Extension Cords And What They Can Carry

Extension cords are treated as interchangeable, and they are not. A cord has a current rating determined by its conductor size and its length, and exceeding it produces heat rather than a shutdown.

Resistance turns current into heat

Every conductor resists the flow of current slightly, and that resistance converts some electrical energy into heat along the whole length of the cord.

The heat produced rises sharply with current, so doubling the load on a cord produces considerably more than double the heating.

Unlike a short circuit, this happens gradually and quietly, and the breaker at the panel is protecting the house wiring rather than the cord plugged into it.

Gauge and length work against each other

Wire gauge numbers run backward: a lower number means a thicker conductor and less resistance per foot.

Length multiplies resistance, so a long cord of a given gauge carries less safely than a short one, and it also drops more voltage along the way.

This is why a long cord that feels adequate for a lamp is unsuitable for a saw or a heater, and why cords carry both a gauge and a maximum length.

High-draw appliances are the real risk

Motors and heating elements draw far more current than electronics. A portable space heater, a window air conditioner or a shop vacuum sits near the top of a household circuit's capacity.

Fire safety guidance in the United States generally advises against running heaters on extension cords at all, and plugging them directly into a wall outlet instead.

Where a cord must be used with a heavy load, a short cord of adequate gauge is the whole of the answer.

Coiled cords cannot shed heat

A cord run out straight loses heat to the surrounding air along its whole length. Coiled on a reel, the inner turns are insulated by the outer ones.

Cords under rugs and behind furniture have the same problem, with the added issue that damage from foot traffic goes unnoticed.

Why cords are temporary by design

Extension cords are not built for permanent installation. Their insulation is thinner than in-wall cable and they are meant to be inspected each time they are used.

A cord stapled to a baseboard or run through a doorway is being used as fixed wiring without any of the protection fixed wiring has.

When the same cord is in the same place year after year, the useful conclusion is that an outlet is missing, and adding one is the durable fix.