Voltage Drop Calculator: Determine Wire Size & Loss for NEC-Compliant Circuits

What is Voltage Drop?

Voltage drop occurs when electrical voltage decreases as current flows through a circuit due to resistance, impedance, or both. This phenomenon is an inherent property of any conductor but must be kept within safe limits to ensure proper functioning of electrical systems.

By calculating voltage drop, you can:

• Maintain proper operation of devices and systems.
• Prevent overheating of wires and components.
• Avoid underpowered or inefficient equipment.

The National Electrical Code (NEC) recommends limiting voltage drop to 3% for branch circuits and feeders, ensuring efficiency and safety.

Why is Voltage Drop Important?

Excessive voltage drop can result in:

• Poor equipment performance: Devices may not operate efficiently or correctly.
• Overheating: Wires can overheat, potentially causing damage or fire.
• Energy waste: Voltage drop represents lost energy, increasing your electricity bill.
• Reduced lifespan of equipment: Inconsistent voltage can shorten the lifespan of electrical devices.

Keeping voltage drop within acceptable limits ensures safe and cost-effective operation of electrical systems.

Voltage Drop Formulas for Single-Phase and Three-Phase Circuits

The calculator uses the following formulas to determine voltage drop.

For single-phase circuits, the voltage drop (V) is calculated using:

For three-phase circuits, the formula becomes:

Calculating Circular Mils (CM) for Wire Sizing:

To determine the required wire size (in circular mils), we use:

The Permitted Voltage Drop is typically 3% of the system voltage.

Power Loss Calculation:

Efficiency Calculation:

Step-by-Step Guide: Calculate Voltage Drop and Wire Size (AWG)

Our calculator simplifies the process of determining voltage drop and choosing the correct wire size. Here's how to use it:

• Wire Length: Enter the total length of the wire run in feet.
• Current: Input the current in amperes (amps) that the circuit will carry.
• Wire Gauge: Select the American Wire Gauge (AWG) size of the wire.
• Conductor Material: Choose between copper and aluminum.
• Circuit Type: Indicate whether it's a single-phase or three-phase circuit.
• System Voltage: Enter the system's voltage (e.g., 120V, 240V).

Click "Calculate," and the tool will provide the following results:

• Voltage Drop: The amount of voltage lost in the circuit (in volts).
• Voltage at Load: The actual voltage available at the end of the wire run.
• Drop Percentage: The percentage of voltage lost compared to the system voltage.
• Wire Area: The cross-sectional area of the wire in circular mils.
• Is Acceptable: Indicates whether the voltage drop is within the generally recommended 3% limit.
• Recommendations: Suggestions for improving the circuit if the voltage drop is excessive.
• Power Loss: The amount of power lost due to voltage drop, measured in watts.
• Efficiency: The overall efficiency of the circuit, expressed as a percentage.

Example Calculations

Scenario: You need to run a 145-foot copper wire to power a shed that will draw 60 amps on a 240V single-phase circuit. You're considering using a 4 AWG wire.

1. Voltage Drop with 4 AWG Wire:

Phase Factor = 2

Conductor Constant (K) = 12.9 (copper)

Current (I) = 60 amps

Length (L) = 145 feet

Wire Area (CM) = 41740 (from a wire gauge table for 4 AWG)

2. Is this Voltage Drop Acceptable?

Permitted Voltage Drop = 240V * 0.03 = 7.2 volts

5.37 volts (actual drop) < 7.2 volts (permitted) = Yes, it's acceptable.

3. What if we needed to use a 120V circuit in this same scenario?

Permitted voltage drop would be: 120 * 0.03 = 3.6

Using the same formula above, we get 5.37 volts of drop, which is greater than 3.6. Thus, it is not acceptable.

Using the formula for circular mils, we get:

Looking at the wire size table, we see that we need a bigger wire size of 2 AWG, which has a CM of 66,360.

Is Voltage Drop 3% or 5%?

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and feeders for optimal efficiency. However, a total voltage drop of up to 5% (including both feeder and branch circuits) is sometimes considered acceptable, though it may still affect equipment performance.

It is essential to minimize the voltage drop as much as possible. Excessive drop can damage equipment. For low-voltage circuits (e.g., sensors), design splits safely with our Voltage Divider Calculator.

How do you calculate drop out voltage?

"Drop out voltage" usually refers to the minimum input voltage at which a voltage regulator can maintain its specified output voltage. It's not directly related to the voltage drop in a circuit caused by wire resistance.

To find a voltage regulator's dropout voltage, you should consult its datasheet provided by the manufacturer.

Is 10% Voltage Drop OK?

No, a 10% voltage drop is generally not okay. It significantly exceeds the recommended 3% limit and can lead to serious performance issues, overheating, and potential damage to equipment.

Is 6% Voltage Drop Bad?

A 6% voltage drop is also higher than the recommended 3% limit. While it might not be as severe as a 10% drop, it's still likely to cause noticeable problems with equipment performance and efficiency. It's best to address the cause of the voltage drop and aim for a lower percentage.

Additional Tips to Minimize Voltage Drop

• Use the correct wire size: This is the most crucial factor. Our calculator helps you determine the appropriate size.
• Shorten wire runs: If possible, reduce the distance between the power source and the load.
• Use higher voltage: Higher voltage systems experience less voltage drop for the same power level.
• Consider copper over aluminum: Copper has lower resistance than aluminum, resulting in a lower voltage drop for the same wire size.

By using our Voltage Drop Calculator and understanding the principles behind it, you can ensure your electrical projects are safe, efficient, and perform at their best. Don't let voltage drop compromise your equipment or inflate your energy bills.