📉 Voltage Drop Calculator
Voltage drop = 2 × current × the wire's resistance-per-metre × the one-way run length (√3 instead of 2 for three-phase) — and it should stay under about 3%.
4.57 V
Voltage drop
3.81%
Percent drop
115.43 V
Voltage at load
Voltage drop is the voltage lost to wire resistance over the run: drop = 2 × current × resistance-per-metre × one-way length (√3 instead of 2 for three-phase). The US National Electrical Code recommends keeping it under 3% on a branch circuit (5% total). Aluminium has about 1.6× the resistance of copper for the same gauge. Use for DC, solar and mains runs — but follow local electrical code and a licensed electrician for real installations. 🔒 In your browser.
How the voltage drop calculator works
Every wire has resistance, so some voltage is lost between the source and the load. The calculator takes the resistance per metre of your chosen AWG gauge (higher for aluminium than copper), multiplies by the current and by the total conductor length — twice the one-way run for DC and single-phase (out and back), or √3 times the run for three-phase — to get the volts dropped. It also shows that as a percentage of your system voltage and the voltage actually reaching the load.
The US National Electrical Code recommends keeping voltage drop under 3% on a branch circuit (and 5% total including the feeder); solar installers often target 2%. Excess drop dims lights, slows motors and wastes energy as heat. To reduce it, use a thicker (lower-gauge) wire, shorten the run, use copper instead of aluminium, or raise the system voltage. This is a design aid — always follow your local electrical code and use a licensed electrician for real installations.
Frequently asked questions
How do I calculate voltage drop?
Multiply the current by the wire's resistance and by the total conductor length. For DC or single-phase that's 2 × current × resistance-per-metre × one-way length; for three-phase use √3 instead of 2. This tool does it from the AWG gauge, length, current and material.
What is an acceptable voltage drop?
The NEC recommends under 3% on a branch circuit and 5% total; many solar and low-voltage designers aim for 2% or less. The calculator flags when your result exceeds 3%.
How do I reduce voltage drop?
Use a thicker wire (a lower AWG number), shorten the cable run, switch from aluminium to copper, or run at a higher voltage. Doubling the wire cross-section roughly halves the drop.
Does aluminium wire have more voltage drop?
Yes — aluminium has about 1.6 times the resistance of copper for the same gauge, so it drops proportionally more voltage. You generally need to go up a couple of gauge sizes to match copper's performance.