Voltage Drop Calculator
Precision electrical engineering for safe and efficient wiring.
Voltage Drop Calculator
Running a long wire from your breaker panel to a shed, shop, or outdoor outlet? Voltage drop is a real problem — too much and your equipment won't work properly. This calculator tells you exactly how much voltage you'll lose and whether your wire gauge is adequate.
What Is Voltage Drop?
Voltage drop is the reduction in voltage along a wire caused by the wire's resistance. The longer the wire and the more current it carries, the greater the voltage drop. Most electrical codes require voltage drop to stay below 3% for branch circuits and 5% total (feeder + branch).
How to Use This Calculator
- Enter the voltage of your circuit (120V, 240V, etc.).
- Enter the current in amps.
- Enter the one-way wire length in feet.
- Select the wire gauge (AWG) or conductor material (copper or aluminum).
- Click Calculate to see voltage drop, percent drop, and recommendation.
Voltage Drop Formula
Single-phase: VD = (2 × K × I × L) ÷ CM
- VD = Voltage drop (volts)
- K = Resistivity constant (12.9 for copper, 21.2 for aluminum)
- I = Current in amps
- L = One-way wire length in feet
- CM = Circular mils of the conductor (from wire tables)
Example Calculation
A 120V, 20A circuit running 100 feet with 12 AWG copper wire:
- CM for 12 AWG = 6,530
- VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 51,600 ÷ 6,530 = 7.9 V
- Percent drop = 7.9 ÷ 120 = 6.6% — exceeds the 3% recommendation
- Solution: upgrade to 10 AWG or reduce the run length
Why Use This Calculator?
Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction. It also wastes electricity as heat. Knowing the voltage drop before you run cable saves the cost of rewiring later.
Common Mistakes to Avoid
- Using total circuit length instead of one-way length — the formula uses one-way length because the return path is accounted for in the ×2 multiplier.
- Ignoring ambient temperature — resistance increases with temperature. For high-temperature environments (attics, conduit in direct sun), derate the conductor accordingly.
- Choosing wire gauge based only on ampacity — for long runs, you may need a larger gauge than the ampacity tables require, purely to control voltage drop.
Frequently Asked Questions
What is the maximum allowable voltage drop?
The NEC recommends (but doesn't require) a maximum of 3% drop for branch circuits and 5% for the combined feeder and branch circuit. Many engineers design to 2% for sensitive loads.
Does voltage drop cause a fire hazard?
Voltage drop itself isn't directly a fire hazard, but undersized wires that draw excessive current do get hot and can be a hazard. Always select wire based on both ampacity and voltage drop.
What wire gauge should I use for a 100-foot run at 20 amps?
For a 120V, 20A circuit at 100 feet with 3% or less drop, use 10 AWG copper. At 240V, 12 AWG may suffice depending on exact load.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has higher resistivity than copper (~65% more), so the same gauge aluminum wire will have more voltage drop than copper for the same run.
Can I use this for 3-phase circuits?
For 3-phase circuits, the formula uses a multiplier of √3 (≈1.732) instead of 2. The calculator includes a 3-phase option.
Conclusion
Voltage drop is one of the most overlooked aspects of electrical planning — but it's easy to calculate and easy to fix if caught early. Use this calculator before you run any long wire run to make sure you're using the right gauge from the start.
Related: Ohm's Law Calculator | Resistor Calculator | Electricity Calculator
Electrician Tip
For long-distance runs (like a detached garage), it is often cheaper to run a higher voltage (e.g., 240V instead of 120V) to reduce current and voltage drop, then step it down if necessary!
