The NEC, National Electrical Code, specifies what type and size of wires are to be used for certain load conditions. These load conditions are based upon voltage and amperage. Most, if not all, municipalities base their local codes upon the regulations set forth by the NEC. A wire sizing calculator is available online to find the correct size wire for any application. Does this Spark an idea?
Watts
Nearly all hot water heaters are rated by wattage. This designates the amount of power that is consumed for heating the water in any given time period. If a hot water heater is rated for 5000 watts, then the heater will consume 5000 watts per hour of operation. Electric bills are calculated in the same manner but described on the bill as KWH or Kilo Watt Hour. One-kilo watt is equal to 1000 watts. A 5000-watt hot water heater then is the same as 5-kilo watts. Most modern day hot water heaters are fed from the main electrical panel from a 220-volt two pole circuit breaker.
Volts and Amps
To find which gauge wire is to be used for a hot water heater, Find the amperage used by that hot water heater. A basic electrical equation is volts times amperage is equal to wattage or (v x a = w). To find amperage from wattage simply divide the wattage by the supplied volts. In the above example of a 5000-watt heater, divide this by 220 volts. The resultant amperage is 22.7 amps. Round this number up to 23 amps to keep it simple.
Wire
According to the NEC wire sizing chart, Table 310, a 12-gauge wire will only safely carry 20 amperes. In electrical wiring never undersize the wire for the circuit. Use the next higher gauge wire to carry the electrical power. In this case it will be a 10-gauge wire which is rated for conducting 30 amperes of power. A general rule of thumb for wire sizes and amperage ratings is as follows: 12-gauge is equal to 20 amps, 10-gauge will carry 30 amperes, an 8-gauge wire will conduct 40 amperes of power, and in most applications, a 6-gauge wire will safely handle 50 amperes. Any power load larger than 50 amperes, and the NEC must be consulted. Special rules or regulations may apply for connecting these high amperage devices. In most cases hot water heaters will use either a 10-gauge or 8-gauge wire for conducting electrical power.
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