Voltage Drop Calculator
Enter the wire gauge, one-way distance, amperage, and system voltage to calculate the voltage drop across your electrical circuit. Results show if you meet the NEC 3% recommendation.
What is Voltage Drop?
A voltage drop calculator determines how much voltage is lost in an electrical conductor due to its inherent resistance as current flows through it. Every wire has resistance that converts a small portion of electrical energy into heat, reducing the voltage available at the load end of the circuit. Excessive voltage drop causes lights to dim, motors to overheat, and sensitive electronics to malfunction. The National Electrical Code (NEC) Section 210.19 Informational Note 4 recommends that voltage drop on branch circuits not exceed 3 percent, and the combined voltage drop of feeder and branch circuit not exceed 5 percent. While these are recommendations rather than requirements in most jurisdictions, they represent industry best practice and are often enforced by inspectors. Voltage drop is calculated using the formula VD = 2 x L x I x R, where L is the one-way distance in feet, I is the current in amperes, and R is the conductor resistance per foot. The factor of 2 accounts for the current traveling out on the hot conductor and returning on the neutral (for single-phase circuits). For three-phase circuits, the factor is 1.732 instead of 2. Conductor resistance per foot varies by wire gauge and material. Copper conductors at 75 degrees Celsius have the following resistances per foot: 14 AWG = 0.00313 ohms/ft, 12 AWG = 0.00197 ohms/ft, 10 AWG = 0.00124 ohms/ft, 8 AWG = 0.000778 ohms/ft, 6 AWG = 0.000491 ohms/ft, 4 AWG = 0.000308 ohms/ft, and 2 AWG = 0.000194 ohms/ft. Aluminum conductors have approximately 1.6 times higher resistance than copper of the same gauge. The relationship between wire gauge and resistance follows a logarithmic pattern. Each decrease of 3 gauge numbers (e.g., from 12 to 9 or from 10 to 7) approximately doubles the cross-sectional area and halves the resistance. This means upgrading by 3 gauge sizes will cut your voltage drop roughly in half. Voltage drop is particularly critical for long runs such as outbuildings, detached garages, well pumps, and landscape lighting. A circuit that performs fine at 30 feet may have unacceptable voltage drop at 150 feet with the same wire gauge. Always calculate voltage drop before running long circuits and upsize wire as needed to maintain the 3 percent recommendation.
How to Calculate
- Select the wire gauge (AWG) installed or planned for the circuit
- Measure the one-way distance from the electrical panel to the farthest outlet or device
- Enter the circuit amperage (use breaker rating or calculated load)
- Enter the system voltage (120V for standard circuits, 240V for appliances)
- Check that the voltage drop percentage is below 3% for branch circuits
- If voltage drop exceeds 3%, consider upsizing the wire gauge by 1-2 sizes
Formula
Voltage Drop (volts) = 2 x distance (ft) x amperage (A) x resistance_per_ft (ohms/ft) Voltage Drop (%) = (voltage_drop / system_voltage) x 100 Voltage at End = system_voltage - voltage_drop Resistance per foot (copper at 75C): - 14 AWG: 0.00313 ohms/ft - 12 AWG: 0.00197 ohms/ft - 10 AWG: 0.00124 ohms/ft - 8 AWG: 0.000778 ohms/ft - 6 AWG: 0.000491 ohms/ft - 4 AWG: 0.000308 ohms/ft - 2 AWG: 0.000194 ohms/ft The factor of 2 accounts for current flowing through both the hot and neutral conductors (round-trip distance).
Example Calculation
For 12 AWG wire, 100 ft one-way distance, 15 amps, 120V system: Voltage Drop = 2 x 100 x 15 x 0.00197 = 5.91 volts Voltage Drop % = (5.91 / 120) x 100 = 4.93% Voltage at End = 120 - 5.91 = 114.09 volts Result: 4.93% voltage drop exceeds the 3% NEC recommendation. You should upgrade to 10 AWG wire for this run. With 10 AWG: VD = 2 x 100 x 15 x 0.00124 = 3.72 volts = 3.10%. Still slightly over 3%, but acceptable for most applications. For strict compliance, use 8 AWG (VD = 2.33 volts = 1.95%).
Frequently Asked Questions
Is the 3% voltage drop rule a code requirement or recommendation?
The NEC 3% voltage drop for branch circuits and 5% total are informational notes (recommendations), not mandatory requirements. However, many inspectors enforce them, and excessive voltage drop causes real performance problems. Design to 3% as standard practice. Some jurisdictions have adopted the 3% and 5% limits as local amendments.
Why does my voltage drop double when I double the distance?
Voltage drop is directly proportional to distance (VD = 2 x L x I x R). If you double the one-way distance, you double the voltage drop. This is why long runs to outbuildings, barns, or well pumps often require significantly larger wire than the same circuit would need at a shorter distance.
How does 240V compare to 120V for voltage drop?
At 240V, the same wattage load draws half the amperage compared to 120V. Since voltage drop depends on amperage, a 240V circuit has half the absolute voltage drop. Additionally, the percentage drop is calculated against the higher base voltage. A 240V circuit running the same wattage at the same distance will have roughly one-quarter the percentage voltage drop compared to 120V.
Does this calculator work for DC circuits like solar?
Yes. The formula is identical for DC circuits: VD = 2 x L x I x R. DC solar circuits from panels to inverters are particularly sensitive to voltage drop because lost voltage directly reduces system power output. Solar industry standard is 1-2% maximum voltage drop between panels and inverter.
What are the symptoms of excessive voltage drop?
Common symptoms include: dimming or flickering lights (especially when motors start), motors running hot or failing to start, electronic equipment resetting or malfunctioning, slow-charging devices, and tripping breakers under normal loads. If end-of-line voltage drops below 108V (on a 120V circuit), most equipment will not operate correctly.
Should I calculate voltage drop for the full breaker rating or actual load?
Calculate for the actual expected load or the highest continuous load on the circuit. Using breaker rating gives worst-case results but may lead to unnecessarily large wire. For circuits with known loads (like a specific appliance), use the actual device amperage. For general-purpose circuits, use 80% of breaker rating as a reasonable design load.