Voltage Drop Calculator

Calculate voltage drop, drop percentage and voltage at the load for copper or aluminium wire by gauge, current, phase and run length. Free, instant, no signup.

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Formula: Vdrop = k × I × (R/1000 × L) • k = 2 (1φ) or √3 (3φ)
  • I = load current (amps)
  • R = conductor resistance per 1000 ft
  • L = one-way run length (ft)

How to use the Voltage Drop Calculator

  1. Enter your values. Fill in the fields with your numbers.
  2. Calculate. Press Calculate to run the voltage drop calculator.
  3. Use the result. Copy the result or try a related tool next.

Why use our Voltage Drop Calculator

Instant results. Enter your figures and the voltage drop calculator returns an answer in seconds.
Free & private. Runs in your browser — no signup, and nothing is sent to a server.
Accurate. Uses standard formulas so you can rely on the numbers.

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About the Voltage Drop Calculator

The Voltage Drop Calculator estimates how much voltage a conductor loses between the panel and the load over a given distance. Every wire has resistance, so by the time current reaches a far-off pump, light, or EV charger, the voltage at the device is lower than what left the breaker. You enter the conductor material (copper or aluminum), wire size, one-way run length, current in amps, and the system type (DC, single-phase, or three-phase AC), and the tool returns the volts dropped, the voltage delivered at the load, and the drop expressed as a percentage of source voltage.

Reach for this tool whenever a run is long, the current is high, or both. Drops inside a typical house are usually negligible, but feeding a detached garage, a well pump 300 to 500 feet away, landscape lighting, or a sub-panel in an outbuilding is exactly where conductors get undersized. The National Electrical Code recommends keeping voltage drop under 3% on a branch circuit and under 5% across the combined feeder and branch path. Checking the number before you pull wire prevents dim lights, motors that struggle to start, and the cost of re-running an undersized cable.

Under the hood the calculator uses the standard resistivity formula. For single-phase and DC circuits the drop is 2 x I x rho x L / A, and for three-phase it swaps the leading 2 for the square root of 3 (about 1.732), since the return path differs. Here I is current, L is one-way length, A is the conductor cross-sectional area, and rho is resistivity: roughly 1.72 x 10^-8 ohm-meters for copper and 2.65 x 10^-8 for aluminum. Larger wire means more cross-section, lower resistance, and less drop. That is why upsizing the conductor is the usual fix.

Results are an engineering estimate, not a substitute for a stamped design or local code sign-off. The math assumes a steady load current at a typical conductor temperature and does not model motor inrush, harmonics, power factor below unity, or elevated ambient heat, all of which can worsen real-world drop. For motor and pump circuits, plan for the startup surge, not just running amps. Everything is computed in your browser, so the wire sizes, lengths, and loads you type are never uploaded or stored on a server.

Frequently asked questions

What is an acceptable voltage drop?

The NEC recommends a maximum of about 3% on a branch circuit and 5% across the feeder and branch circuit combined. These figures are guidance in Informational Notes rather than hard rules, but most electricians treat them as best practice, and local inspectors may enforce them.

Why do I enter one-way length instead of the round trip?

You enter the one-way distance from the source to the load, and the formula accounts for the return path automatically: it multiplies by 2 for single-phase and DC, and by the square root of 3 for three-phase. Doubling the length yourself would overstate the drop.

Copper or aluminum, which has less voltage drop?

For the same wire size, copper drops less voltage because it has lower resistivity (about 1.72 versus 2.65 x 10^-8 ohm-meters for aluminum). Aluminum can still be used, but you typically need to step up one or two wire sizes to match copper's performance on a long run.

How do I reduce voltage drop that's too high?

The most common fix is using a larger conductor, since more cross-sectional area means less resistance. You can also shorten the run by relocating the panel closer to the load, split the load across multiple circuits, or, where feasible, raise the supply voltage.

Does voltage drop waste energy?

Yes. The voltage lost in the wire is dissipated as heat, known as I-squared-R loss because it rises with the square of the current. On long, heavily loaded runs this wasted heat adds up as higher energy costs and warmer conductors, so reducing drop improves efficiency.

From our blog

How to Count Your Days Sober Accurately (and Make the Number Work for You)

By the Super Simple Digital Tools Team · Updated June 2026

Counting sober time sounds simple, but small choices about how you count change every number that follows. The two things that matter most are your start date and how the calculator turns elapsed time into units. Get those right once and you have a count you can trust for years, share with a sponsor, and check against the chips or coins you collect along the way.

Start with the date. The widely used convention is to count the first full day of abstinence as day one, which usually means the calendar day after your last drink or use. Whatever rule you choose, write it down and never adjust it later. A common source of confusion is quietly moving the date by a day or two; that one change ripples through your total days and every anniversary, so consistency beats precision second-guessing.

Next, understand the two outputs. Total days is a clean tally you can announce at a meeting. The years-months-days view is a calendar reading, which is why thirty days does not always equal one month and why a leap year can nudge the totals. Neither is wrong. Use total days for chip counts and the calendar breakdown when you want to say how many months or years you have strung together.

Milestones give the count a purpose. In the first year, recovery communities commonly mark 24 hours, 30, 60, and 90 days, then half a year, nine months, and the one-year mark, with annual milestones after that. Knowing your day count lets you see the next milestone approaching instead of being surprised by it, so you can plan to attend a meeting, tell a friend, or simply pause and acknowledge the progress.

Finally, treat the number as a tool, not a verdict. A high count is encouraging, but a relapse and a fresh start do not erase what you learned. Reset the date honestly if you need to, keep tracking, and use the figure on hard days as evidence that staying sober is possible because you have already done it. The calculator counts the time; the work and the meaning behind it are yours.

  • Enter the exact calendar day after your last drink or use, not a rounded estimate, so every total stays accurate.
  • Bookmark or note your day count before meetings where chips or coins are handed out, so your number matches the milestone.
  • Set a reminder a few days before big marks like 90 days or one year to plan how you'll acknowledge them.
  • If you relapse, set a new honest start date and keep tracking; the earlier progress still counts as experience.

Read the full guide →

Tool by the Super Simple Digital Tools Team. Reviewed by our editorial team. Free to use, no signup required.

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