Estimate voltage drop, voltage at the load, percentage loss, maximum circuit distance, or the minimum conductor size needed to meet a selected voltage-drop target.
This calculator provides a resistance-based engineering estimate. It does not replace conductor ampacity checks, electrical-code requirements, manufacturer data, or professional design.
Calculator
Choose what you want to calculate, then enter the known values below.
Voltage Drop Basics
Voltage drop is the reduction in electrical potential that occurs while current flows through conductors and connections. Every real conductor has resistance, so part of the supplied voltage is lost across the wiring before the electricity reaches the load.
The amount of drop depends heavily on conductor material, cross-sectional area, circuit length, and load current. Copper has lower resistivity than aluminum, a larger conductor has less resistance than a smaller one, a longer run adds resistance, and higher current produces more voltage loss for a given conductor.
Excessive voltage drop can cause equipment to receive less voltage than intended. Depending on the load, that can contribute to dim lighting, poor heater performance, unreliable electronics, or undesirable motor behavior. Voltage-drop calculations are therefore commonly used during circuit planning, especially for long wire runs.
Formula
This calculator estimates conductor resistance from material resistivity and cross-sectional area, then applies a resistance-only voltage-drop approximation.
The factor of 2 accounts for the outgoing and returning conductor path when the entered distance is the one-way run length.
For a balanced three-phase circuit, the common line-to-line resistance-only estimate uses the square root of 3 multiplier.
Resistance per unit length is estimated from conductor resistivity ρ and conductor cross-sectional area A, with a temperature correction applied.
Estimated voltage lost across the conductor run.
Load current in amperes.
Estimated conductor resistance per unit length.
One-way conductor length, not the round-trip distance.
Three Useful Modes
Use this mode when conductor size and circuit distance are already known. The result includes voltage drop in volts, percentage drop, estimated load voltage, circuit resistance, and estimated conductor power loss.
Choose a conductor size and maximum allowed percentage drop to estimate how long the one-way circuit run can be before the resistance-only voltage-drop estimate reaches your selected limit.
Enter circuit distance and a voltage-drop target to find the smallest listed AWG or kcmil conductor that satisfies the selected drop limit. This result does not perform an ampacity check.
Conductor material matters. Copper and aluminum are both widely used electrical conductors, but copper has lower electrical resistivity. If two conductors have the same cross-sectional area and length, the aluminum conductor will generally have more resistance and therefore more voltage drop at the same current.
Wire size matters. A larger conductor has more cross-sectional area, which lowers its resistance. Moving from a smaller AWG conductor to a larger conductor is one of the most direct ways to reduce voltage drop on a long run. Remember that AWG numbering works in reverse: a smaller AWG number represents a larger conductor.
Distance matters. Resistance increases as conductor length increases. That is why a wire size that works comfortably on a short run may produce an undesirable voltage drop when the same load is placed hundreds of feet away. The calculator asks for one-way distance and applies the appropriate circuit multiplier.
Current matters. Ohm's law shows that voltage drop rises with current when resistance remains the same. A circuit that has acceptable voltage drop at a light load can have a much larger drop when the load current increases.
Temperature matters. The resistance of common conductor materials changes with temperature. This tool adjusts the resistance estimate from a 20°C reference using a temperature coefficient for copper or aluminum. The temperature input is a modeling value, not a substitute for conductor temperature-rating or ampacity rules.
AC impedance can matter too. Real AC systems can have reactance in addition to conductor resistance. Power factor, conductor arrangement, conduit material, frequency, and cable construction can affect a more detailed AC voltage-drop calculation. This calculator intentionally uses a transparent resistance-only method, so complex AC installations should be checked with engineering or manufacturer data.
Design Target
There is no single percentage that is automatically correct for every electrical system. The acceptable limit depends on the equipment, circuit type, applicable code, project specification, and design goals.
A 3% target is commonly used as a practical design value for an individual branch circuit, while 5% is often discussed as a total feeder-plus-branch-circuit design target. These values should be treated as planning guidance rather than a universal pass/fail rule.
Sensitive electronics, motors, long feeders, low-voltage systems, renewable-energy wiring, automotive circuits, and other specialized applications may require different limits.
Imagine a 120 V single-phase circuit carrying 20 A over a 100 ft one-way copper run. A smaller conductor will have more resistance and a larger voltage drop. Increasing the conductor size reduces resistance, raises the estimated voltage available at the load, and reduces conductor power loss.
Instead of assuming that a conductor is acceptable because its calculated drop is below a chosen percentage, use the result as one part of the design process. The conductor must still satisfy ampacity and installation requirements.
American Wire Gauge, usually abbreviated AWG, is a conductor sizing system commonly used in North America. One confusing detail is that AWG numbers decrease as conductor diameter increases. For example, 10 AWG is larger than 12 AWG, and 4 AWG is much larger than 10 AWG.
Large conductors eventually move into sizes such as 1/0, 2/0, 3/0, and 4/0 AWG. Beyond those sizes, conductors are often described in circular mils, commonly written as kcmil. The calculator includes several common kcmil sizes so long-distance and higher-current voltage-drop scenarios can be explored.
The wire-size selector in this tool stores the approximate conductor cross-sectional area in square millimeters. The calculator then estimates resistance from the selected material resistivity. Actual manufactured conductors, stranded construction, temperature, joints, terminals, and AC effects can make measured resistance differ from the simplified value.
Accuracy & Safety
A mathematically acceptable voltage drop does not prove that a conductor is safe, code-compliant, or appropriate for the installation.
Frequently Asked Questions
Voltage drop is the reduction in electrical potential that occurs as current flows through the resistance and impedance of conductors and connections. Longer runs, higher current, and smaller conductors generally increase the drop.
A commonly used design target is around 3% for an individual branch circuit and around 5% total for feeder plus branch circuit. The appropriate limit depends on your equipment, installation, code, and project specification.
No. The minimum conductor size mode selects a size based only on the voltage-drop limit. Ampacity, temperature ratings, derating, overcurrent protection, terminal ratings, and applicable electrical codes must be checked separately.
For a balanced three-phase circuit, the common resistance-only line-to-line voltage-drop approximation uses √3 as the circuit multiplier instead of the factor of 2 used for a two-conductor DC or single-phase circuit.
Yes. Enter the one-way conductor length and select feet or meters. The calculator converts the length to meters internally before applying the resistance calculation.
Different tools may use different resistance tables, conductor temperatures, AC reactance, power factor assumptions, conduit data, rounding methods, or manufacturer-specific cable information. Compare the methodology as well as the final number.