The control panel of a 120/240v generator. 120/240 volts means the generator can supply 120 volts, 240 volts, or a combination of both, all at the same time.
Why 120/240 Volts?
Almost anything you plug into an outlet at home is a 120-volt appliance. This applies to lights, kitchen appliances, televisions, and even the charging blocks for your electronics. In North America, 120 volts is the most common voltage for household use. There are folks who still call it 110, and a few will use 115. Many appliances still have 115 volts listed on their appliance tag.
The first systems in America used DC current at 110 volts. Thomas Edison called it a “safe” voltage and went to great lengths to defend that position. He eventually admitted that Alternating Current (AC) was better. In the 1930s, the voltage increased to 115 volts to better accommodate more appliances. By the 1950s, incandescent light bulbs were 120 volts.
Voltage drops over long distances. Using electricity also drops the voltage. The industry standard is 5% over or under the rated voltage. For homes, the 120-volt range is 114 to 126 volts, and for 240-volts it is 228 to 252 volts in a 120/240 volts systems.
At higher voltages, small voltage drops due to distane and use make less of a difference, thus the change from 110 volts, to 115, to 120 volts.
The United States adopted a 120/240 volt split phase system of distribution to homes and most businesses in the 1960s.
120/240 volts became the new standard in 1967 and 1968. The National Electrical Code (NEC) reinforced the standard with changes to the code at the same time.
In 1984, the NEC officially changed the standard voltage to 120 volts for accurate load calculations.
Today, utilities deliver 120/240 volts split phase current to nearly all homes in North America.
Understanding 120/240 Volt Power
In the United States and all of North America, utility companies generate 3-phase current for distribution throughout the country. It’s an efficient way to generate and deliver power. 3-phase electric motors are much more efficient than single-phase motors.
However, delivering 3-phase power requires more distribution lines and equipment. Each change in the voltage requires three transformers instead of one or a dedicated 3-phase transformer. This adds cost to the utility connection and the property’s service entrance equipment.
Homes and many small businesses don’t need 3-phase, but most use a combination of 120 and 240 volt appliances or equipment. Enter the “Split-Phase” system for residential and light commercial applications.
A transformer takes power from a one phase of the 3-phase distribution lines and splits it into two 120 volts lines (L1 and L2) and adds a neutral (N). Voltage measured from either L1 or L2 to N is 120 volts. Voltage measured L1 to L2 is the sum of both lines. 120V + 120V = 240 volts.
240 volts runs central air conditioners, electric dryers, water heaters, electric ranges and cooktops, and sometimes well pumps. There are other, less common applications as well. Some 240-volt appliances have electronics or controls that require 120 volts. By adding a neutral line to the appliance hookup, the controls can use 120-volt power from L1 + N or L2 + N, while the heating elements or motor use L1 + L2.
120/240v Generator Outlet
A 120/240-Volt 50-Amp receptacle with drawing labeled to show how it delivers 120 volts and 240 volts at the same time.
A modern 240-volt outlet has three slots for L1, L2, and Ground (G) while a 120/240-volt outlet has four slots for L1, L2, N, and G.
Before the NEC required grounded outlets in homes, it was common for 120/240-volt outlets to have just three slots for L1, L2, and N. If a 120/240-volt appliance had a metal enclosure, a jumper connected a ground screw to the Neutral terminal in the appliance. This provided the safety path back to the neutral-ground bond in the fuse box. However, if the neutral wire broke, the metal frame could become energized, creating a shock hazard. Older homes may still have the 3-wire 120/240-volt outlets. The NEC does not require an upgrade. When possible, homeowners should have a qualified electrician install a 4-prong outlet for new 120/240-volt appliances.
30Amp 120/240-Volt NEMA L14-30R Receptacle Found on a 120/240v Generator
Modern 120/240-volt outlets have four slots for L1, L2, N, and G.
A 120/240v generator produces the same type of power as the utility service to most homes. It has two hot wires, a ground, and a neutral. The demand for quieter, more fuel-efficient generators with cleaner power has led to the development of the 120/240v inverter generator with power quality on par with or better than utility power.
The 120/240v generator outlet mimics the 4-slot 120/240-volt outlet in a home with the hot wires L1 and L2, the neutral N, and ground G. This makes generator power compatible with tools and appliances that require 120 volts, 240 volts, or 120/240 volts. It is also the safest way to connect the generator to your home through a transfer switch inlet box.
During a power outage, the homeowner connects the generator 120/240v outlet to the inlet box with a generator cable sized for the outlet rating—either 30 amps or 50 amps.
50Amp 120/240-Volt NEMA 14-50R Receptacle Found on Some Generators
L1 and L2 from the generator outlet can carry up to the full current rating of the outlet. For example, a 50-amp outlet can handle 50 amps on L1 and 50 amps on L2, for a total of 100 amps at 120 volts, or 50 amps at 240 volts. Always use a cable rated for the same current rating as the outlet and inlet box.
People often ask about when or if to ground a portable generator. For a generator with a bonded neutral, the short answer is, with a 3-pole transfer switch that transfers L1, L2, and N, connecting the generator ground terminal to the home’s ground rod is the safest way to do it. If you’re just running extension cords into the house or plugging tools or appliances into a generator outlet, grounding the frame is unnecessary.
It is important to understand how the factory configured your generator’s ground—either bonded neutral or floating neutral. If you just plug appliances or tools into the generator outlets, you won’t run into safety problems. Some electronics require a neutral-ground bond. A ground plug solves this issue. They are inexpensive and use one of your generator outlets.
Another way to use a generator with a 120/240v outlet is an extension cord with multiple outlets. All the outlets share the ground and neutral wires, while each 120-volt outlet uses only L1 or L2. It reduces extension cord clutter between the generator and the tools or appliances powered by the generator. This type of 30-amp cord provides the equivalent of 4 standard 15-amp household outlets.
The 120/240v Generator Switch
The heart of a generator is the alternator. Inside are coils of wire moving through a magnetic field, inducing an electric current within the coils of wire. Interconnections between the coils of wire determine the voltage of the current produced. Connecting the coils one way provides a combination of 120 volts and 240 volts. Different connects provide only 120 volts.
It’s important to balance the load connected to the generator so that the coils share the load. An unbalanced load can damage the generator or cause the circuit breaker to trip.
Some applications use only 120 volts. For example, most RVs use 120 volts only. Connecting an RV to a 120/240-volt generator would place the entire load on one coil, and none on the other coil—a severely unbalanced condition. Generators intended for multiple uses, including powering RVs, may include the generator 120/240v switch.
The 120/240v generator switch changes the internal connections of the coils so that the generator only produces 120 volts. All coils share the load equally and ensure balance, even at full load.
Selecting 120 volts powers all the 120-volt outlets, including the TT-30R 30-amp RV outlet, but de-energizes any 120/240v generator outlets.
Parallel Ready: Electronics make it easy to connect inverter generators in parallel, which can double the power available if both are the same size. The parallel cords or kits may provide the 50-amp 120-volt outlet necessary for larger RVs.
120/240v Generator Safety
All generators powered by fossil fuels produce carbon monoxide, a deadly gas. Carbon Monoxide, or CO, is odorless and invisible.
Never operate a generator indoors or in any enclosed space. Opening windows and doors does not remove or lessen the danger. CO detection and shutdown features prevent misuse. They do not make it safe to run a generator indoors. Defeating a CO detection and shutdown module will void the product warranty.
Place your generator outdoors, preferably downwind, at least 10 feet away from any buildings and 20 feet from windows, doors, and other openings. Remember that your generator has the potential to impact your neighbors with both noise and carbon monoxide.
Do not backfeed your generator into your home by connecting it to an appliance outlet. Backfeeding is dangerous and illegal. If backfeeding causes injury to another person, you may be criminally liable for their injury. If it causes a fire, your insurance company may deny your claim. Backfeeding is dangerous.
Follow all Portable Generator Safety Rules
About MJ Logan
MJ Logan began his writing career in third grade creating small booklets with a construction paper cover and two or three sheets of white typing paper, carefully folded in half and glued to the construction paper. His career led from electrical engineering and electronic test to network management, independent software development, and finally back to writing.
