The amp rating of a wire measures the number of amperes of current it can safely carry. Exceeding this rating can destroy the wire, leading to a potential fire. Moreover, building codes require residential wiring to have a certain amp rating that varies between jurisdictions; improper wiring can lead to levied fines in addition to property damage and injury from an overloaded circuit.
Amperes are the measure of the electrical current traveling past a certain point. Electrical current is the flow of electrons through a medium. As particles of matter, electrons take up space and, therefore, create friction when they travel along a wire. Too much friction creates excess heat and can melt the wire. Because of this, the wire needs to be wide enough to allow the electrons adequate movement space.
To determine what amp rating the wiring of a circuit should have, the person should know the total draw of the circuit and the ambient temperature of the location through which the wire is intended to travel. To determine the current draw for an appliance, the person should divide household voltage (120) by the wattage of the appliance, then add all the numbers together. Then, the person should take ambient temperature and consult the most recent version of local electrical codes, cross-referencing the data to find the appropriate wire gauge.