Voltage drop

Voltage drop is the reduction in voltage that happens as current flows along a cable — every conductor has some resistance, so the far end sees slightly less voltage than the source, and the effect grows with higher current, longer cable runs and thinner conductors. It's why lights briefly dim when a big motor (AC compressor, pump) starts and pulls a large inrush, and why a long circuit to an outbuilding or a heavy appliance needs thicker cable to keep the drop within limits. Excessive voltage drop makes equipment underperform, run hot or behave oddly, and can point to an undersized cable or a loose, high-resistance connection. Sizing cable for the load and distance — and fixing loose connections — is a licensed electrician's job.