Wire Size Calculator

What This Calculator Solves

A conductor carrying current over distance develops a voltage loss proportional to its resistance, the current it carries, and the round-trip length of the circuit. This tool converts those three variables into two numbers an electrician or designer needs before committing to a conductor: the voltage drop in volts and the voltage-drop percentage relative to system voltage. It does not select a wire gauge. It tells you whether a resistance value you already have — pulled from a manufacturer table, an engineering reference, or a prior calculation — will keep a specific run inside your voltage-drop target.

The output feeds directly into conductor selection, feeder and branch-circuit layout, and voltage-drop review on long runs to pumps, outbuildings, subpanels, or equipment far from the source. If the calculated percentage exceeds your allowable limit, the next step is to test a lower-resistance conductor (larger AWG or kcmil) in the same calculator, not to accept the result as final.

Inputs and What Each One Controls

• Load current (A) — the current the circuit actually carries, used as the constant multiplier in the voltage-drop formula.
• One-way length (ft) — the distance from source to load. The calculator doubles this internally to represent the two-conductor loop (out and back), since both the ungrounded and grounded/neutral conductors carry the current and both contribute resistance.
• System voltage (V) — the voltage base used to express the drop as a percentage. This should match the actual circuit voltage (120 V, 208 V, 240 V, 277 V, 480 V, etc.), not an assumed default.
• Allowable voltage drop (%) — the design limit you are checking against, commonly 3% for branch circuits or 5% for branch circuit plus feeder combined, per common engineering practice.
• Conductor resistance (ohm/1000 ft) — a user-supplied value. The calculator does not look up AWG or kcmil resistance from any table; you must enter a verified resistance figure and, ideally, log the source, conductor material, and temperature condition in the optional note field. Copper and aluminum resistance values differ substantially, and resistance itself varies with conductor temperature, so an unverified entry produces an unverified result.

Formula and Calculation Logic

The calculator applies the standard single-phase voltage-drop relationship:

Voltage Drop (V) = Current (A) × Resistance (ohm/1000 ft) × [2 × One-Way Length (ft) / 1000]

The percentage is then:

Voltage-Drop % = (Voltage Drop / System Voltage) × 100

Two supporting values close the loop between your target and your input:

• Allowable voltage drop (V) = System Voltage × Allowable % / 100
• Maximum resistance for entered drop limit (ohm/1000 ft) = Allowable Voltage Drop / [Current × (2 × One-Way Length / 1000)]

This last figure is the resistance ceiling your conductor must stay under (or below) to hit the stated percentage target at this current and length. It is the number you compare against a wire’s actual per-1000-ft resistance when picking a size.

Calculation Example

Using the input set shown on the calculator:

Field	Value
Load current	20 A
One-way length	100 ft (200 ft round trip)
System voltage	120 V
Allowable voltage drop	3%
Conductor resistance	1.588 ohm/1000 ft

Applying the formula:

Voltage Drop = 20 × 1.588 × (200/1000) = 6.352 V

Voltage-Drop % = 6.352 / 120 × 100 = 5.2933%

Allowable Voltage Drop = 120 × 0.03 = 3.6 V

Maximum Resistance for 3% Limit = 3.6 / (20 × 0.2) = 0.9 ohm/1000 ft

Voltage-Drop Margin = 3% − 5.2933% = −2.2933 percentage points

The negative margin means the entered conductor resistance (1.588 ohm/1000 ft) is above the 0.9 ohm/1000 ft ceiling required to stay at or under 3% drop. At 20 A over a 100 ft one-way run, this specific resistance value fails the stated target and calls for a lower-resistance conductor, a shorter run, or a relaxed voltage-drop allowance.

Reading the Voltage-Drop Margin

The margin is the fastest way to interpret the result without re-checking the arithmetic by hand. A positive margin means the entered resistance stays under the ceiling and the run meets the stated percentage target. A negative margin, as in the worked example, quantifies how far over the limit the current conductor choice runs — here, by roughly 2.29 percentage points, corresponding to a conductor resistance nearly 76% above the maximum allowed (1.588 vs. 0.9 ohm/1000 ft).

To correct a failing result, re-run the calculator with a lower documented resistance value (a heavier gauge or a shorter length) rather than adjusting the allowable percentage to force a pass. The allowable-drop percentage is a design decision, not a variable to tune around a fixed conductor.

Scope and Field Limitations

This calculator performs circuit voltage-drop arithmetic only. It does not:

• Look up AWG or kcmil resistance values — every ohm/1000 ft figure must come from a source you verify, at the correct conductor material (copper or aluminum) and temperature condition, since resistance rises with conductor temperature and differs significantly between materials.
• Determine ampacity, apply ambient-temperature or conductor-bundling adjustment factors, or check terminal and insulation temperature ratings.
• Select a code-compliant wire size under NEC Article 210, 215, or related sections, or verify raceway fill, overcurrent protection coordination, or any other requirement enforced by the authority having jurisdiction (AHJ).

Use the output as one input into conductor selection: pair the calculated maximum-resistance ceiling with a verified per-1000-ft resistance table for the conductor material and temperature you intend to install, confirm the resulting ampacity independently, and have the final circuit design reviewed against applicable code and the AHJ’s requirements before installation.