Flat extension cords are for light, temporary use.

Yes, shape affects both durability and safety—but not in the way most people think. The difference comes down to what the cord was designed to do.

Flat cords (often called “zip cords”) are cheap, flexible, and easy to hide under rugs or along baseboards. They’re fine for lamps, phone chargers, and other low-wattage stuff. But that flat construction means the two conductors run side-by-side with minimal insulation between them. They’re more prone to pinching, cutting, and overheating if you pull more than 10 amps through them. And burying one under a rug is a bad idea—it traps heat and hides damage.

Round cords are built tougher. The conductors are twisted together inside a thicker, circular jacket. That gives them better crush resistance, better moisture sealing, and the ability to handle higher amperage (think 12-gauge for saws or space heaters). Round is also what you want outdoors or anywhere the cord will get stepped on, dragged, or exposed to weather.

So shape is a shorthand for the job the cord was meant to do. Flat = light indoor duty. Round = heavy or outdoor duty. If you’re asking whether one is safer, the round cord wins for anything that draws real power.

Don’t use a flat cord for your table saw. That’s how garage fires start.