Voltage Drop Calculator

Every wire has some resistance, and that resistance causes a small but real voltage loss over distance — the longer the run and the higher the current, the more voltage is lost before it reaches whatever it’s powering. This calculator estimates that voltage drop from wire length, current, conductor material, and wire size, then checks the result against the commonly cited 3% guideline used in electrical design.

How the Voltage Drop Calculator Works

Enter your system voltage, the load current in amps, the one-way length of the wire run, the conductor material (copper or aluminum), whether the circuit is single-phase or three-phase, and the wire’s size in circular mils. The calculator applies the standard voltage drop formula — incorporating a material-specific resistivity constant, the phase factor, wire length, current, and circular mils — to calculate the voltage lost over that run, then expresses it both as a raw voltage figure and as a percentage of your system voltage.

An Important Input Detail: This Calculator Needs Circular Mils, Not AWG Size

Wire is commonly sold and specified by AWG (American Wire Gauge) number — 12 AWG, 10 AWG, and so on — but this calculator’s formula works directly with circular mils, a different unit describing a wire’s actual cross-sectional area. This calculator doesn’t include an automatic AWG-to-circular-mils lookup, so you’ll need to convert your specific wire gauge to its circular mil equivalent before entering it. A few common reference points: 14 AWG is approximately 4,107 circular mils, 12 AWG is approximately 6,530 circular mils (the calculator’s default value), 10 AWG is approximately 10,380 circular mils, and 8 AWG is approximately 16,510 circular mils. For wire sizes beyond these common examples, a standard AWG-to-circular-mils conversion table (widely available from electrical reference sources) will have the exact figure for your specific wire size.

Why Longer Wire Runs and Higher Current Both Increase Voltage Drop

Voltage drop increases directly with both wire length and current, which makes physical sense once you consider that voltage drop is really just Ohm’s Law applied to the wire’s own inherent resistance: longer wire has more total resistance to overcome, and higher current means more electrical “flow” pushing against that same resistance, both of which increase the voltage lost along the way. This is exactly why voltage drop becomes a real practical concern specifically for long wire runs carrying substantial current — a short run with light current loses a negligible amount, while a long run feeding a high-current load can lose enough voltage to actually affect how the connected equipment performs.

The 3% Guideline: Why It Exists and What Happens If You Exceed It

The National Electrical Code (NEC) recommends keeping voltage drop within about 3% for branch circuits (and up to 5% combined for a full feeder-and-branch-circuit run) as a practical guideline for reliable equipment operation, though it’s worth noting this is a recommendation for good design practice rather than always a strict, universally enforced code requirement in every jurisdiction and situation. Excessive voltage drop can cause motors to run hotter and less efficiently, lights to dim or flicker, and electronic equipment to behave unpredictably or fail to start properly — all consequences of connected equipment receiving meaningfully less voltage than it was designed to operate on. This calculator checks your specific run against that 3% threshold directly and flags whether your result falls within or outside the generally recommended range.

Copper vs. Aluminum: Why Conductor Material Changes the Result Significantly

Copper has meaningfully lower electrical resistance than aluminum for the same wire size, which is why this calculator uses a different resistivity constant for each material — aluminum wiring experiences noticeably more voltage drop than an identically sized copper wire carrying the same current over the same distance. This is one of several reasons aluminum wiring, despite being lighter and less expensive than copper, generally requires a larger wire gauge than copper would for an equivalent application, in part specifically to keep voltage drop within acceptable limits.

Single-Phase vs. Three-Phase: Why the Phase Factor Changes the Math

This calculator’s phase setting applies a different multiplier depending on whether the circuit is single-phase (using a factor of 2, accounting for the round trip through both the supply and return conductor) or three-phase (using a factor of √3, reflecting the different mathematical relationship between line and phase values in a three-phase system). Selecting the wrong phase setting for your actual circuit type will produce a meaningfully incorrect voltage drop estimate, since the two formulas diverge by a real, non-trivial factor rather than a small rounding difference.

The Underlying Ohm’s Law Relationship

Voltage drop calculations are fundamentally an application of Ohm’s Law — voltage equals current multiplied by resistance — applied specifically to a wire’s own resistance over a given length, rather than to a circuit component like a resistor or a load device. If you want to explore that foundational relationship more generally, outside the specific wire-length context this calculator addresses, the Ohms Law Calculator solves for voltage, current, resistance, or power from any two known values in a basic circuit.

Estimating the Cost of Running a Circuit Over Time

Once you know a circuit’s current draw and voltage, that combination determines its power consumption in watts, which is the starting point for estimating what running that circuit actually costs over time. The Electricity Calculator takes power draw and usage hours to project electricity cost per day, month, or year — a natural next step once you’ve confirmed a circuit’s voltage drop is within acceptable limits and want to understand its ongoing operating cost.

Resistor Values in Smaller Electronic Circuits

While this calculator addresses voltage drop over a wire run at typical building-electrical scales, a related but distinct calculation comes up in smaller electronic circuits using discrete resistor components rather than long wire runs. The Resistor Calculator decodes standard color-band resistor values directly, which is useful for a different scale of circuit design than the wire-run voltage drop this calculator addresses.

Frequently Asked Questions

How do I convert my wire’s AWG size to circular mils for this calculator?

This calculator requires circular mils directly rather than AWG size. Common reference points: 14 AWG ≈ 4,107 circular mils, 12 AWG ≈ 6,530 circular mils, 10 AWG ≈ 10,380 circular mils, and 8 AWG ≈ 16,510 circular mils. For other wire sizes, a standard AWG-to-circular-mils conversion table has the exact figure.

What voltage drop percentage is considered acceptable?

The National Electrical Code commonly recommends keeping voltage drop within about 3% for branch circuits, and up to 5% combined for a full feeder-and-branch-circuit run, as a practical guideline for reliable equipment operation — though this is a design recommendation rather than always a strictly enforced requirement in every situation.

Why does aluminum wire have more voltage drop than copper?

Aluminum has meaningfully higher electrical resistance than copper for the same wire size, which directly increases voltage drop over an equivalent length and current. This is one reason aluminum wiring typically requires a larger gauge than copper for an equivalent application.

What happens if voltage drop on a circuit is too high?

Excessive voltage drop can cause motors to run hotter and less efficiently, lights to dim or flicker, and electronic equipment to behave unpredictably or fail to start properly, since the connected equipment is receiving meaningfully less voltage than it was designed to operate on.

Why does the phase setting (single vs. three-phase) change the result?

Single-phase and three-phase circuits use different mathematical factors in the voltage drop formula — single-phase uses a factor of 2 for the round trip through supply and return conductors, while three-phase uses a factor of √3 reflecting the different relationship between line and phase values. Selecting the wrong phase type for your actual circuit produces a meaningfully incorrect result.