For string-level wiring between individual panels, 10 AWG (6 mm²) is the industry default and handles the vast majority of residential configurations without issue. Proper solar panel wire sizing is critical for system safety, efficiency, and compliance with electrical codes. Whether you're installing. This calculator gives you the exact AWG gauge for any solar circuit based on current, distance, and voltage. It also calculates voltage drop, power loss, and fuse size. If the wires are too thin or improperly sized, you risk suffering significant energy losses, which directly translates into reduced return on investment (ROI). Worse still, improper. A solar wire that's too small does three bad things: Going one size larger than you think you need is one of the cheapest performance upgrades on a DIY solar install — copper costs are linear-ish, but performance gains are much larger.