🔌Voltage Drop Calculator

About this tool

Choose mm² and metres or AWG and feet, the conductor metal, wire size and phase. Then enter the one-way cable length, the current, the supply voltage, the conductor temperature and your target maximum drop. The calculator shows the drop in volts and percent, the voltage at the load and the smallest listed size that meets your target.

How voltage drop is calculated

Voltage drop is current × the cable's resistance. In single-phase and DC circuits current flows out and back, so the resistance is for twice the one-way length. For three-phase line-to-line drop it is √3 × the length. Resistance is resistivity × length ÷ cross-section. Copper is 0.0172 and aluminium 0.0282 Ω·mm²/m at 20 °C, corrected here to the conductor's working temperature. A 100 ft run of 12 AWG copper carrying 16 A at 120 V and 75 °C drops about 6.2 V, or 5.2%. In metric, 30 m of 2.5 mm² copper carrying 16 A at 230 V drops about 3.4%, and moving up to 4 mm² brings it down to about 2.2%. Common targets are 3% for a branch circuit and 5% in total, from the US NEC's informational note, or 3% for lighting and 5% for other uses under the UK's BS 7671. Honest limits: reactance is ignored, which matters for large cables and motors, and voltage drop is only one check alongside current rating, protection and installation method. Electrical work can be dangerous and is regulated in many places, so use a qualified electrician. The Ohm's law calculator handles the basic relationships, the electricity cost calculator prices the energy and the solar panel calculator sizes systems where long DC runs are common.

Frequently asked questions