120V is the AC voltage on a single hot wire in your home with respect to neutral (or ground). With resistance in the wiring in your house, this 120V will likely have dropped to 115V by the time it gets to the appliance you are powering.
At the end of a long extension cord you could even drop to 110V.
This is why you'll see the different terms used. In fact many appliances or devices will be rated to 110V or 115V which basically tells you they are tested to operate down to a lower voltage. This gives you assurance that at the end of a long circuit or extension cord it will still operate fine.
In the United States, utility companies are required to provide a split-phase 240V feed to your house. This consists of two legs of 120VAC that are 180 degrees out of phase with one another.
A Quick History of Our Electrical Grid
The picture below depicts one 120V leg in red and another in yellow. The 0V line in the middle is the voltage of your neutral line.
AC power oscillates at 60 cycles per second (60 Hz) in the U.S. Other parts of the world use a 50Hz standard.
The AC voltage oscillates from +120V to -120V. With the two legs feeding your house being out of phase, can you pick up 240V AC by using both legs rather than one leg and neutral.
The Difference in Outlets
When you use both legs to feed a circuit, like a range or water heater, you end up with twice the voltage (a line that oscillates from +240V to -240V). The neutral wire is not utilized in a 240V circuit as the current is fed by one leg and returned on the other leg.
Putting Volts, Amperes and Ohms Together
A word of caution..when you see something labeled 208V or 480V these are not the same as what is described above. These voltages refer to 3-phase power systems, more typically found in commercial or industrial applications for large motors and other equipment.
These power systems are 3-phase where 208V is the voltage between two phases of a Y-connected circuit that is 120V from neutral to any single phase.
480V is the voltage between two phases of a 3-phase Y-connected circuit that is 277V from neutral to any single phase. Do not attempt to connect a motor or any other equipment designed for 3-phase power to a 2-phase power system like that feeding your house.
It will not work and you will damage the equipment and be sorely dissappointed.
Don’t Get Confused
Homeowners should be aware of the voltage of their laptops and other electronic devices to protect against electrical overloads.
What are the different voltages: 110/115/117/120/125/220/240? One thing where things might get a bit confusing is the different numbers people bandy about for the voltage of a circuit. One person might talk about 110V, another 117V or another 120V.
These are all, in fact, exactly the same thing… In North America the utility companies are required to supply a split-phase 240 volt (+-5%) feed to your house.
This works out as two 120V +- 5% legs. Additionally, since there are resistive voltage drops in the house wiring, it’s not unreasonable to find 120V has dropped to 110V or 240V has dropped to 220V by the time the power reaches a wall outlet.
Especially at the end of an extension cord or long circuit run.
So, Which Is It 110 volts or 120 volts?
For a number of reasons, some historical, some simple personal orneriness, different people choose to call them by slightly different numbers.
This FAQ has chosen to be consistent with calling them “110V” and “220V”, except when actually saying what the measured voltage will be. One thing that might make this a little more understandable is that the nameplates on equipment often show the lower (ie: 110V instead of 120V) value.
What this implies is that the device is designed to operate properly when the voltage drops that low. 208V is *not* the same as 240V. 208V is the voltage between phases of a 3-phase “Y” circuit that is 120V from neutral to any hot.
Volts, Amperage, and Ohms
480V is the voltage between phases of a 3-phase “Y” circuit that’s 277V from hot to neutral. In keeping with 110V versus 120V strangeness, motors intended to run on 480V three phase are often labeled as 440V.
Learn more about What does an electrical service look like? Many people refer to the voltage at the electrical outlets as 110 volts. Others routinely call these 120-volt outlets. If you plug almost any appliance into these plugs, the appliance works fine in either instance.
Do some power companies deliver 110 volts and others 120 volts? What is the real story about 110V vs. In the United States, there is no difference between a 110-volt outlet and a 120-volt outlet.
The 240 Volt Part of the Puzzle
The standard followed by power companies is to deliver electrical service to homes at 120 volts plus or minus a ten percent variance. Appliances in the US market are designed to operate within these variances.
The voltage supplied to your home’s electrical outlets is only part of the story. There are several other components of your electrical service that you must understand. Many of us take the quality and dependability of the electrical service we enjoy in the United States for granted.
Read on for a deeper grasp of our electrical service’s historical and technical side. In the 1880s, the electrification of the US was just beginning.
Amperage – Capacity of Work Available
Thomas Edison introduced an early electrical grid system based on direct current generation and electrical transmission. These direct current systems had a major problem.
DC current is hard to transmit over wires for very long distances. In Edison’s early designs, the electrical grid required a power generation station almost every.
Voltage – The Potential to Do Work
With the help of George Westinghouse, Nikola Tesla pioneered the use of alternation current to deliver power to homes and businesses.
The war of currents had begun. Alternating current gained the advantage over DC. AC current voltages can be easily stepped up and down.
This makes transmission of huge amounts of electricity over long distances possible.
The national electrical grids that we enjoy today are direct descendants of the systems Tesla and Westinghouse pioneered.
Over the course of time, power generation stations in the US delivered various voltages to homes and businesses.
Since the prevalent usage was to provide light, the difference in voltages across the landscape wasn’t much of a problem.
Ohms – The Resistance to Work
As the availability of electrical appliances began to grow, it because apparent that this range of voltages was a problem. Eventually, as the grids grew and electrical usage in homes and businesses expanded, standardization across the grid became the norm.
At the user end of the grid, the difference between 100 volts and 120 volts is so minute that they are unnoticeable by the end-user.
Eventually, a national standard for electrical service delivery was adopted.