Voltage Drop Calculator
Calculate voltage drop in a wire run from source voltage, current, AWG, and distance.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Voltage drop is the reduction in voltage along a wire due to its resistance. This calculator computes total drop for a two-conductor run (current travels out and returns).
- Enter source voltage. The voltage at the power supply or breaker panel (e.g., 12 V for automotive, 120 V or 240 V for mains).
- Enter current. The current drawn by the load in amps.
- Select AWG gauge. The wire gauge you are using or considering.
- Enter one-way distance. The distance in feet from the source to the load. The calculator accounts for both the outgoing and return conductors (doubles the distance).
- Read the results. Voltage drop, voltage remaining at the load, and drop as a percentage of source voltage.
About Voltage Drop
Voltage drop = I × R, where R = 2 × distance × resistance per foot. The resistance per foot comes from the AWG table. The NEC recommends that voltage drop not exceed 3% for branch circuits and 5% total for feeders plus branch circuits. For 12 V DC systems (automotive, marine, solar), even a 3% drop reduces voltage to 11.64 V, which can affect sensitive electronics.
The solution is to use a larger wire (lower AWG number) or shorten the run. If the drop is over 3%, this calculator helps you find the right wire gauge by trying lower AWG values until the drop is acceptable. Use the Wire Gauge Calculator to check the ampacity of any gauge before finalizing your selection.
Frequently Asked Questions
What is an acceptable voltage drop for a circuit?
The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total (feeder plus branch). For a 120 V circuit, 3% = 3.6 V drop, leaving 116.4 V at the load. For 12 V DC systems, 3% = 0.36 V drop, leaving 11.64 V. Sensitive electronics like audio equipment and LED drivers may require less than 2% to operate correctly and efficiently.
Why does voltage drop matter for LED lighting?
LED drivers and strips are sensitive to voltage. A 12 V LED strip running near minimum voltage due to a large drop may flicker, appear dim, or not light at all. For long LED strip runs, use a larger wire gauge or power the strip from both ends. Constant-current LED drivers compensate for some drop, but constant-voltage drivers (most strips) do not.
How do I reduce voltage drop in a long wire run?
Use a larger wire (lower AWG number) to reduce resistance. Shorten the run distance if possible. Increase the source voltage if the equipment allows (e.g., using 24 V instead of 12 V halves the current for the same power, reducing drop by 75%). Run multiple parallel conductors to reduce effective resistance. For very long runs, consider using a local voltage regulator near the load.
Does voltage drop cause wires to get hot?
Yes. Voltage drop is caused by wire resistance, and that resistance dissipates power as heat: P = I² × R. A wire with a 3% voltage drop on a 10 A circuit dissipates significant power as heat. This is why undersized wire can overheat and become a fire hazard. Always size wire for both acceptable voltage drop and adequate ampacity (current-carrying capacity).