Estimate voltage loss for AC/DC circuits. Optimize wire size for Copper & Aluminum conductors according to NEC recommendations.
Select whether your system is AC or DC, and choose Single Phase or Three Phase. Most residential home circuits are Single Phase AC, while large industrial motors often use Three Phase AC.
Choose your conductor material (Copper or Aluminum). Copper is the standard for household wiring, while aluminum is often used for service entrance cables. Select the wire gauge (AWG or kcmil) you intend to use.
Enter the one-way length of the wire run (the distance from the breaker panel to the device). Enter the total load current in Amps, and the source voltage (e.g., 120V for outlets, 240V for dryers).
Adjust the Power Factor (default is 1) or Temperature if you are calculating for specific environmental conditions.
Click "Calculate Voltage Drop". If the result is Orange (over 3%) or Red (over 5%), consider increasing your wire size (lowering the AWG number).
Voltage drop is one of the most critical yet often overlooked factors in electrical system design. Whether you are wiring a detached garage, installing solar panels, or running power to a large industrial motor, understanding voltage drop is essential for safety, efficiency, and code compliance. This comprehensive guide will explain the physics behind voltage drop, how to calculate it manually, and the specific National Electrical Code (NEC) standards you must follow.
In simplest terms, voltage drop is the reduction in electrical potential (Voltage) as electric current travels through the passive elements (wires) of an electrical circuit. Every wire, regardless of its material or thickness, has some amount of internal resistance. This resistance opposes the flow of current.
You can visualize this using the "Water Pipe Analogy." Imagine a garden hose connected to a spigot. The water pressure at the spigot is high (Voltage Source). As water travels through a very long or very narrow hose, friction against the hose walls reduces the pressure. By the time the water reaches the nozzle, the pressure (Voltage at Load) is lower than it was at the source. In electricity, that "lost pressure" is lost energy, usually converted into heat along the wire.
While a small amount of voltage drop is unavoidable, excessive drop causes significant problems:
The National Electrical Code (NEC) provides specific recommendations for voltage drop to ensure system efficiency. While these are often found in "Informational Notes" (meaning they are recommendations rather than strict safety code rules), they are treated as mandatory by professional engineers and inspectors to ensure proper system performance.
For branch circuits (the wiring from your final breaker panel to the outlet or light fixture), the NEC recommends that voltage drop should not exceed 3%. This ensures that the device plugged into the outlet receives at least 97% of the rated voltage.
For feeder circuits (the heavy cables running from the main service entrance to a sub-panel), the recommended limit is also 3%.
The most important rule is the combined total. The voltage drop of the Feeder + Branch Circuit combined should not exceed 5%.
| System Component | Max Voltage Drop | Example (120V System) | Example (240V System) |
|---|---|---|---|
| Branch Circuit | 3% | Max drop: 3.6V | Max drop: 7.2V |
| Feeder | 3% | Max drop: 3.6V | Max drop: 7.2V |
| Total Combined | 5% | Max drop: 6.0V | Max drop: 12.0V |
Four main variables determine how much voltage is lost in a circuit. Changing any one of these can solve a voltage drop problem.
Different metals conduct electricity differently. Copper is a superior conductor with lower resistance per foot compared to Aluminum.
Rule of Thumb: To get the same conductivity as copper, you typically need to use an aluminum wire that is two sizes larger (e.g., replace #8 Copper with #6 Aluminum). Aluminum is cheaper and lighter, making it popular for long service feeders, but it requires careful sizing.
The thinner the wire, the higher the resistance. In the American Wire Gauge (AWG) system, lower numbers mean thicker wires. A 12 AWG wire is thicker than a 14 AWG wire. Using a larger wire (upsizing) provides a wider "highway" for electrons to flow, reducing resistance and voltage drop.
According to Ohm's Law ($V = I \times R$), voltage drop is directly proportional to the current (Amps). If you double the current flowing through a wire, the voltage drop doubles. This is why high-power devices need thicker wires than low-power LED lights.
Resistance increases linearly with distance. A 100-foot wire has exactly twice the resistance of a 50-foot wire of the same gauge. Voltage drop calculations must always consider the one-way distance, but the formula often doubles this (multiply by 2) because the current has to travel to the load and back to the source.
While online calculators are convenient, understanding the manual formulas helps you troubleshoot.
This is the most common formula used for home wiring and 12V automotive projects.
Used for industrial motors and commercial buildings.
Note: In three-phase systems, we use the square root of 3 (approx 1.732) instead of 2, because the return current is shared among the phases out of sync.
Temperature: Resistance increases as wires get hotter. If you are running conduit across a hot rooftop exposed to direct sunlight, the resistance of the copper increases. Our calculator allows you to adjust the temperature (standard is 25°C / 77°F).
Power Factor (PF): In AC circuits with inductive loads (like motors), the current and voltage are not perfectly in sync. A Power Factor of 1.0 is ideal (purely resistive). Motors often have a PF of 0.8 or 0.9. Lower power factors actually increase the effective current draw, potentially increasing voltage drop.
If your calculation shows a drop higher than 3% or 5%, here are your options:
Yes, slightly. In AC systems, steel conduit (ferrous metal) can increase voltage drop due to "inductive reactance." PVC or Aluminum conduit does not have this magnetic effect. For small branch circuits, this effect is negligible, but for large feeders, it matters.
LEDs are very sensitive to voltage fluctuations. If a heavy load (like a fridge or pump) turns on and causes a momentary voltage drop on the same circuit, the LEDs may dim or flicker. Dedicated circuits for lighting help solve this.
Sometimes utility companies provide "taps" on transformers to slightly boost voltage, but in a residential setting, you cannot safely increase the source voltage beyond the standard 120V/240V provided. The only safe variable you can control is the wire resistance (size).
Disclaimer: The "K" method formula shown above is an approximation. This calculator uses the more precise NEC Chapter 9, Table 8 resistance and reactance values for higher accuracy in AC calculations. Always verify designs with a licensed electrician.
This calculator is provided for educational and estimation purposes only. Results depend on accurate input data and standard resistance values (NEC Chapter 9, Table 8). Real-world conditions such as conduit type, insulation quality, and harmonics can affect actual results. Always consult a licensed electrician or engineer for critical electrical system designs.