-line source voltage, line current, one-way cable length, conductor data, power factor, and reactance. The calculator uses the balanced three-phase √3 voltage-drop approximation. Severe phase unbalance, harmonics, motor starting, and source impedance require additional analysis beyond this simplified conductor calculation. Use project-specific conductor and equipment data whenever the result supports installation, equipment selection, or code-sensitive decisions.

Yes for preliminary DC solar-circuit voltage-drop calculations when the correct current, one-way length, conductor size, material, and temperature are used. PV installations also require DC voltage limits, conductor ampacity, temperature corrections, equipment ratings, overcurrent protection, grounding, rapid-shutdown considerations where applicable, and the governing electrical code. Voltage drop alone cannot establish PV conductor compliance.

Yes. Battery circuits are DC, so the two-wire relationship can be used for a positive and negative conductor path. Low-voltage battery systems can be especially sensitive because a small drop represents a large percentage of source voltage. High currents also make cable resistance and connection resistance important, so terminals, lugs, and busbars may need separate consideration.

Yes for preliminary conductor drop estimates. EV charging circuits can be single-phase or three-phase depending on the installation and equipment. Use the actual continuous current, source voltage, conductor length, conductor material, and AC power factor/reactance where appropriate. Final EVSE conductor sizing must still comply with ampacity, continuous-load, protection, temperature, and local electrical-code requirements.

Yes. For normal running conditions, enter the motor's line current, voltage, conductor length, material, power factor, and AC impedance data. Starting conditions may produce much larger temporary voltage drop because inrush current is higher. Motor circuits also require ampacity, overload, short-circuit, motor-controller, and code checks that are separate from voltage-drop calculations.

Yes, if total circuit resistance is known directly. Use the Known Resistance mode with source voltage, current, and total resistance. For an AC circuit where impedance effects matter, a simple total resistance may be insufficient because reactance and power factor contribute to the voltage drop. Manufacturer cable impedance or measured circuit data can provide a better basis.

Different calculators may use different conductor resistance tables, reference temperatures, AC reactance assumptions, one-way versus loop length, copper or aluminum data, parallel conductor handling, or power-factor conventions. Some tools use DC resistance only while others use AC impedance. Check the resistance basis, temperature, circuit type, and whether length is one-way before comparing numerical results.

No. Voltage drop and ampacity are different design checks. A conductor can have a low enough voltage drop but still be too small for allowable current based on insulation rating, ambient temperature, bundling, terminations, or code rules. Conversely, a conductor may meet ampacity but produce too much voltage drop on a long run. Both checks are required.

No. Voltage drop can identify a conductor size that meets a chosen voltage-performance target, but that size must still satisfy ampacity, overcurrent protection, insulation temperature rating, installation method, conduit or raceway requirements, short-circuit withstand, terminations, and applicable code. The calculator's Minimum Conductor Size result is therefore only a voltage-drop selection, not final design approval.

Follow the electrical code, standards, and project requirements that govern the actual installation. Common design guidance often discusses targets such as about 3% branch-circuit and 5% combined feeder-plus-branch voltage drop, but those values should not be presented as a universal mandatory rule for every circuit. Verify the applicable NEC edition, local amendments, equipment instructions, and engineering criteria.