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Cable load, or wire load, is a measure of the amount of electrical energy in watts that a power cable can safely handle. The basic formula for figuring cable load in watts is amps of current multiplied by the voltage. Resistance and other factors are why a power cable can handle only a specific amount of electrical energy before the heat created by the wire's natural resistance to the electricity passing through it begins to cause the wire to overheat, creating dangerous conditions.
A safe cable load capacity is measured in watts, but it is necessary to figure the safe amperage capacity of the circuit as well. This is done by multiplying cable load and then multiplying this number by 0.8 to get the safe amperage capacity for the wiring. When cable load and amperage capacity are not carefully observed, wiring insulation breakdowns and electrical malfunction are inevitable.
Precise cable load is determined by a number of factors, including the resistance, length and insulation type of the wire as well as the location of the wiring and the ambient temperature where the device will be used. The different types of metal used in manufacturing the wire and the type of wire are also accounted for when figuring a safe cable load. Some metals are better conductors of electrical current, so wires made from these metals will produce less resistance to the electrical current and will operate at cooler temperatures.
When electrical current travels in a power cord, it does not pass through the actual metal wire. Instead, it travels on the surface of the wire. This is why stranded wire, which is composed of many smaller wires braided together, has the ability to carry a greater cable load than a solid wire that has the same diameter and length but has a smaller total surface area. The stranded wire produces less resistance to the flow of electricity and, in turn, less heat, thus raising its safe watt and amperage load capacity.
Using a smaller wire than what is called for in a schematic can be dangerous. When figuring safe cable loads for circuits used to power electrical equipment, appliances and other projects where the voltage drop may vary, a person should always err on the side of caution and use larger-diameter wiring when possible. One should also avoid bundling wires together when installing wiring in a building, because the safe cable load of wire bundles is greatly reduced because the heat is not able to escape.