Solar Wire Size Calculator
Find the minimum conductor size for a solar circuit from current, run length and the voltage drop you are willing to accept.
Results
Two things decide cable size
The first is current carrying capacity: the cable must not overheat. The second is voltage drop, and on solar runs it is almost always the binding constraint. A cable perfectly capable of carrying 30 A may still be far too thin over a 20 metre run at 12 V.
Real installations run hot
Cable in conduit, in a roof space, in direct sun or bundled with other cables carries less than its table rating. Derating factors of 0.7 to 0.8 are common, and ignoring them is how correctly calculated cable still ends up warm.
Common mistakes
Choosing cable from an ampacity table alone. On solar runs the limiting factor is nearly always voltage drop, and a cable that comfortably carries the current can still lose an unacceptable share of it over distance.
Questions people ask
Can I use ordinary mains cable outdoors? No. Array wiring needs UV-stable, double-insulated solar cable rated for the temperatures a roof reaches.
Is oversizing wasteful? Slightly more expensive up front, and it pays back in lower losses for the life of the system. Going one size up is rarely a decision anyone regrets.