Voltage Drop Calculator — Quick & Accurate Online Calculator & Guide

How Voltage Drop Works

Voltage Drop = Current × (Resistance/1000 ft × Total Wire Length)

Voltage drop is calculated using the wire resistance (from AWG tables) multiplied by the round-trip current path length. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% for feeder plus branch combined.

R

Wire Resistance — Copper and aluminum have different resistivity values. Resistance increases with length and decreases with wire gauge (smaller AWG number = thicker wire = lower resistance).

NEC 3%

NEC Recommended Max — The National Electrical Code recommends voltage drop not exceed 3% for branch circuits and 5% for feeder + branch combined.

Conductor

Conductor Material — Copper has about 40% lower resistance than aluminum for the same gauge, but aluminum is lighter and cheaper. Aluminum requires special termination connectors.

How to Use

1. Enter the source voltage and load current.
2. Enter the one-way wire run length in feet.
3. Select the wire gauge and conductor material.
4. The calculator checks compliance with the 3% NEC recommended maximum.
5. Check the power loss (wattsLost) to understand energy waste over time.
6. If voltage drop exceeds 3%, try a larger gauge wire or shorter run.

Quick Reference

Common Uses

• •Sizing electrical wire gauges for residential, commercial, or industrial wiring projects to meet NEC code requirements
• •Planning long wire runs to outbuildings, subpanels, outdoor lighting, or well pumps where voltage drop is a concern
• •Choosing between copper and aluminum conductors based on resistance, cost, and installation requirements for a given circuit length

Understanding the Result

Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction. The NEC recommends a maximum 3% voltage drop for branch circuits. If your calculation shows more than 3% drop, upsize the wire gauge. Each jump in AWG (e.g., 12 to 10) reduces resistance by approximately 20%. Long wire runs — to outbuildings, outdoor lighting, or subpanels — are the most common sources of voltage drop problems. Real-world example: running 120V to a shed 100 feet away with a 15A load on 14 AWG copper. One-way equals 100 ft, round-trip equals 200 ft. Resistance of 14 AWG copper is 3.14 Ω per 1,000 ft, so total resistance equals 0.628 Ω. Voltage drop equals 15A times 0.628 Ω which is 9.42V, or 7.85 percent — well over the 3% NEC recommendation and likely to cause dim lights and slow motor starting. Upsizing to 10 AWG copper (1.24 Ω per 1000ft) drops resistance to 0.248 Ω and voltage drop to 3.72V (3.1 percent) — just within acceptable range. For a 240V well pump 300 feet from the house running at 10A, even 10 AWG copper produces a drop of 7.44V (3.1 percent). 8 AWG drops it to 4.67V (1.9 percent) — a safer choice that also reduces energy waste over the lifetime of the installation.

Worked Examples

Frequently Asked Questions

Why does voltage drop matter?

Voltage drop causes motors to draw excess current (potentially causing overheating and failure), reduces lighting output (incandescent bulbs dim noticeably below 95% rated voltage), and can cause sensitive electronics to malfunction or fail to start. In data centers, even a 2% voltage drop at the rack level can cause server power supply instability.

Should I use the one-way or round-trip distance?

Enter the one-way distance — the calculator automatically doubles it to account for both the hot and neutral (or return) conductors of the complete circuit. For 3-phase circuits, the calculation differs and requires a different formula. Multiply the single-phase result by 0.866 for 3-phase approximations.

Does aluminum wire need special handling?

Yes. Aluminum wire requires anti-oxidant compound at connections, special AL-CU rated outlets and switches, and torque-wrench tightening. Aluminum expands and contracts more than copper with temperature changes, which can loosen connections over time — a known fire hazard if not installed correctly.

How does temperature affect voltage drop?

Wire resistance increases by about 0.4% per degree Celsius above 25C (77F). In a hot attic at 60C (140F), resistance is about 14% higher than our room-temperature calculation. Cold weather slightly decreases resistance. The NEC ampacity tables include temperature derating factors (Table 310.15(B)(1)) for installations in high-temperature environments.

What gauge wire do I need for a 200-amp residential service?

For 200A residential service entrance, typical requirements are 2/0 AWG copper or 4/0 AWG aluminum. However, service entrance calculations are different from branch circuit voltage drop — they involve NEC load calculations, local utility requirements, and are not simply determined by voltage drop alone. This calculator is designed for branch circuits and feeders, not service entrances. Always consult a licensed electrician for service panel installations.

Pro Tips

• →For continuous loads (3+ hours, like EV chargers, lighting, or HVAC), NEC requires sizing the circuit to 125% of the load. Always account for this before using the calculator values.
• →Use the power loss result (wattsLost) to estimate energy waste over time. A 50W continuous loss at $0.15/kWh wastes about $66/year. Over a 30-year installation, that is nearly $2,000 — potentially more than the cost difference of upsizing the wire.
• →For 3-phase circuits, multiply the voltage drop by 0.866 (the square root of 3 divided by 2). Our calculator assumes single-phase. 3-phase circuits have lower voltage drop because the neutral carries less current.
• →Always measure the actual one-way distance (along the wire path, not as the crow flies). Wire must follow joists, conduit bends, and service loops — a "50 ft straight line" run might actually need 70 ft of wire. Add 10-15% buffer to your measurement.
• →If your calculated drop is borderline (2.5-3.5%), consider upsizing the wire. The marginal cost of the next gauge is usually modest compared to the labor of replacing it later, and the energy savings compound over the life of the building.

Limitations to Know

• •This calculator uses standard DC resistance values for copper and aluminum conductors at 25C (77F) ambient temperature. Resistance increases by about 0.4% per degree Celsius, so in hot attics (60C/140F) the actual voltage drop will be ~14% higher than calculated.
• •The calculator assumes single-phase AC circuits. For 3-phase circuits, multiply the voltage drop by 0.866. It does not account for power factor (reactance), which becomes significant for wire sizes larger than 2 AWG or for motor loads with low power factor.
• •Conduit fill, ambient temperature derating, and termination temperature ratings (60C vs 75C column in NEC ampacity tables) are also not considered. Always consult a licensed electrician for critical installations and verify compliance with your local electrical code requirements.

Sources

• Voltage Drop
• Voltage
• National Electrical Code, Article 210.19