Most electric dryers and air conditioners require larger 30 amp circuits. So, if we have a 120 volt system and a 15 amp current, we can flow a maximum of 120 x 15 or 1,800 watts on a typical lighting or appliance circuit. When too many lights or appliances are attached to the electrical system, it will overload and overheat. This can cause the wire insulation to melt and ignite, resulting in an electrical fire. The amount of electrical current flowing through wire is affected by resistance.
Resistance causes increased heat in the wire.
Heat is the byproduct that makes some appliances work, such as an iron, toaster, stove or furnace. Large current faces high resistance when moving through a small wire. This generates lots of heat. That's how an incandescent light bulb works. Resistance through the light filament causes it to heat up which gives off a bright light.