Voltage Drop Calculator - NEC Compliant Wire Size & Drop

Voltage Drop Calculator

Calculate wire size suitability, voltage loss, and maximum run length.

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How to Use This Calculator

  1. Select Phase & Material: Choose whether your circuit is Single Phase (typical home wiring) or 3-Phase (commercial), and whether you are using Copper or Aluminum wire.
  2. Select Wire Size: Choose the AWG (American Wire Gauge) size you intend to use.
  3. Input Specs: Enter the source voltage (e.g., 120V), the load current (Amps), and the one-way distance to the load.
  4. Calculate: Click "Calculate Drop" to see if your wire is sufficient or if you need to upsize to prevent power loss.

Example Calculation

Let's manually calculate the voltage drop for a typical home workshop circuit to see how the math works in practice.

Scenario:

  • Circuit: 120V Single Phase
  • Load: 20 Amps
  • Distance: 50 feet
  • Wire: 12 AWG Copper

Step 1: Find Resistance

From standard NEC tables, 12 AWG Copper has a resistance of roughly 1.93 Ohms per 1,000 feet.

Step 2: Apply Formula

VD = (2 × Length × Resistance × Amps) / 1000
VD = (2 × 50 × 1.93 × 20) / 1000

Step 3: The Result

Voltage Drop: 3.86 Volts
Percentage: (3.86 ÷ 120) × 100 = 3.22%

Result: 3.22% > 3%. This is slightly high. You might consider upsizing to 10 AWG to get below 3%.

Understanding Electrical Voltage Drop

Voltage drop describes the reduction in electrical potential along the path of a current flowing in an electrical circuit. It is an unavoidable physical phenomenon caused by the internal resistance of the conductors. Just as water pressure decreases as water flows through a long garden hose due to friction, electrical "pressure" (voltage) decreases as current travels through a wire.

This drop is governed fundamentally by Ohm’s Law (\(V = I \times R\)), where \(V\) is voltage drop, \(I\) is the current load, and \(R\) is the resistance of the wire. The longer the wire run, the higher the total resistance, and consequently, the greater the voltage loss. While copper is an excellent conductor, it is not perfect; its resistance creates heat energy as electrons force their way through, stealing voltage from the final appliance.

Why does this matter? If voltage drops too low (typically below 3% of the source voltage), sensitive electronics may reset, lights will dim or flicker, and motors (like those in AC units or saws) will draw excess current to compensate, leading to overheating and burnout. The National Electrical Code (NEC) recommends a maximum drop of 3% for branch circuits to ensure safety and efficiency. To correct high voltage drop, electricians "upsize" the wire—using a thicker gauge (lower AWG number) to lower electrical resistance.

Resources

Wire Data Reference

Standard NEC data for uncoated wire at 75°C. "Ohms" is resistance per 1,000 feet.

Swipe table to see details
Size (AWG) Typical Max Amps Copper (Ω/kft) Aluminum (Ω/kft) Common Application
14 AWG 15 A 3.07 Ω - Lighting & General Outlets
12 AWG 20 A 1.93 Ω 3.18 Ω Kitchen/Bath Outlets
10 AWG 30 A 1.21 Ω 2.00 Ω Dryers, Water Heaters
8 AWG 40-50 A 0.764 Ω 1.26 Ω Electric Ranges, Ovens
6 AWG 55-65 A 0.491 Ω 0.808 Ω Hot Tubs, EV Chargers (L2)
4 AWG 85 A 0.308 Ω 0.508 Ω Subpanels (small)
2 AWG 115 A 0.194 Ω 0.319 Ω Service Entrance (100A)
1/0 AWG 150 A 0.122 Ω 0.201 Ω Service Entrance (150A)
2/0 AWG 175 A 0.0967 Ω 0.159 Ω Service Entrance (200A)
4/0 AWG 230 A 0.0608 Ω 0.100 Ω Large Main Feeders

Frequently Asked Questions

What is a voltage drop calculator and why should I use it?
A voltage drop calculator is an online, user-friendly tool that helps you determine how much voltage is lost along an electrical circuit as current flows through a conductor. By measuring this voltage loss for a given wire gauge, length and load, you can design safer, more efficient systems, avoid overheating, and keep your installation closer to professional and standard-compliant practice.

What inputs do I need to use this voltage drop calculator?
You simply enter a few key input values: the type of circuit (DC / single phase), the wire material (for example copper), the wire gauge (AWG), the source voltage, the load amperage or power, and the one-way distance from the source to the load. The calculator then automatically applies the correct formula and gives you an instant result, showing whether your chosen wire size is suitable.

What does “one-way distance” mean in the calculator?
“One-way distance” is the length of conductor from the panel or supply to the load device. You don’t need to double it manually because the voltage drop calculator already applies the proper electrical formula that accounts for the full run of the circuit. This makes planning simple and helps you estimate real-world performance more accurately.

What is considered an acceptable voltage drop?
Many professional guidelines suggest keeping voltage drop under about 3% for most branch circuits, but local rules may differ. After you enter your data, the calculator shows both the voltage loss and the percentage so you can compare and adjust your design. If the percentage is too high, you can reduce the drop by selecting a larger wire gauge, shortening the distance, or lowering the load.

How do I know if my chosen wire size (AWG) is sufficient?
Once you select the wire gauge and run the calculation, the tool outputs the final voltage at the load along with the percentage drop. If the result is within your target limit, your wire is normally considered adequate for that circuit and distance. If not, you can interactively compare different gauges and quickly identify a more efficient, safe option.

What is the difference between Copper and Aluminum in this calculator?
Each wire material has a different electrical resistance, which directly affects voltage drop. Copper typically has lower resistance than aluminum, so for the same amperage and length it usually performs more efficiently. When you select the material in the calculator, it automatically applies the correct resistance values so your estimate stays accurate and reliable.

How does the calculator handle Amps vs. Watts for the electrical load?
If you know the load in Amps, you can enter the amperage directly. If you only know Watts, you enter that instead and the calculator converts it to current using the source voltage. This automatic conversion makes the tool more dynamic and customizable, so you can work with whichever numbers you have available and still get a precise result.

What’s the difference between Single Phase and Three Phase formulas?
Single-phase and three-phase systems use slightly different formulas because the way current flows through the circuit is not the same. For a typical single-phase run, the equation uses a factor of 2, while a three-phase system uses √3 (about 1.732). The calculator applies the correct formula in the background, so you only need to select the appropriate system type and let it handle the math.

Can this tool calculate the maximum distance I can run a circuit?
Yes. In Max Distance mode, you enter your circuit details—wire gauge, material, source voltage, load amperage, and acceptable voltage drop—and the calculator estimates the longest one-way distance you can safely run. This is especially useful when you want to design an efficient layout before you physically install any wire.

Does this calculator replace electrical codes or a licensed electrician?
No. While the voltage drop calculator is a powerful, interactive aid for estimating resistance effects and improving efficiency, it does not replace national or local codes, nor the expertise of a licensed electrician or engineer. Always verify your design against applicable standards and consult a professional when you’re unsure about the safety or compliance of an electrical installation.

Electrical Safety Disclaimer: This calculator provides estimates based on standard NEC formulas and typical wire properties. It is intended for planning and educational purposes only. Actual voltage drop can vary based on installation conditions, temperature, conduit type, and specific wire specifications. Always consult a licensed electrician and verify your design against local electrical codes before installation.

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