Voltage Drop Calculator
Enter wire material, gauge, length and current to calculate voltage drop, percentage drop and the recommended minimum wire size.
Reviewed by the ToolNestr Editorial Team — July 2026
How voltage drop is calculated
Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor due to its resistance. For DC and single-phase AC circuits, the formula is Vd = 2 × L × I × R / 1000, where L is the one-way wire length in feet, I is the current in amperes, and R is the conductor resistance in ohms per 1000 feet. The factor of 2 accounts for the supply and return paths.
The percentage voltage drop is calculated as (Vd / Vsystem) × 100%. The NEC recommends keeping voltage drop below 3% for branch circuits and 5% for combined feeder and branch circuits.
Worked example
What different voltage drop ranges mean
Voltage drop is categorized by its impact on system performance. Lower is always better for efficiency and equipment health.
Electrician
Verify wire sizing meets code requirements for residential and commercial installations before pulling cables.
Solar Installer
Minimize I\u00b2R losses in PV array wiring to maximize energy harvest from your solar panel installation.
Audio System Designer
Ensure speaker wire gauge is adequate for long runs to avoid audible power loss and damping factor degradation.
Homeowner
Plan garage, workshop or outdoor wiring projects with confidence that your circuit will perform safely and efficiently.
| AWG | Copper (Ω/1000 ft) | Aluminum (Ω/1000 ft) | Max amps (Cu, 60°C) |
|---|---|---|---|
| 18 | 6.385 | 10.450 | 16 |
| 16 | 4.016 | 6.572 | 25 |
| 14 | 2.525 | 4.131 | 40 |
| 12 | 1.588 | 2.599 | 64 |
| 10 | 0.999 | 1.635 | 102 |
| 8 | 0.628 | 1.028 | 161 |
| 6 | 0.395 | 0.646 | 256 |
| 4 | 0.248 | 0.406 | 407 |
| 3 | 0.197 | 0.322 | 512 |
| 2 | 0.156 | 0.256 | 646 |
| 1 | 0.124 | 0.203 | 814 |
| 0 | 0.098 | 0.161 | 1025 |
| 00 | 0.078 | 0.128 | 1025 |
| 000 | 0.062 | 0.101 | 1025 |
| 0000 | 0.049 | 0.080 | 1025 |
How to use the Voltage Drop Calculator
Enter wire specs
Choose the wire material (copper or aluminum) and select the AWG gauge. Thicker wire has lower resistance.
Enter the load
Provide the one-way wire length in feet, the load current in amps, and your system voltage. The calculator handles the rest.
Check the drop
Review the voltage drop, percentage drop, and power loss. If the drop exceeds 3%, consider a larger wire gauge.
Tips for minimizing voltage drop
Keep drop under 3% for branch circuits
The NEC recommends a maximum of 3% voltage drop on branch circuits. For critical equipment, aim for 1\u20132% to ensure reliable operation.
Keep total drop under 5%
The combined voltage drop of the feeder and branch circuit should not exceed 5%. Plan your wiring runs with both segments in mind.
Use larger gauge for long runs
For runs longer than 100 feet, step up one or two AWG sizes to keep voltage drop acceptable. The extra cost is usually less than the cost of problems caused by excessive drop.
Consider copper for critical circuits
Copper has about 40% lower resistance than aluminum. For circuits where voltage drop is critical, the extra cost of copper is often justified by better performance.
Why voltage drop matters
Voltage drop is not just about lost energy \u2014 it directly affects the performance and lifespan of electrical equipment. Motors run hotter and slower, lights dim, electronic devices may malfunction, and battery charging becomes incomplete. In extreme cases, excessive voltage drop can cause equipment failure and create safety hazards due to overheated wires.
NEC recommendations
The National Electrical Code (NEC) provides guidelines in Article 215 and 310 for acceptable voltage drop. The recommended limits are 3% for branch circuits and 5% for feeders plus branch circuits combined. While these are recommendations rather than absolute requirements in most jurisdictions, exceeding them often leads to inspection failures and equipment problems.
Wire sizing basics
Selecting the right wire gauge involves balancing three factors: current-carrying capacity (ampacity), voltage drop, and cost. Start with the minimum gauge required for ampacity, then check voltage drop over the planned distance. If the drop is too high, increase the wire size. The calculator helps you iterate through these options quickly.
Frequently asked questions
What is voltage drop?
Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor due to its resistance. It is calculated as Vₓ = I × R, where I is current and R is the total resistance of the wire.
What is the maximum allowable voltage drop?
The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% for the combined feeder and branch circuit. For critical loads like motors and medical equipment, even tighter limits may apply.
What factors affect voltage drop?
Voltage drop depends on the conductor material (copper has lower resistance than aluminum), wire gauge (thicker wire = lower resistance), length of the wire run (both directions = total circuit length), and the amount of current flowing.
Why does wire length matter?
Voltage drop is proportional to the total circuit length, which includes both the supply and return paths. A 100-foot run of wire actually means 200 feet of conductor length when calculating voltage drop.
When should I use aluminum vs copper wire?
Copper has lower resistance and is more durable, making it better for most installations. Aluminum is lighter and cheaper, used mainly for large feeders and service entries, but requires special connectors and anti-oxidation treatment.
How does wire gauge (AWG) affect voltage drop?
Larger AWG numbers mean thinner wire with higher resistance. A 14 AWG wire has about 2.5Ω per 1000 ft, while a 4 AWG wire has about 0.25Ω per 1000 ft. Using a thicker gauge dramatically reduces voltage drop.
Does voltage drop matter for low-voltage circuits?
Yes, even more so. A 1V drop on a 120V circuit is less than 1%, but the same 1V drop on a 12V circuit is over 8%. Low-voltage systems like landscape lighting, audio systems, and solar panels are especially sensitive.
How do I calculate voltage drop for 3-phase systems?
For three-phase AC, the formula is Vₓ = √3 × I × L × R / 1000 (where L is one-way length in feet and R is resistance per 1000 ft). The √3 factor accounts for the phase relationship.
What is power loss due to voltage drop?
Power loss (I²R loss) is the energy dissipated as heat in the wires. It is calculated as P = I² × R. This wasted energy reduces system efficiency and can cause wire heating in high-current circuits.
Can voltage drop damage equipment?
Yes. Motors running at reduced voltage draw higher current, overheat and may fail. Sensitive electronics may malfunction or reset. Lighting dims noticeably, and battery chargers may not reach full charge.
Sources & references
This tool uses standard formulas and reference values from:
- • NFPA 70 — National Electrical Code (NEC), conduit fill, box fill and conductor ampacity. nfpa.org
- • IPC-2221, Generic Standard on Printed Board Design (trace width / current). ipc.org
- • NIST reference constants and unit definitions; IEEE standards where applicable. nist.gov
For educational and preliminary use. Verify against full engineering calculations and the governing standard before any real design.