Wire gauge determines the safe operating current in an extension cord.
The gauge of a wire refers to its cross-sectional area. American wire gauge, or AWG, uses numbers of increasing size to identify thinner wires. Cross-sectional area influences the resistance of a wire to a current flowing along it. As area decreases, resistance increases and the wire becomes hotter. The gauge expresses the thickness of a wire so AWG relates to the maximum safe current load in the wire. Finding the AWG for an extension cord is a straightforward process.
Instructions
1. Establish the total number of watts drawn by the appliances connected to the extension cord. Most appliances and power tools display a plaque giving details of their operating voltage and maximum wattage. If more than one appliance will run contemporaneously, combine their wattages. For example, an extension cord powering a 100-watt light, a 2,000-watt electric drill and a 1,000-watt heater must cope with a combined wattage of 100 + 2,000 + 1,000, or 3,100 watts.
2. Determine the voltage supply to be used with the cord. Cords used with 220-volt receptacles must have 220 volt ratings. Using 110 volt cord with higher voltages is unsafe. Use Ohm's Law, watts/voltage = current, to find the current drawn by the appliance. For example, 3,100 watts/110 volts equals a current of 28.2 amps.
3. Calculate the AWG using the formula AWG = 50 - (10 * log (700 * current)). For example, to find the AWG for a wire to carry 30 amps, AWG = 50 * (10- log (700*30). This reduces to 50 * (10 - log (21,000)), or 50 - 10*4.3. The AWG is, therefore 50 - 43, or 7.
4. Decrease the AWG by one step for every additional 100 feet of cable to compensate for voltage drop. If the extension cord is 100 feet long, and the calculations suggest using 12 gauge, use 11 gauge cord. If it is 200 feet long, use 10 gauge, and so on.
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