Voltage Drop Calculator

Calculate voltage drop in electrical wiring based on wire gauge, length, current, and material. Ensure your circuits meet NEC recommendations.

About this calculator

This calculator estimates how much voltage a circuit loses to wire resistance over a run, using standard copper/aluminum resistance-per-1000-feet tables and the round-trip (out-and-back) wire length. Wire Resistance comes purely from Wire Length, Wire Gauge, and Wire Material -- it has nothing to do with Source Voltage or Current -- and Voltage Drop is simply Current multiplied by that resistance, so raising Current or Wire Length always raises Voltage Drop, while Source Voltage itself never moves the absolute Voltage Drop figure at all. Source Voltage matters for two of the other outputs, though: Voltage Drop % expresses the same drop as a share of your source voltage (so a fixed drop looks smaller as a percentage on a 240V circuit than on a 120V circuit), and Voltage at Load is simply Source Voltage minus Voltage Drop.

The NEC (National Electrical Code) publishes voltage-drop guidance -- commonly cited as 3% for branch circuits and 5% combined for feeder plus branch circuits -- as a recommended design practice rather than a universal numeric code requirement in every jurisdiction, which is why the Assessment output is framed as guidance to check against your local code rather than a pass/fail compliance verdict. Aluminum wire is modeled here at roughly 1.6 times copper's resistance for the same gauge, reflecting aluminum's lower conductivity, so switching Wire Material to aluminum increases Voltage Drop and Power Loss for an otherwise identical run.

Inputs

Source Voltage

Current

Wire Length (one-way)

Wire Gauge (AWG)

Wire Material

Tab to move between fields

Results

Voltage Drop

4.76 V

Voltage Drop %3.97%

Voltage at Load115.24 V

Wire Resistance0.32 Ω

Power Loss71.46 W

AssessmentMarginal (3-5%)

Figures current as of 2023. Source: National Fire Protection Association, NFPA 70, National Electrical Code (NEC), Informational Notes to 210.19(A) and 215.2(A)(1) on voltage drop

How to Use This Calculator

Enter source voltage (V) and current (A).
Set wire length (one-way, ft) and select AWG gauge from the dropdown.
Review voltage drop (V), voltage drop (%), and voltage at the load (V).
NEC limits voltage drop to 3% for branch circuits and 5% combined for feeders + branch circuits.
If voltage drop exceeds limits, increase wire gauge (lower AWG number) or reduce run length.

How the result changes with Current

Current	Voltage Drop
7.5	2.38 V
11	3.49 V
23	7.3 V
38	12.07 V

What each input means

Source Voltage: Electrical potential difference in volts.
Current: Electrical current in amperes.
Wire Length (one-way): Total length of the wire.
Wire Gauge (AWG): Wire gauge (AWG).

How this is calculated

Formula

Voltage Drop = Current × Wire Resistance. Wire Resistance = (Resistivity × 2 × Length) / 1000. NEC recommends ≤3% drop for branch circuits.

Worked example, using the default values

Identify Input Parameters
5 parameters
Source Voltage = 120, Current = 15, Wire Length (one-way) = 100, Wire Gauge (AWG) = 12, Wire Material = 0 = 5 input(s) provided
Calculate Voltage Drop
Voltage Drop
4.76 = 4.76
Calculate Voltage Drop %
Voltage Drop %
3.97 = 3.97%
Calculate Voltage at Load
Voltage at Load
115.24 = 115.24

Figures and sources

NEC (NFPA 70) voltage-drop design guidance — 3% branch circuit / 5% combined feeder-plus-branch (2023) — National Fire Protection Association, NFPA 70, National Electrical Code (NEC), Informational Notes to 210.19(A) and 215.2(A)(1) on voltage drop

Engine last updated September 3, 2026. Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why doesn't raising Source Voltage change the Voltage Drop in volts?

Voltage Drop is Current times Wire Resistance, and neither of those depends on Source Voltage -- Wire Resistance comes from Wire Length, Wire Gauge, and Wire Material, and Current is whatever you enter directly. Source Voltage only enters the picture for Voltage Drop % (the same drop expressed as a percentage of a bigger or smaller source voltage) and Voltage at Load (Source Voltage minus the drop), so it changes how the loss LOOKS relative to your system, not the physical amount of voltage lost in the wire.

Does the NEC require the 3% and 5% voltage drop limits shown here?

The National Electrical Code (NFPA 70) publishes 3% for branch circuits and 5% combined for feeders plus branch circuits as recommended design guidance for efficient operation, not as a blanket numeric requirement enforced identically in every jurisdiction. Local amendments and specific equipment requirements can differ, so treat the Assessment output here as a useful design check rather than a guarantee of code compliance -- verify against your local electrical code.

Why does switching to aluminum wire increase my voltage drop?

This calculator models aluminum conductor as roughly 1.6 times the electrical resistance of copper at the same gauge, which reflects aluminum's lower conductivity compared to copper. Since Voltage Drop is Current times Wire Resistance, that higher resistance directly increases both Voltage Drop and Power Loss for an otherwise identical wire length, gauge, and current -- which is why aluminum runs are commonly upsized relative to an equivalent copper run.

Why does the calculator use round-trip wire length instead of the length I enter?

Wire Length (one-way) is the distance from the source to the load, but current has to travel out to the load AND back to complete the circuit, so the actual length of conductor carrying current is double what you enter. This calculator doubles your Wire Length internally before computing Wire Resistance, which is standard practice for voltage drop calculations on a two-conductor circuit.

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