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Voltage Drop Calculator

Voltage drop over a cable run, against the 3% limit.

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Every size on this run

SizeΩ per 1000 ftDrop%Within 3%

Maximum run at 3%

How far this conductor can carry your load before the drop becomes excessive.

CurrentMax run at 3%Max run at 5%

Voltage drop

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Details

Drop in volts—
Voltage at the load—
Total conductor length—
Power lost in the cable—

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Smallest conductor that stays within 3%

Voltage drop is a length problem, not a current-rating problem — a conductor perfectly rated for the load can still be inadequate over a long run. Three percent on a branch circuit is guidance in most codes rather than a hard requirement. Real wiring work belongs to a qualified electrician.

Quick Tool Overview

The Voltage Drop Calculator works out how much voltage a cable run loses from conductor size, length, current and material, checks it against the three percent branch-circuit recommendation, and suggests a larger size when the run exceeds it.

What it does
Calculates voltage drop over a cable run
You give it
Voltage, current, run length, conductor size and material
You get back
Voltage drop, percentage and a size recommendation
Best for
Long runs, outbuildings and lighting circuits
Calculators Free, unlimited use No sign-up Runs in your browser

About the Voltage Drop Calculator

Calculate how much voltage a cable run loses, from the conductor size, its length, the current it carries and whether it is copper or aluminium. The result is checked against the three percent recommendation for a branch circuit and five percent overall.

Voltage drop is a length problem rather than a safety-current problem, which is why it catches people out. A conductor perfectly rated for the current can still be inadequate over a long run, because the drop is proportional to distance. The usual symptoms are dim lighting, motors running hot and equipment resetting under load. The fix is almost always to go up a size — and the calculator suggests the smallest conductor that would bring the run within limits.

Key features of the Voltage Drop Calculator

Checked against the limit

Three percent on a branch circuit and five percent overall, with a clear pass or fail.

Suggests a larger conductor

When the drop is too high, the smallest size that would fix it is named.

Copper and aluminium

Aluminium needs about two sizes larger for the same drop, and the tool accounts for it.

Single and three phase

Three phase uses √3 rather than doubling for the return path.

AWG and mm²

Both conductor sizing systems are supported.

How to use the Voltage Drop Calculator

  1. 1

    Enter the circuit

    System voltage, whether it is single or three phase, and the current drawn.

  2. 2

    Set the run length

    One-way distance. The calculator doubles it for single-phase return.

  3. 3

    Choose the conductor

    AWG or mm² size, and copper or aluminium.

  4. 4

    Check against the limit

    The drop as a percentage, with a recommended size if it exceeds three percent.

Frequently asked questions

Three percent on a branch circuit and five percent total from the service to the furthest outlet is the usual recommendation. It is guidance in most codes rather than a hard requirement.

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