Voltage Drop Calculator

About this calculator

Running a long cable to a workshop, pump, or outbuilding always loses some voltage to the resistance of the wire itself, and if the drop is too large, motors run hot, lights dim, and sensitive electronics misbehave, yet sizing a conductor by eye or rule of thumb often ends up either dangerously undersized or wastefully oversized. This calculator supports DC, single-phase AC (round-trip, ×2 factor) and three-phase AC (×√3 factor) systems, taking the cable's one-way length in meters or feet, the load current, the conductor's cross-sectional area in mm² or AWG, and the resistivity of its material (copper or aluminum) to compute exactly how many volts are lost between source and load. It then expresses that loss as a percentage of your supply voltage, the number electrical codes actually reference, and flags it as low, moderate or high — a rough guide only, not a code-compliance check, since actual limits vary by jurisdiction and circuit type. Instead of consulting cable-sizing tables or guessing a wire gauge and hoping, you get an immediate, precise answer, and can quickly test what a larger cross-section, a different system type, or a shorter run would do to the result.

How to use this

1. 1Enter the one-way cable length from source to load.
2. 2Enter the load current in amps and the conductor's cross-sectional area.
3. 3Select the conductor material (copper or aluminum) and enter the supply voltage, then read the voltage drop and percentage drop.

Troubleshooting

• Should I enter the one-way or round-trip cable length?

  Enter the one-way length only, the formula already accounts for the round trip (current flows out and back) by doubling the length internally.

• What percentage voltage drop is acceptable?

  A common guideline is under 3% for a single branch circuit and under 5% for the combined feeder and branch circuit, check your local electrical code for the exact limit that applies to your installation.

Frequently asked questions