What is the voltage drop formula?

For a single-phase or DC two-wire run the drop is Vdrop = 2 × L × I × ρ / A, where L is the one-way length in metres, I is the current in amps, ρ is the conductor resistivity in Ω·m, and A is the cross-section in m². Example: 20 A down 50 m of 4 mm² copper gives 2 × 50 × 20 × 1.68e-8 ÷ 4e-6 ≈ 8.4 V. A three-phase line replaces the 2 with √3.

Why does the formula multiply length by 2?

Current has to flow out to the load on the supply conductor and back on the return conductor, so it crosses the full run length twice. A 50 m run therefore presents 100 m of conductor to the current, which is why the single-phase formula carries a factor of 2. A balanced three-phase circuit has no separate neutral return, so its coefficient drops to √3 ≈ 1.732 instead of 2.

What resistivity do copper and aluminium have?

At about 20 °C copper is roughly 1.68e-8 Ω·m and aluminium is roughly 2.65e-8 Ω·m. Aluminium has about 58% more resistance for the same size, so an aluminium run of the same gauge drops about 1.58 times the voltage a copper run would. That is why aluminium feeders are usually specified one or two sizes larger than the copper equivalent to hit the same drop target.

What is the 3% recommended limit?

Electrical guidance commonly suggests keeping the voltage drop on a branch circuit at or under 3%, and the feeder plus branch combined at or under 5%. On a 230 V system 3% is about 6.9 V; on a 120 V system it is about 3.6 V. The calculator flags any result above 3% so you can step up a wire size, shorten the run, or split the load before the drop starts dimming lights or stressing motors.

Why does a larger cross-section reduce the drop?

Resistance is inversely proportional to cross-sectional area, R = ρL/A, so doubling the area halves the resistance and halves the drop. Going from 4 mm² to 8 mm² copper on that same 20 A 50 m run takes the drop from about 8.4 V down to about 4.2 V. Each step up in wire size is the most direct lever you have when a run comes in over the 3% limit.

How does three-phase change the result?

A balanced three-phase line carries current on three conductors with no neutral return, and the line-to-line drop works out to √3 × L × I × ρ / A rather than 2 × L × I × ρ / A. For the same current, length and conductor the three-phase drop is √3 ÷ 2 ≈ 0.866 of the single-phase value, so three-phase distribution is inherently more efficient for moving the same power over a distance.

Size a long branch circuit before pulling wire

You are running a 20 A circuit 50 m out to a workshop. Punch in the current, length and your candidate 4 mm² copper, and the tool shows a drop of about 8.4 V on a 230 V system, which is past the 3% limit. Bump the size to 6 mm² and watch the percentage drop under the line before you ever cut a length of cable.

Decide between copper and aluminium feeders

An aluminium feeder is cheaper per metre but carries about 58% more resistance than copper of the same size. Enter the run both ways, compare the drop and the load-end voltage side by side, and you can tell whether the aluminium needs to step up a gauge or two to land at the same percentage the copper would.

Check a three-phase distribution run

Feeding a three-phase motor down a long run, switch the circuit type to three-phase and the coefficient changes from 2 to √3. The drop comes out about 0.866 of the single-phase number for the same conductor, which often lets you keep one size smaller while still landing the load-end voltage where the motor wants it.

Verify a low-voltage DC run for solar or 12 V

Low-voltage DC is unforgiving: 3% of 12 V is only 0.36 V, so even a short run of thin wire can starve the load. Set the system voltage to 12 V, type your panel-to-controller current and length, and the percentage tells you immediately whether you need fatter cable or a shorter path.

Voltage Drop Calculator: Wire and Cable Drop, 3% Check

Wire and cable voltage drop from current, run length, conductor size and material, with the 3% limit check, copper vs aluminium, single-phase and three-phase, browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Enter the load current, the one-way run length and the conductor size. The tool computes the round-trip voltage drop, the percentage of system voltage lost, and the voltage left at the load.

Quick examples

Load current I (A)One-way run length L

Conductor size byCross-section A (mm²)Conductor materialCircuit typeSystem voltage Vs (V)

Result

Voltage drop

8.4 V

Drop percentage

3.65 %

Voltage at load

221.6 V

Area used: 4 mm²

Over the 3% recommended limit — go up a wire size or shorten the run.

What this tool does

Free voltage drop calculator for sizing wire and cable on real installations. Enter the load current in amps, the one-way run length in metres or feet, and the conductor cross-section in mm² or as an AWG gauge, and the tool returns the round-trip voltage drop in volts, the percentage of system voltage lost, and the voltage that actually reaches the load. The core formula is Vdrop = 2 × L × I × ρ / A for a single-phase or DC two-wire run, where the factor of two counts both the supply and the return conductor; a balanced three-phase line uses √3 instead. Pick copper (ρ ≈ 1.68e-8 Ω·m) or aluminium (ρ ≈ 2.65e-8 Ω·m), or type your own resistivity. Anything past the common 3% branch-circuit recommendation is flagged so you know to go up a wire size or shorten the run. Everything runs in your browser with one-click copy and a shareable URL. 100% client-side, no upload.

Tool details

Input: Files + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Developer; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Voltage Drop Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Size a long branch circuit before pulling wire
You are running a 20 A circuit 50 m out to a workshop. Punch in the current, length and your candidate 4 mm² copper, and the tool shows a drop of about 8.4 V on a 230 V system, which is past the 3% limit. Bump the size to 6 mm² and watch the percentage drop under the line before you ever cut a length of cable.
Decide between copper and aluminium feeders
An aluminium feeder is cheaper per metre but carries about 58% more resistance than copper of the same size. Enter the run both ways, compare the drop and the load-end voltage side by side, and you can tell whether the aluminium needs to step up a gauge or two to land at the same percentage the copper would.
Check a three-phase distribution run
Feeding a three-phase motor down a long run, switch the circuit type to three-phase and the coefficient changes from 2 to √3. The drop comes out about 0.866 of the single-phase number for the same conductor, which often lets you keep one size smaller while still landing the load-end voltage where the motor wants it.
Verify a low-voltage DC run for solar or 12 V
Low-voltage DC is unforgiving: 3% of 12 V is only 0.36 V, so even a short run of thin wire can starve the load. Set the system voltage to 12 V, type your panel-to-controller current and length, and the percentage tells you immediately whether you need fatter cable or a shorter path.

Common pitfalls

Forgetting the round-trip factor. The current crosses the run twice, so a single-phase drop uses 2 × length, not just the one-way length. Dropping the factor of two halves your answer and lets an undersized cable look like it passes.
Entering the total wire length instead of the one-way run. The formula already doubles the one-way length internally. If you type the full there-and-back figure, the tool doubles it again and reports twice the real drop.
Using copper resistivity for an aluminium run. Aluminium carries about 58% more resistance, so reusing the copper number underestimates the drop by roughly a third and can leave the load chronically under-volted.

Privacy

Every calculation, the drop formula, the percentage, the load-end voltage and the AWG-to-mm² conversion, is plain JavaScript that runs in your browser tab. No current, length or cable spec ever leaves the page and nothing is logged. The one caveat: the shareable URL encodes your inputs in the query string, so a link pasted into chat records those values in the recipient server access log. For a private job, use the copy button and paste the text instead of sharing the URL.

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Voltage Drop Calculator: Wire and Cable Drop, 3% Check

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Voltage Drop Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Size a long branch circuit before pulling wire

Decide between copper and aluminium feeders

Check a three-phase distribution run

Verify a low-voltage DC run for solar or 12 V

Common pitfalls

Privacy

FAQ

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