Voltage Drop Calculator
Check how much voltage you lose across a wire run before you send the estimate. Enter voltage, amps, conductor material, wire size, and run length to get a fast field-use answer.
Voltage Drop Calculator
Check voltage drop, estimated load-side voltage, and whether the selected conductor stays inside your target.
Formula used
Single-phase: Vdrop = 2 x A x R x one-way feet / 1000
Run length is one-way. The phase multiplier accounts for the conductor path.
This electrical calculator is built for practical estimating and scoping, not deep engineering workflow. It gives electricians a quick way to sanity-check longer runs, panel-fed equipment, detached structures, and EV charging circuits before the proposal goes out.
When should you use a voltage drop calculator?
Use this tool when the run is long enough that wire size could change equipment performance, startup behavior, or your final material list. It is especially useful for feeders to garages, barns, detached shops, outdoor equipment, and EV chargers.
It also helps during quoting. If a long run pushes the drop above your target, you can catch the conductor change early instead of discovering it after the job is sold.
How this calculator works
The calculator uses common conductor resistance values in ohms per 1000 feet. For single-phase runs it uses the standard factor of 2. For three-phase runs it uses 1.732. From there, it estimates voltage drop in volts and converts that to a percentage of your nominal system voltage.
Once the drop is calculated, the tool compares the result with your target percentage and checks whether a larger conductor in the same material would get the run back under that threshold.
From quick math to cleaner electrical proposals
Voltage drop is often one of the details that separates a rough quote from a confident one. When you know a long run needs a conductor bump, you can price it correctly, explain it clearly, and avoid awkward change-order conversations later.
Learn how to build professional contractor estimatesExample run
240V, 40A, 150-foot one-way run, 8 AWG copper, single-phase
Vdrop = 2 x 40 x 0.778 x 150 / 1000
Vdrop = 9.34V
Drop percent = 9.34 / 240 x 100 = 3.89%
That is above a 3% target, so the run would likely need a conductor bump if that is the design goal. The calculator recommends the next larger wire size that gets the run under your chosen threshold.
What affects voltage drop most?
Run length
Longer runs add resistance and usually drive the biggest change in drop percentage.
Load current
Higher amperage increases voltage drop quickly, which is why EV charging circuits and larger equipment runs need extra attention.
Conductor size and material
Larger conductors have lower resistance. Aluminum also behaves differently than copper, so the calculator lets you compare both.
System voltage
The same lost volts represent a smaller percentage on higher-voltage systems, which is why voltage matters in the final answer.
Frequently asked questions
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