Size Your Solar Wiring Right the First Time

Enter your system voltage, current, and wire run length. Get the minimum copper AWG, actual voltage drop, and a cost comparison between two nearby gauges. Built for off-grid homesteads, RVs, and small photovoltaic systems.

Wire Size Simulator

Pick a preset below or enter your own numbers. All fields use real copper AWG resistance values at 75°C.

Maximum amps through this wire (panel Imp or controller output)
Distance from panel to controller or battery. The simulator doubles this for round-trip.
2-3% is standard for panel-to-controller runs

Enter your values and hit Calculate to see results.

How Voltage Drop Works in Solar Wiring

What voltage drop actually means

Every foot of wire has resistance. When current flows, some voltage is lost as heat. A 3% drop on a 24V system means 0.72 volts never reaches your charge controller. That power is gone. For a 15A panel, that is 10.8 watts heating up your wire instead of charging your batteries.

Why round-trip length matters

Current flows out on the positive conductor and returns on the negative. A 30-foot run from panel to controller actually means 60 feet of wire resistance. Many first-time builders forget this and end up with double the drop they expected.

When to aim lower than 3%

Panel-to-controller runs can tolerate 2-3%. Controller-to-battery runs should stay under 1% because the wire carries the full charge current at battery voltage. Battery interconnects should also be kept tight. Short runs with high current are where most people get into trouble.

Copper vs aluminum

This simulator uses copper resistance values. Aluminum wire has about 60% more resistance for the same gauge. If you are using aluminum, you will need to go one or two gauges larger. Copper stranded PV wire is the standard for solar for good reason.

Common Wiring Mistakes on Small Solar Builds

Forgetting to double the wire length for round-trip

This is the number one error. You measure 20 feet from panel to controller and enter 20. But the current travels 40 feet total. Your actual voltage drop is double what you planned for.

Using the panel Voc instead of system voltage

Your panel might say 22.4V open circuit, but if it is charging a 24V battery bank through an MPPT controller, use 24V for the calculation. The controller converts the higher panel voltage down, and the current is determined by wattage divided by battery voltage.

Mixing up one-way and total cable purchased

If you buy a 100-foot spool, you can run 50 feet one-way. The remaining 50 feet is the return path. Some people buy cable based on one-way distance and then come up short.

Ignoring temperature and conduit derating

Wire resistance increases with temperature. If your cable runs through a hot attic or is bundled with other wires, the effective resistance goes up. For runs in conduit above 30°C ambient, go one gauge larger than the simulator suggests.

Sizing wire based on cost alone

Going one gauge heavier than the minimum costs a few extra dollars but buys you headroom for future panel additions, lower losses on cloudy days when every watt counts, and a cooler wire. The cost difference between 10 AWG and 8 AWG on a 30-foot run is usually under $15.

Worked Example: 200W Panel on a 24V RV

You have a 200W panel going to an MPPT charge controller, then to a 24V battery bank. The panel Imp is about 5.8A. The run from panel roof to controller is 25 feet. You want to stay under 3% drop.

  1. System voltage: 24V (battery bank)
  2. Current: 5.8A (panel Imp)
  3. One-way length: 25 ft
  4. Acceptable drop: 3%

Round-trip is 50 feet. At 5.8A, 14 AWG gives you 4.1% drop (too high). 12 AWG gives 2.6% (good). 10 AWG gives 1.6% (great but more copper than needed). The simulator recommends 12 AWG as the minimum. The cost difference between 12 AWG and 10 AWG for 50 feet of copper is about $6. For most RV builds, 12 AWG is the right call here.