Voltage Drop Calculator — Free Online Electrical Tool

How to Use the Voltage Drop Calculator

1. Select your supply voltage: Choose the nominal voltage for your circuit from the dropdown menu. Common options include 120V for standard residential outlets, 240V for large appliances like dryers and ranges, 208V for commercial three-phase systems, and 480V for industrial equipment. The supply voltage determines both the actual voltage drop and the critical percentage calculation.
2. Enter the current draw: Input the expected maximum current in amperes. Use the circuit breaker rating or the nameplate current of the equipment being powered. For circuits feeding multiple loads, add up all the individual current draws. Overestimating current slightly provides a safety margin in your calculation.
3. Enter the one-way wire length: Measure or estimate the distance in feet from the electrical panel to the load. Enter only the one-way distance; the calculator automatically doubles this to account for both the hot and neutral (or return) conductors. Include any vertical runs and account for routing around obstacles.
4. Select wire gauge and material: Choose the AWG wire gauge you plan to use or are evaluating. Select copper or aluminum as the conductor material. The calculator retrieves the correct circular mil area and resistivity for accurate results.
5. Review and interpret results: The results panel shows the voltage drop in volts and as a percentage of supply voltage. A green indicator means the drop is within the NEC-recommended 3% maximum, while a red indicator warns of excessive drop. The receiving-end voltage tells you exactly what voltage will reach your load.

If the voltage drop exceeds 3%, try selecting a larger wire gauge (lower AWG number), reducing the circuit length, or switching from aluminum to copper conductor. You can also consider using a higher supply voltage if your system supports it.

Voltage Drop Formula

Variables Explained

• V_drop: The voltage lost across the wire conductors, measured in volts. This is the difference between the supply voltage at the panel and the voltage delivered to the load at the far end of the circuit.
• L: One-way wire length in feet. The factor of 2 in the formula accounts for both the supply conductor and the return conductor, since current must flow through both.
• I: Current flowing through the circuit in amperes. Higher current produces proportionally more voltage drop because the power lost as heat in the conductor equals I-squared times R.
• R: Resistivity of the conductor material in ohm-circular mils per foot. Copper has a resistivity of 10.37 and aluminum has 17.02 at 75 degrees Celsius, the standard operating temperature assumed by the NEC.
• CM: Circular mil area of the wire gauge. This is the cross-sectional area of the conductor; larger values mean less resistance and less voltage drop per unit length.

Step-by-Step Example

Calculate the voltage drop for a 20A load on a 120V circuit using 12 AWG copper wire with a 100-foot run:

1. Identify values: L = 100 ft, I = 20 A, R = 10.37 (copper), CM = 6,530 (12 AWG)
2. Apply the formula: V_drop = (2 x 100 x 20 x 10.37) / 6,530
3. Calculate numerator: 2 x 100 x 20 x 10.37 = 41,480
4. Divide: 41,480 / 6,530 = 6.35 V
5. Calculate percentage: (6.35 / 120) x 100 = 5.29%

At 5.29%, this exceeds the NEC-recommended 3% maximum. You should upgrade to 10 AWG wire (10,380 CM), which would reduce the drop to about 3.99V (3.33%), or consider shortening the run. Using 8 AWG (16,510 CM) would further reduce it to 2.51V (2.09%).

Practical Examples

Example 1: Mark's Detached Garage Wiring

Mark is running a 20-amp, 120V circuit from his main panel to a detached garage 150 feet away. He initially planned to use 12 AWG copper wire. Using the voltage drop calculator:

• V_drop = (2 x 150 x 20 x 10.37) / 6,530 = 9.53V
• Percentage: (9.53 / 120) x 100 = 7.94% — far exceeds the 3% recommendation
• Upgrading to 8 AWG: (2 x 150 x 20 x 10.37) / 16,510 = 3.77V = 3.14%
• Upgrading to 6 AWG: (2 x 150 x 20 x 10.37) / 26,240 = 2.37V = 1.97%

Mark decides on 6 AWG copper wire to keep the drop well under 3%, ensuring his power tools in the garage receive adequate voltage for reliable operation. The extra wire cost is a worthwhile investment for the long run distance.

Example 2: Sarah's Commercial Kitchen Circuit

Sarah is installing a 240V, 40-amp circuit for a commercial oven located 75 feet from the electrical panel. She is evaluating 8 AWG copper wire:

• V_drop = (2 x 75 x 40 x 10.37) / 16,510 = 3.77V
• Percentage: (3.77 / 240) x 100 = 1.57%

At just 1.57%, the voltage drop is well within the 3% recommendation. The 240V supply voltage works in Sarah's favor here, as the same current on a 120V circuit would produce a 3.14% drop. The higher supply voltage allows her to use smaller wire while maintaining acceptable voltage regulation.

Example 3: David's Barn Lighting with Aluminum Wire

David wants to run lighting circuits to his barn 200 feet from the main panel. To save on material costs, he is considering aluminum wire for the 120V, 15-amp circuit. He compares 10 AWG aluminum versus 12 AWG copper:

• 12 AWG copper: V_drop = (2 x 200 x 15 x 10.37) / 6,530 = 9.53V = 7.94%
• 10 AWG aluminum: V_drop = (2 x 200 x 15 x 17.02) / 10,380 = 9.83V = 8.19%
• 6 AWG aluminum: V_drop = (2 x 200 x 15 x 17.02) / 26,240 = 3.89V = 3.24%
• 4 AWG aluminum: V_drop = (2 x 200 x 15 x 17.02) / 41,740 = 2.45V = 2.04%

David selects 4 AWG aluminum wire, which keeps the voltage drop at 2.04% while being more affordable than the equivalent copper conductor for this long distance. He ensures all connections use anti-oxidant compound and aluminum-rated terminals for a safe, long-lasting installation.

Example 4: Elena's Industrial Motor Feed

Elena is sizing conductors for a 480V, 30-amp industrial motor located 250 feet from the distribution panel. She evaluates 10 AWG copper wire:

• V_drop = (2 x 250 x 30 x 10.37) / 10,380 = 14.99V
• Percentage: (14.99 / 480) x 100 = 3.12%

While slightly over 3%, the 480V system keeps the percentage reasonable even at 250 feet. Upgrading to 8 AWG copper would reduce the drop to 9.43V (1.96%), well within limits. Elena chooses 8 AWG to provide margin for motor starting surge currents, which can be 6 to 8 times the running current and temporarily increase voltage drop significantly.

Wire Gauge Reference Table

Tips and Complete Guide

Understanding NEC Voltage Drop Recommendations

The National Electrical Code provides voltage drop guidance through informational notes rather than mandatory requirements. The two key recommendations are: branch circuits should not exceed 3% voltage drop at the farthest outlet, and the combined drop of feeders plus branch circuits should stay under 5%. Many local building codes adopt these recommendations as enforceable standards. When planning electrical installations, always design for the 3% branch circuit limit as your primary target. For sensitive electronic equipment, some engineers design for even tighter margins of 2% or less to ensure optimal equipment performance and longevity.

Temperature Effects on Voltage Drop

Wire resistance increases with temperature, which means voltage drop gets worse as conductors heat up under load. The standard NEC calculations use resistivity values at 75 degrees Celsius, which is the rated insulation temperature for most common wire types (THWN, THHN). In hot environments such as attics or sun-exposed conduit runs, actual wire temperatures can exceed this assumption, increasing voltage drop beyond calculated values. Conversely, underground or climate-controlled installations may experience lower temperatures and slightly better performance. For critical installations, consider applying a temperature correction factor to your voltage drop calculations.

Voltage Drop and Energy Efficiency

Voltage drop represents real energy loss in the form of heat generated in the conductors. This waste directly increases your electricity costs. For a 20A circuit running continuously at 5% voltage drop on 120V, you lose 6V x 20A = 120 watts — that is roughly 1,051 kWh per year, costing about $130 at national average rates. Reducing the drop to 2% through proper wire sizing saves approximately 72 watts continuously, recovering the additional wire cost within a few years. For commercial and industrial installations with many long circuits, proper voltage drop management can save thousands of dollars annually in energy costs while also improving equipment reliability.

When to Use Larger Conductors

Beyond meeting NEC recommendations, there are situations where oversizing conductors is beneficial. Long runs to detached structures, circuits feeding motor loads with high starting currents, and installations where future load increases are anticipated all warrant larger wire. The incremental cost of going up one or two wire sizes is often modest compared to the labor cost of pulling new wire later. Additionally, larger conductors run cooler, last longer, and reduce fire risk. For solar panel installations and battery bank connections, minimizing voltage drop is especially critical since every fraction of a percent lost reduces the system's overall energy harvest.

Common Mistakes to Avoid

• Measuring total wire length instead of one-way distance: The formula already accounts for the return conductor by multiplying by 2, so enter only the one-way distance from panel to load. Entering the round-trip distance would double-count and produce an erroneously high result.
• Ignoring the return conductor in multi-wire branch circuits: In a standard circuit, current flows through both the hot and neutral conductors, and both contribute to voltage drop. The formula's factor of 2 handles this correctly for single-phase circuits.
• Using the wrong resistivity for the wire temperature: Standard calculations assume 75 degrees Celsius. If your wire insulation is rated for 60 or 90 degrees, the actual resistivity may differ. Always verify the conductor rating matches your calculation assumptions.
• Forgetting motor starting current: Motors can draw 6 to 8 times their rated running current during startup. While brief, this surge causes a proportional spike in voltage drop that can prevent the motor from starting or trip protective devices. Size conductors to keep voltage drop acceptable during startup, not just steady-state operation.
• Not accounting for power factor: The standard formula assumes a power factor of 1.0. For circuits with significant reactive loads (motors, transformers), the actual voltage drop may differ. For precision, use the full impedance-based formula that includes both resistance and reactance components.

Frequently Asked Questions