Wiring residential homes with 240 volts is a necessity, for powering some heating and cooling equipment as well as some large appliances. The 220-volt circuits as they were known prior to the 1960s are now commonly known as 240-volt circuits and 110-volt circuits are now 120-volt circuits. However, people still use the old 110/220 volt terms in conversation, but in reality, those have not been used since the 1960s and 1970s in most locations.
The 240 volts feeding the main electric panel runs on two different wires each carrying 120 volts with respect to a third wire called the neutral and common to both. Each 120-volt line runs 180 degrees out of phase with the other. With the neutral as common to both, if a multimeter was used to measure across the neutral line and any one of the two colored power (hot) lines, the reading would be 120 volts. If it was measured across the two power (hot) lines, it would be at 240 volts. This allows half the amount of amperage to power a 1500 Watts heater, for instance, as opposed to running a 1500 Watts portable heater plugged into a 120 volts outlet since the current used (in amps) is calculated by dividing the Wattage (1500 in each case) by the voltage which is 240 in the first and 120 in the second case.
Ways to Wire
There are two ways to wire up an appliance on 240 volts, depending on the appliance’s needs. If the appliance is a stove and requires 120 volts to operate a clock, certain controls, and control board along with 240 volts to run the elements, a three-wire cable will be needed to hook it up with two hot wires and a neutral which will provide the 120 volts across either one of the hotline. On the other hand, if a hot water heater is being hooked-up, a two-wire cable with two hotlines will be required, without a neutral wire.
Two-wire and Three-wire Cables
Most of today’s common appliances and fixtures operate off 120-volt wiring. A lot of high wattage appliances, however, will require 240 volts hookup to operate at a lower power consumption rate. A 240 volts hookup is usually provided through a two-wire cable such as 12/2, 10/2, or 8/2 or three-wire cable such as 12/3, 10/3, or 8/3, the last digit being the number of wires in the cable excluding the ground wire. In 12/2, 10/2, and 8/2 cables, both wires are hot and carrying 240 volts between them with the white neutral line absent. In a 12/3,10/3, and 8/3 cable, the two colored wires (can be black, red, or blue insulation) are always hot and the one white wire is always neutral, with the green or copper ground wire not counted in the coding of the cable.
Any 240 volts circuit will be protected through a two-pole breaker, which is essentially two breakers positioned side-by-side in a larger casing with the arms of both mechanically tied together so that if one is tripped, both sides of the line go off. The National Electrical Code (NEC) now requires that this type of wiring be used predominantly in residential home construction. That is why most of today’s dryers and oven ranges come equipped with a 4-prong plug.