Which voltage drop should I allow?

For inverter cables 1 to 2 % is common, for lighting and signal circuits 3 %. The lower your limit, the thicker the cable.

Why does this differ from a table online?

Tables assume a fixed length and a fixed allowed drop. This calculator uses your length, current and limit, and the same ampacity data as the check in the editor.

Does this apply to 230V?

The formula holds, but at 230V the relative drop is far smaller and separate wiring regulations apply to size and protection.

What about temperature and bundling?

These figures assume free air at roughly 30 °C. In a hot engine bay or inside a bundle the conductor should be sized up.

Calculator

Wire size calculator for 12V and 24V

Enter the power or current, the cable length and the voltage drop you allow. You get the smallest standard cross-section that carries the current and stays within that limit.

Free, no account needed
12V, 24V, 48V and 230V
Result while you type
Carries over into your diagram

System voltage

Calculate with

Load power WCable length (one way) mAllowed voltage drop %Conductor material

Recommended cable

35 mm²

Current: 50 A
Voltage drop: 0.2 V
Drop percentage: 1.67 %
Power loss: 10 W
Cable can carry: 150 A

Based on free-air copper cable at around 30 °C, counting both the supply and return conductor.

The result is guidance based on your input. Manufacturer instructions and local regulations always take precedence; you remain responsible for the installation.

How conductor size is decided

There are two limits. The first is ampacity: how many amps the cable carries without overheating. The second is voltage drop: how much voltage you lose over the outgoing and return conductor. At 12V that second limit is almost always the strictest, so a cable that 'handles the current' can still be too thin.

Length counts twice

Current flows there and back, so a one-way run of three metres means six metres of conductor in the formula.

Copper or aluminium

Aluminium conducts about 60 % as well as copper, so at the same size you lose more voltage.

Take the strictest limit

Use the thicker of the two results. In the editor that happens automatically, per connection in your diagram.

From calculation to complete diagram

Designing a complete electrical system? In WireElectric you draw every component and cable in one diagram, enter the length per cable and get the cross-section checked per connection instead of one calculation at a time.

You can start drawing without an account.

Open WireElectric

WireElectric editor showing a 12V campervan diagram: LiFePO4 battery, MEGA fuse, busbars, MultiPlus-II and SmartSolar MPPT with cables between the terminals.

Questions about wire gauge

Which voltage drop should I allow?: For inverter cables 1 to 2 % is common, for lighting and signal circuits 3 %. The lower your limit, the thicker the cable.
Why does this differ from a table online?: Tables assume a fixed length and a fixed allowed drop. This calculator uses your length, current and limit, and the same ampacity data as the check in the editor.
Does this apply to 230V?: The formula holds, but at 230V the relative drop is far smaller and separate wiring regulations apply to size and protection.
What about temperature and bundling?: These figures assume free air at roughly 30 °C. In a hot engine bay or inside a bundle the conductor should be sized up.

Other calculators

Every calculator answers one question about your installation. WireElectric then runs the same calculations for every connection in your complete diagram.