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American Homes Use The 3 Wire System

North American Homes use a three-wire supply with many good points when installed and properly maintained. However, there are problems with this system, especially when “do-it-yourself” individuals do the work. There are two basic problems. In North America, the electricity supply comes to the home as three wires. Ground connections are created at home using driven ground rods, buried plates, or a metal water line. The third wire is bonded to this ground connection. The three wires come from a transformer’s “center tapped” secondary winding. Two outer legs have 240 volts across them. The third wire is the center tap, so there are 120 volts between this wire and either of the other two wires. The third wire carries only the unbalanced load and not the full load current. The center tap is connected to a grounding conductor at the transformer. The frequency is 60 Hertz. Since there are two “Live” wires, there are two main fuses or, more usually, one Double Pole circuit breaker. Undoubtedly, the North American system attempts to provide more safety than the European system. Portable appliances and light bulbs use 120 volts. Fixed appliances and heavy heating loads use 240 volts. In Europe, light bulbs and portable appliances use 240 volts at 50 Hertz on a two-wire system. More on this website

What are the problems with this system? The first problem comes because we need to use double-pole circuit breakers for the fixed load circuits. If this is done properly, there is no problem. However, there are cases where two single-pole circuit breakers are used instead of one double-pole breaker. The appliance stops working if one of these single pole breakers is tripped or switched off. The appliance is still “Live” because the second circuit breaker is still on. Trained people know how to test for this and avoid a shock. The do-it-yourself individual may receive a shock. If the correct circuit breaker is used, both poles will trip, and there is no problem. This is one of those cases where “making it work” does not mean “making it safe” A typical situation may involve a person who has decided to move an electrical baseboard heater (perhaps to install a new carpet or tile). This person may lower the setting of the thermostat to the lowest setting. This will stop heat from being produced, but there is still one live wire at the heater. Most untrained people would assume that there are no live wires present, just as would be the case with a light fixture that is switched off. Looking at a 240-volt thermostat, you will notice that it probably does not have an OFF position marked. This is because it must completely disconnect the heater if it says OFF. In other words, it must be a 2 pole thermostat. Most thermostats are only a single pole.

The second problem is caused by a poor neutral (third wire) connection. The severity of this problem depends on where the poor connection is located. For example, if it happens at a receptacle (plug) in the kitchen, you may destroy a small appliance. This is how it happens. Remember that we have 240 volts available. Each half of the duplex receptacle in a kitchen plug is on a different “live” leg, and they have a common neutral (3 wires). If the neutral wire is disconnected or has a poor connection, we have two 120-volt appliances plugged in to form a series circuit. Suppose we have a clock radio plugged into the bottom outlet and a kettle into the top outlet. The clock radio has a resistance of about 30 times (and maybe more) than the kettle. When the neutral is removed from the circuit, we have a series circuit, and the current is common to both appliances. The voltage drop across each appliance will be the common current multiplied by the resistance. If the clock radio has 30 times the resistance, it will have most 240 volts across it. We will get 8 volts across the kettle and 232 volts across the clock radio. Guess what happens to the clock radio. The U.S.A. has dealt with the kitchen receptacle problem by using 20 Amp 120 Volt two-wire circuits, whereas Canada still allows 3 wire circuits for the kitchen with the option of using 20 Amp 120 Volt circuits. If the poor connection is back at the main panel and on the incoming supply, many devices that draw low currents can be destroyed. This includes expensive computer equipment, sound equipment, telephone equipment, etc. There will be damage if you have a large load on one “live” leg and a relatively small one on the other “live” leg. If your light bulbs seem to burn out too quickly, this could cause the problem. The problem is more common in homes that use Aluminum wire. Many untrained individuals do not fully understand how important the grounding connection is to the system. People have been known to remove the main grounding wire while gardening because they thought it was unimportant (only the ground).