Electrical Parameters
All fields update results instantly as you type.
Conductor Settings
Choose material, circuit type, and your allowable voltage drop limit.
Material Type
Phase / System
Acceptable Voltage Drop Limit
Recommended Wire Size
--
--
Exceeds Standard 4/0 AWG Limits: The calculated load requires conductors larger than 4/0 AWG. This is outside the scope of standard residential and light commercial wiring. Consult a licensed electrical engineer for conductor sizing - parallel conductors or larger kcmil cable (e.g. 250 kcmil, 350 kcmil) may be required.
Actual Voltage Drop
--
Volts
Drop Percentage
--
% of source voltage
Voltage at Equipment
--
Volts
Enter amps, distance, and voltage above to see the recommended wire size.
Key Terms Explained
Voltage Drop
The reduction in electrical voltage as current travels through a conductor due to the wire's resistance. Measured in volts or as a percentage of source voltage.
AWG (American Wire Gauge)
A standardized system for wire diameter in North America. Counter-intuitively, a smaller AWG number means a thicker wire - 4 AWG is much thicker than 14 AWG.
Circular Mils (CM)
The unit used to measure wire cross-sectional area in North American electrical calculations. One circular mil equals the area of a circle with a 0.001-inch (1 mil) diameter.
Ampacity
The maximum continuous current a wire can safely carry without exceeding its temperature rating. Ampacity is a separate concern from voltage drop - a wire must satisfy both limits.
Conductor
Any material that carries electric current - in practice, the copper or aluminum wire inside insulation. The word is used in codes and specifications in place of "wire."
K-Factor (Specific Resistivity)
A constant that describes how resistive a conductor material is. Copper K = 12.9, aluminum K = 21.2 (ohms per circular mil-foot). Higher K means more resistance and more voltage drop per foot.
Branch Circuit
The wiring between the last overcurrent device (breaker or fuse) and the outlets, fixtures, or equipment it serves. The NEC recommends a 3% maximum voltage drop on branch circuits.
Feeder
The wiring between the service entrance (main panel) and a sub-panel or distribution point. The NEC recommends a 5% maximum voltage drop from service to end-use outlet, combining feeder and branch circuit drop.

The Complete Guide to Wire Sizing and Voltage Drop

Choosing the right wire gauge is one of the most critical steps in any electrical project. Too small, and you lose voltage before it reaches the load - motors overheat, lights dim, and equipment fails prematurely. Too large, and you waste money on unnecessary copper or aluminum. This guide explains the math behind wire sizing so you can use this calculator with confidence.

How to Use This Calculator

Start by selecting your system voltage from the dropdown - use the DC options for solar, automotive, and low-voltage systems, and the AC options for residential and commercial circuits. Enter the total amperage the circuit will carry under full load. For the one-way distance, measure from the power source to the furthest load (the calculator handles the two-way math internally using the standard formula). Choose your conductor material and circuit type, then set your acceptable voltage drop limit - 3% for branch circuits and 5% for feeders are the NEC-recommended defaults.

Results update in real time as you change any field. The recommended wire size is the smallest standard AWG or kcmil size whose actual voltage drop falls within your chosen limit. The three metric cards below the hero show the exact voltage drop in volts, the drop as a percentage, and the voltage your equipment actually receives.

The Math Behind Wire Sizing

Wire sizing for voltage drop starts by calculating the required cross-sectional area in Circular Mils (CM). For single-phase and DC circuits, the formula is: CM = (2 x K x Amps x Distance in feet) / Allowable Voltage Drop in Volts. For three-phase circuits, the 2 is replaced by 1.732 (the square root of 3), because three-phase power distribution is inherently more efficient. K is the resistivity constant for the material - 12.9 for copper and 21.2 for aluminum. Once the minimum required CM is calculated, the calculator scans the AWG table to find the next standard size up, then computes the actual drop for that real wire size.

Copper vs. Aluminum: When to Use Each

Copper remains the default choice for most residential branch circuits. It is easier to terminate (aluminum requires antioxidant compound and rated connectors), does not oxidize as aggressively, and packs more conductivity into a smaller diameter. For runs of 100 feet or less at moderate amperage, the size difference between copper and aluminum is often only one AWG step.

Aluminum becomes cost-effective for large feeder runs and service entrance conductors. A 200-amp service entrance conductor in aluminum might be 2/0 AWG vs. 1/0 AWG in copper - the aluminum cable weighs less, is easier to pull in conduit, and costs significantly less per foot. Most utility companies and electrical contractors use aluminum for service conductors routinely. The key rule: always use lugs and connectors rated for aluminum when terminating aluminum wire.

Single-Phase vs. Three-Phase

Single-phase power is the standard in North American residences - two hot legs and a neutral, delivering 120V or 240V. Three-phase power is used in commercial and industrial settings because it delivers power more efficiently: for the same conductor size and current, three-phase carries 1.732 times more power than single-phase. This efficiency factor appears directly in the voltage drop formula - three-phase drops less voltage than single-phase for the same wire, amps, and distance.

FAQ

Every wire has electrical resistance. The longer the wire, the more resistance it presents to current flow. That resistance causes a portion of the voltage to be lost as heat before it reaches the end device - the same way friction slows water moving through a long hose. The amount of drop depends on three things: the wire length, the current (amps) flowing through it, and the wire's cross-sectional area (gauge). Thicker wire has less resistance, so less voltage is lost. Shorter runs and lower current also reduce drop.
Copper is the more conductive material - its resistivity constant (K) is 12.9 ohms-cmil/ft compared to aluminum's 21.2 ohms-cmil/ft. This means copper carries the same current with less voltage drop for a given wire size. Aluminum wire must be one to two AWG sizes larger than copper to deliver equivalent performance. Aluminum is lighter and less expensive, so it is commonly used for large feeder runs and service entrance cables. Copper is preferred for branch circuits and most residential wiring because of its smaller size, ease of termination, and lower risk of oxidation issues at connections.
Undersized wire causes two problems simultaneously. First, the voltage at the equipment drops below what it needs to operate correctly. Motors run hotter and burn out faster, LED drivers flicker or fail prematurely, and sensitive electronics may malfunction or produce incorrect output. Second, the wire itself heats up because all that lost voltage is converted to heat in the conductor. This degrades insulation over time and is a fire hazard. The NEC establishes ampacity limits to prevent conductor overheating - wire sizing for voltage drop is a separate, additional consideration on top of those limits.
The NEC recommends (but does not mandate) that branch circuits be designed for no more than 3% voltage drop from the panel to the outlet, and that the total drop from the service entrance to the furthest outlet not exceed 5%. These limits exist to ensure equipment operates within the tolerance bands that manufacturers design for. A motor rated for 120V is typically tested down to 108V (10% drop) but performs best between 114V and 126V. The 3%/5% guideline provides a comfortable engineering margin that accounts for simultaneous load on multiple circuits and keeps equipment running efficiently without premature wear.
This calculator determines wire size based on voltage drop math alone. It does not check ampacity limits, conduit fill, ambient temperature derating, the number of current-carrying conductors, or local code amendments - all of which affect the final safe wire selection for a real installation. For any permanent wiring, panel work, service entrance sizing, commercial applications, or loads above what standard 4/0 AWG covers, consult a licensed electrician. Voltage drop sizing is just one of several factors a professional evaluates together.
Engineering Estimate Only. This calculator computes wire size based solely on the voltage drop formula using standard K-factor constants. It does not account for ampacity limits per NEC Table 310.16, ambient temperature derating, conduit fill reduction factors, the number of current-carrying conductors, or local code requirements. Results are for educational and preliminary planning purposes only. Always verify wire sizing with a qualified licensed electrician before installation.