Undersized wire causing voltage sag under load

Voltage sag under load (lights dim when something big kicks on, inverter alarms during heavy draw) even with a healthy battery often traces back to undersized wire gauge for the actual current and cable length involved — voltage drop over distance is real and gets worse with thinner wire and higher current. A voltage drop calculator with your actual wire length, gauge, and expected current will tell you fast whether your wiring is adequate. How long is the run and what gauge wire is currently installed?