Why do American homes have both 120-volt outlets and 240-volt outlets? The answer is the 120/240-volt split-phase system. Almost every house in the United States uses it. This post explains, in simple steps, how one transformer gives a home two voltages at the same time.
What Is Split-Phase Power?
Most US homes receive single-phase power at 60 Hz from the utility. The supply is called 120/240-volt, three-wire, single-phase service. It is not two-phase power. It is one phase that has been split into two halves by a transformer.
The pole transformer outside a house has a secondary winding rated at 240 volts. The middle of that winding is tapped and connected to a wire called the neutral. That centre tap is grounded at the transformer and again at the service panel in the home. The two ends of the winding are the two hot legs, called L1 and L2.
The Parts of the System
The pole transformer. Steps the utility distribution voltage down to 240 volts on its secondary winding. Its centre tap is the key to the whole system.
Hot leg L1. One end of the secondary winding. It measures 120 volts to the neutral.
Hot leg L2. The other end of the secondary winding. It also measures 120 volts to the neutral, but it is 180 degrees out of phase with L1, so L1 to L2 measures the full 240 volts.
The neutral. The centre-tap wire. It is the return path for 120-volt circuits, and it is grounded at the transformer and at the service panel.
The equipment ground. A bare copper or green wire. It is a safety path only and should not carry current in normal use.
How Do We Get 120 V and 240 V?
The maths is simple. From either hot leg to the neutral you measure 120 volts. From L1 to L2 you measure the full 240 volts, because the two halves add together. So one transformer gives you two useful voltages: 120 volts for light loads and 240 volts for heavy loads.
Standard wall outlets, lights and small appliances run on 120 volts. Large loads such as electric ranges, clothes dryers, water heaters, central air conditioners and EV chargers usually run on 240 volts. A 240-volt load draws about half the current of a 120-volt load of the same power, so the wires and breakers can be smaller. That is the main reason heavy appliances use 240 volts. For example, a 4500-watt water heater draws 4500 / 240 = 18.75 amps at 240 volts, but it would draw 37.5 amps at 120 volts.
Diagram: In a split-phase system, each hot leg measures 120 V to the grounded neutral, and the two hot legs together measure 240 V.
What Do the Three Wires Do?
The service brings three wires into the house: hot L1, hot L2 and neutral. In the panel, L1 and L2 connect to two separate bus bars. A normal single-pole breaker connects to one bus bar and gives a 120-volt circuit. A double-pole breaker connects to both bus bars and gives a 240-volt circuit.
The neutral is the return path for 120-volt circuits. It carries only the unbalanced current. If L1 and L2 carry equal 120-volt loads, the neutral current cancels out to almost zero. That is why balancing the loads between the two legs matters in a panel. The neutral is not the same as the ground wire. The ground wire, which is bare copper or green, is a safety path only. It should not carry current in normal use, and it is bonded to the neutral only at the main service panel.
Difference Between Neutral and Ground
Neutral (white or grey wire): a current-carrying conductor. It is the normal return path for 120-volt circuits, and it carries the unbalanced current. It is grounded at the transformer and bonded to ground only at the main service panel.
Ground (bare copper or green wire): a safety conductor only. It carries no current in normal use. Its job is to give fault current a low-resistance path back to the source so the breaker trips quickly if a live wire touches a metal case.
In short, the neutral works every day, and the ground waits for a fault. Mixing them up, or bonding them together at a sub-panel, is a common and dangerous mistake.
Wire Colours and Voltage Range
In US homes, the hot wires are usually black for L1 and red for L2. The neutral is white or grey, and the equipment ground is bare copper or green. The nominal voltage is 120 volts, but the utility is allowed a range. Under ANSI C84.1, the normal delivery range is 114 to 126 volts for a 120-volt service, and 228 to 252 volts for a 240-volt service. So a reading of 118 volts at an outlet is normal, not a fault.
Diagram: A home breaker panel. Single-pole breakers feed 120 V circuits, while double-pole breakers, such as the 30 A dryer breaker, feed 240 V circuits.
Key Takeaways
US homes use 120/240-volt split-phase power at 60 Hz. A centre-tapped transformer gives 120 volts from each hot leg to neutral and 240 volts between the two hot legs. Small loads use 120 volts, heavy loads use 240 volts, the neutral carries only the unbalanced current, and the ground wire is for safety only. Once you can draw this system from memory, breaker panels, outlet circuits and appliance ratings all start to make sense.
Objective Questions
1. What is the standard residential supply voltage and frequency in the United States?
a) 230 V, 50 Hz
b) 120/240 V, 60 Hz
c) 120 V, 50 Hz
d) 240 V, 50 Hz
Answer: (b) 120/240 V, 60 Hz
Explanation: Almost every home in the United States uses 120/240-volt, three-wire, single-phase service at 60 Hz.
2. In a split-phase system, what is the voltage between the two hot legs, L1 and L2?
a) 120 V
b) 208 V
c) 240 V
d) 480 V
Answer: (c) 240 V
Explanation: From L1 to L2 you measure the full 240 V of the transformer secondary, because the two 120 V halves add together.
3. What current does the neutral wire carry in a split-phase system?
a) The full current of both hot legs added together
b) Only the unbalanced current between L1 and L2
c) No current at all, ever
d) The same current as the ground wire
Answer: (b) Only the unbalanced current between L1 and L2
Explanation: The neutral is the return path for 120 V circuits, and it carries only the unbalanced current. If L1 and L2 carry equal 120 V loads, the neutral current cancels out to almost zero.
4. Under ANSI C84.1, what is the normal voltage delivery range for a 120-volt service?
a) 100 to 110 V
b) 114 to 126 V
c) 120 to 132 V
d) 110 to 120 V
Answer: (b) 114 to 126 V
Explanation: Under ANSI C84.1, the normal delivery range is 114 to 126 V for a 120 V service, and 228 to 252 V for a 240 V service.
5. A 240-volt electric water heater is rated at 4500 watts. How much current does it draw?
a) 9.4 A
b) 18.75 A
c) 37.5 A
d) 45 A
Answer: (b) 18.75 A
Explanation: Current I = P / V = 4500 / 240 = 18.75 A.
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