A voltage drop calculator helps electricians and DIYers verify that a wire run loses an acceptable amount of voltage before reaching the load. The National Electrical Code recommends keeping voltage drop below 3% on branch circuits and below 5% total. Excessive drop wastes energy, dims lights, causes motors to overheat, and can void equipment warranties.
Wire Run Parameters
Distance from panel to load — not total wire length
Continuous load — check breaker or device nameplate
Calculation Breakdown
Gauge Comparison (same run)
| Gauge | Resistance (Ω/1000ft) | Voltage Drop (V) | Drop % | NEC Status |
|---|
How to Use the Voltage Drop Calculator
Voltage drop is the reduction in electrical potential along a wire caused by the wire's own resistance. Every conductor has some resistance, and when current flows through it, a small portion of the voltage is lost as heat. For short wire runs, the drop is negligible — but for long runs to outbuildings, workshops, outdoor lighting, or large appliances, voltage drop can be enough to cause equipment problems and code violations. This voltage drop calculator lets you check any wire run in seconds.
Step 1: Select Your Wire Gauge
Choose the AWG (American Wire Gauge) you are planning to use or already have installed. Smaller AWG numbers mean larger, lower-resistance wire. The dropdown shows common gauges from 14 AWG (standard 15A household circuits) through 4/0 AWG (heavy service entrance conductors). If you are not yet sure which gauge to use, start with your circuit breaker size as a guide — a 20A breaker typically uses 12 AWG minimum.
Step 2: Choose Copper or Aluminum
Copper is the most common conductor for household wiring and has lower resistance per foot than aluminum. Aluminum wiring is sometimes used for service entrances, sub-panels, and large feeder runs where the weight savings matter. The calculator automatically applies the correct resistance values for each material. As a rule of thumb, aluminum carries about 1.6× more resistance than copper at the same gauge, so aluminum runs will show a higher voltage drop percentage.
Step 3: Enter One-Way Wire Length
Enter the distance from your panel (or sub-panel) to the load — this is the one-way length. The voltage drop formula automatically doubles this to account for both the hot and neutral conductors in the circuit. Do not measure your total wire length; measure only the straight-line distance (or actual routing distance) from source to load.
Step 4: Enter Load Current and System Voltage
Enter the continuous load in amps. For motors and compressors, use the nameplate FLA (Full Load Amps). For lighting or mixed loads, add up the wattages and divide by the voltage (P = V × I). Select your system voltage — most residential circuits in the U.S. are 120V for outlets and 240V for appliances like dryers, ranges, and EV chargers.
Step 5: Review the NEC Compliance Result
The calculator shows your voltage drop in volts and as a percentage, the actual voltage arriving at your load, and whether your run passes the NEC 3% branch circuit recommendation or 5% total limit. The gauge comparison table shows what drop you would get with larger gauges — making it easy to pick the smallest wire that still meets code. When in doubt, size up: larger wire costs more upfront but runs cooler and lasts longer.
Understanding the Voltage Drop Formula
The formula used is Vdrop = (2 × L × I × R) / 1000, where L is one-way length in feet, I is current in amps, and R is the conductor resistance in ohms per 1,000 feet. The result is in volts. Dividing by your source voltage and multiplying by 100 gives the drop percentage. For aluminum wire, the resistance values are scaled by a factor of approximately 1.64 relative to copper.
Frequently Asked Questions
Is this voltage drop calculator free?
Yes, completely free with no signup required. All calculations run in your browser using JavaScript — no data is sent to any server. Use it as many times as you need for any wire run.
Is my data safe when using this tool?
Absolutely. Everything runs locally in your browser. No wire length, gauge, or load data is stored or transmitted anywhere. You can even use the calculator offline once the page has loaded.
What is the NEC voltage drop recommendation?
The National Electrical Code (NEC) recommends keeping voltage drop to no more than 3% on any individual branch circuit, and no more than 5% combined across the feeder and branch circuit. Exceeding these limits can cause equipment to underperform, overheat motors, and shorten the life of connected devices.
What formula does the voltage drop calculator use?
The calculator uses the standard formula: Vdrop = (2 × L × I × R) / 1000, where L is the one-way wire length in feet, I is the current in amps, and R is the wire resistance in ohms per 1,000 feet. The factor of 2 accounts for both the hot and neutral (return) conductors.
What is the difference between copper and aluminum wire resistance?
Aluminum wire has higher electrical resistance than copper — approximately 1.6 times more resistance per foot. This means aluminum wire of the same gauge will produce more voltage drop. When using aluminum, you typically need to upsize by one or two AWG gauges to match the voltage drop of a copper wire run.
How do I reduce voltage drop on a long wire run?
The most effective ways to reduce voltage drop are: use a larger wire gauge (lower AWG number), shorten the wire run where possible, increase the system voltage (e.g., use 240V instead of 120V — the same load at 240V draws half the current and drops only one-quarter the voltage), or split the load across multiple circuits.
Why does a higher voltage (240V) produce less voltage drop percentage?
Voltage drop percentage is Vdrop divided by supply voltage. At 240V, a device drawing 10A at 120V only draws 5A at 240V, which cuts the current in half. Since voltage drop is proportional to current (V = I × R), the drop in volts is also halved, and when divided by the higher supply voltage the percentage is much lower.
What wire gauge should I use for a long outdoor circuit?
For runs longer than 100 feet, you almost always need to upsize at least one gauge beyond the minimum required for the ampacity. This calculator shows you whether your chosen gauge passes the NEC 3% threshold and recommends the smallest gauge that does comply, so you can plan your wiring correctly before purchasing materials.