The formula
How it works
Long cable runs lose a little voltage as current flows through the wire’s resistance. This calculator estimates that drop for copper wire from its gauge, the current and the run length — useful for checking a circuit stays within the usual 3% limit.
FAQ
Why does voltage drop matter?
Too much drop means devices get less voltage than they need, causing dim lights, weak motors and wasted energy as heat in the cable. Electrical codes usually recommend keeping it under 3% for a branch circuit.
How do I reduce voltage drop?
Use a thicker wire (a lower AWG number), shorten the run, or raise the supply voltage. Doubling the wire’s cross-section roughly halves the drop.
Why does the calculator use twice the one-way length?
Current has to travel out to the load and back to the source to complete the circuit, so the total resistance seen is that of both conductors together — hence the factor of 2 applied to the one-way run length you enter.
Does this apply to three-phase circuits too?
Three-phase voltage drop uses a slightly different formula, typically with a factor of √3 instead of 2, because the return current is shared differently across the phases. This calculator is built for standard single-phase circuits.
Does wire temperature affect the result?
Yes — resistance rises as copper gets hotter, so a wire working hard in a warm environment will drop a bit more voltage than the room-temperature resistance values used here suggest. The standard AWG resistance table is a good estimate for typical conditions.
What if my current varies over time, like with a motor?
Use the highest current the circuit will realistically draw — such as motor starting current or peak load — since that is when voltage drop is largest and most likely to cause problems. Steady-state drop will usually be lower.
Does aluminum wire give the same result as copper?
No — aluminum has higher resistance than copper of the same gauge, so it drops more voltage for a given current and length. This calculator’s resistance table is for copper only; aluminum runs typically need a thicker gauge to match copper’s performance.
About the voltage drop calculator
This calculator estimates the voltage lost along a run of copper wire, caused by the wire’s own resistance. Every conductor resists the current flowing through it a little, and over a long cable that resistance eats into the voltage that reaches the far end. Knowing the drop matters because equipment needs a minimum voltage to work properly, and electrical codes set limits to keep circuits safe and efficient.
How to use it
Pick the wire size in AWG, then enter the current the circuit carries, the one-way length of the run, and the source voltage. The calculator returns the voltage drop in volts, as a percentage, and the voltage that actually arrives at the load. For example, 20 amps down 50 feet of 12 AWG copper drops about 3.2 volts on a 120-volt supply — around 2.6%, just inside the usual 3% guideline. A longer run or thinner wire pushes it higher.
The formula
For a single-phase circuit, , where is the one-way length in feet, is the current in amperes and is the wire’s resistance per 1,000 feet. The factor of 2 accounts for the current travelling out and back along both conductors. Dividing the drop by the source voltage and multiplying by 100 gives the percentage, the figure codes usually limit.
Where it is used
Electricians use it to size cable for long runs — outbuildings, garden lighting, solar arrays and industrial machinery — so equipment gets enough voltage and cables do not overheat. It guides the choice between wire gauges when a cheaper thin cable would drop too much. The same principle applies to low-voltage systems like car wiring and LED strips, where even a small drop is a large fraction of a 12-volt supply.