What is the safe maximum length for a 16-gauge extension cord with a 10-amp tool?

Stick to 50 feet for a 16-gauge cord with a 10-amp tool. Voltage drop is real — at 10 amps, every 50 feet drops about 3-4% of voltage, and going to 100 feet causes a 7-8% drop that can slow the tool, overheat the motor, and void warranties. Most tool manufacturers recommend keeping drop under 5%. If you need more than 50 feet, step up to 14-gauge (good to 75-100 ft) or 12-gauge (100+ ft).