Quick answer
Voltage is the electrical pressure between two points that pushes current through a circuit, measured in volts (V).
NIST calls it electric potential difference and defines one volt as one watt per ampere. A US home gets two voltages from one service: 120 volts for lights and outlets, and 240 volts for dryers, ranges, water heaters and EV chargers.
Quick facts
The key facts about Voltage, with sources:
- SI unit
- Volt (V); 1 V = 1 W/A 1
- Named after
- Alessandro Volta, who made the first battery 1
- US outlets
- 120 V for most household appliances 2
- Big appliances
- 240 V for dryers and electric cooktops 2
- EV Level 2
- 240 V at home, 208 V at many businesses 3
- Grid lines
- High voltage for long distances, stepped down near homes 4
Key takeaways
- Voltage is the push; current (amps) is the flow it causes. Without voltage, no current moves.
- US homes get 120 V and 240 V from the same service. Most outlets are 120 V; heavy appliances use 240 V.
- For the same power, doubling the voltage halves the current, so 240 V loads use smaller wire for their size.
- The grid uses very high voltage for long lines because it is cheaper, then steps it down for safety near homes.
- Any household voltage can drive a deadly current. Testing and wiring belong to a licensed electrician.
Voltage explained without the jargon
Think of a garden hose. Water pressure pushes water through the hose. Voltage is the same idea for electricity. It is the pressure that pushes electrons along a wire. Higher pressure can push more flow through the same path.
The formal name is electric potential difference. NIST, the U.S. standards body, lists the volt as the SI unit of electric potential difference, and defines it as one watt per ampere 1. The unit honors Alessandro Volta, who built the first battery 1.
Voltage always sits between two points. A 120-volt outlet has 120 volts between its hot slot and its neutral slot. A bird on one power line feels no voltage because both feet touch the same wire. Touch two points at different voltages, and current can flow through you.
How voltage, current and resistance fit together
NIST defines the ohm, the unit of resistance, as one volt per ampere 1. Turn that around and you get Ohm’s law: volts equal amps times ohms. Push harder (more volts) through the same resistance, and more current flows.
Power ties in too. Since one volt equals one watt per amp, watts equal volts times amps 1. This is why big appliances run on 240 volts. A 4,800-watt water heater would need 40 amps at 120 volts, but only 20 amps at 240 volts. Less current means less heat in the wire.
Volts = Amps × Ohms · Watts = Volts × Amps
Volts (V)- The push between two points
Amps (A)- The current that flows
Ohms (Ω)- Resistance; 1 Ω = 1 V/A by NIST definition
Watts (W)- Power; 1 V = 1 W/A
Source: [1]
How one service gives a home both 120 V and 240 V
- Utility transformer on the pole or pad → three wires to the house: two hot legs and one neutral
- Hot leg to neutral → 120 V for outlets and lights
- Hot leg to hot leg → 240 V for dryers, ranges, water heaters and EV chargers
- Both legs enter the electrical panel → single-pole breakers make 120 V circuits, double-pole breakers make 240 V circuits
Voltages you will meet, from battery to power line
| Voltage level | Where it shows up | Notes |
|---|---|---|
| Low DC (a few volts to tens of volts) | Batteries, USB chargers, doorbells, thermostats | Usually direct current |
| 120 V AC | Most household appliances in US homes 2 | Level 1 EV charging uses it 3 |
| 208 V AC | Many apartment buildings and businesses | Commercial Level 2 chargers 3 |
| 240 V AC | Dryers, cooktops, water heaters, home Level 2 chargers 2 3 | Uses both hot legs |
| Distribution voltage | Lines along your street | Stepped down by the transformer near you 4 |
| Transmission voltage | Large towers across country | High voltage for efficient long-distance travel 4 |
Example: same heater, two voltages
Take a 4,800-watt load, like a water heater element. Virginia Tech Extension lists 40-gallon water heaters at 4,500 to 5,500 watts 2. Amps equal watts divided by volts 1.
| Supply voltage | Math | Current drawn |
|---|---|---|
| 120 V | 4,800 ÷ 120 | 40 A |
| 240 V | 4,800 ÷ 240 | 20 A |
| 208 V (some apartments) | 4,800 ÷ 208 | About 23 A |
Half the current at 240 V means the wire carries half the load for the same work.
Note that a heater element rated at 240 V puts out less heat at 208 V, because its resistance stays the same.
Check the nameplate before any swap, and let an electrician size the circuit with the wire size calculator logic.
Where the voltage is printed
Appliance nameplates list the rated voltage, often as 120 V or 120/240 V. Range and dryer labels may also list a 208 V rating for apartments. EV chargers list both.
Your breaker gives a clue. A single-pole breaker, one handle wide, feeds a 120-volt circuit. A double-pole breaker, two handles tied together, usually feeds a 240-volt circuit. Reading the panel label is fine. Opening the cover is not.
On a quote, you might see "120/240 V single-phase" for the service. A subpanel or EV job will list the voltage of the new circuit and its amps.
Higher voltage versus lower voltage
What higher voltage gives you
What it costs or risks
- A higher push can drive more current through a body or a fault.
- Needs a double-pole breaker and a dedicated circuit in a home.
- Lower voltage is safer near people, which is why the grid steps it down before it reaches homes 4.
What voltage alone cannot tell you
Voltage does not tell you how much power a device uses. A 120-volt night light and a 120-volt space heater share a voltage but differ by more than 100 times in watts. For that you need amps or watts.
Voltage also sags. Long wire runs and heavy loads cause voltage drop, so the voltage at the far end can be lower than at the panel. Lights that dim when a motor starts are a common sign.
What voltage changes about the cost of a job
We do not publish prices. These factors explain why a 240-volt job quote differs from a 120-volt one. See what drives an electrical project price.
- Breaker type. A 240-volt circuit needs a double-pole breaker, which takes two panel spaces.
- Wire. Bigger loads need thicker wire, whatever the voltage.
- Panel room. A full panel may need a tandem breaker, a subpanel or an upgrade.
- Distance. Long runs can need a larger wire size to limit voltage drop.
- Lifespan. Voltage itself does not wear parts out. Heat from loose connections and overloads does.
How an electrician adds a 240-volt circuit
Changing a home’s voltage setup is licensed work. Knowing the steps helps you read the quote.
- Confirm the appliance voltage and amps from its nameplate or manual.
- Check panel space for a double-pole breaker and the service capacity.
- Size the wire and breaker for the load, including any continuous-load rules.
- Pull a permit with the local building department.
- Run the circuit, install the breaker and the outlet or disconnect.
- Test, label the panel directory and pass the inspection.
Keeping voltage problems from building
- Watch for lights that dim or flicker when big loads start. Note when it happens.
- Plug sensitive electronics into a surge protector rated for the job.
- Keep cords and plugs in good shape. Replace cracked cords rather than taping them.
- Ask about whole-home surge protection if spikes have damaged gear before.
- Report repeated low or high voltage to your utility. Their side of the meter is their job.
Voltage risks and warning signs
Voltage drives current through anything that closes the loop, including a person. OSHA says shocks happen when the body touches both wires, one wire and the ground, or a metal part that became live 5. Wet skin makes it worse, because wet skin conducts 5.
Signs of a voltage problem include flickering or dimming lights, half the house losing power, appliances running weak, or lights getting oddly bright. Some of these point to a loose neutral, which can push the wrong voltage to outlets. That is an urgent call to the utility or an emergency electrician.
A tester is not a safety guarantee
Do not open outlets or panels to measure voltage yourself. Meters and testers can read wrong in untrained hands. If lights surge bright or half the house goes dark, turn off sensitive loads and call your utility or an electrician.
Rules that touch voltage
The National Electrical Code, NFPA 70, sets wiring rules by voltage. For example, 2023 NEC 210.19 covers branch-circuit conductors for circuits up to 1,000 volts AC or 1,500 volts DC; higher-voltage branch circuits moved to a new Article 235 6. The 2026 edition can be read free from NFPA 7.
Your utility sets the service voltage at the meter. The NEC governs everything after it. A state or city must adopt an NEC edition before it applies, and adoption dates differ.
Voltage compared with related terms
| Term | Measures | Unit | Water-hose analogy |
|---|---|---|---|
| Voltage | Push between two points | Volt | Water pressure |
| Amperage | Flow of charge | Amp | Gallons per minute |
| Resistance | Opposition to flow | Ohm | A narrow or kinked hose |
| Watt | Rate of work | Watt | How fast a water wheel turns |
| Frequency | How often AC reverses | Hertz | Water sloshing back and forth |
Myths about voltage
- Myth 240 V uses more electricity than 120 V.
- Reality Energy use depends on watts and hours, not voltage. A 240 V load draws half the amps of a 120 V load of the same watts.
- Myth Low voltage is always safe.
- Reality OSHA ranks harm by the current that flows, and wet skin lets more through 5. Treat any household circuit as dangerous.
- Myth Voltage is the same everywhere in the house.
- Reality Voltage drops along long or overloaded runs. Far outlets can read lower.
- Myth A 208 V appliance and a 240 V appliance are interchangeable.
- Reality Some are rated for both. Many are not. Check the nameplate.
What changes by state
Household voltage is set nationally by utility practice, so 120/240 V is the norm across states. What changes is the code edition your inspector enforces and who may do the work.
Minnesota, for example, reviewed the 2026 NEC in committee in September 2025 before adopting it 8. Check your edition and your state license rules.
When voltage matters to you
Voltage matters when you buy a large appliance, an EV charger or a heat pump. Check whether it needs 120 V or 240 V and what circuit it calls for. The appliance circuit lookup lists common ones.
Voltage also matters if lights flicker or surge. That points to a wiring or utility issue, not a bulb problem.
Match the plug before you buy
A 240-volt appliance has a different plug shape from a 120-volt one. If the appliance will not fit your outlet, do not use an adapter. Ask an electrician for the right circuit.
Next steps
- Voltage drop calculatorHow far a circuit can run
- EV charger readiness checkIs your panel ready for 240 V charging?
Questions about Voltage
What is voltage in simple terms?
Voltage is the push that moves electricity through a wire. It works like water pressure in a hose. The higher the pressure, the more flow it can drive through the same path. It is measured in volts, and NIST defines one volt as one watt per amp.
What voltage is a US household outlet?
A standard US outlet is 120 volts. Large appliances such as clothes dryers and electric cooktops use 240 volts, as Virginia Tech Extension notes. The same service supplies both. Each hot leg to neutral gives 120 volts, and the two hot legs together give 240 volts.
What is the difference between 120V and 240V?
A 240-volt circuit pushes twice as hard, so it draws half the amps for the same watts. That lets heavy loads like water heaters, ranges and EV chargers run on reasonable wire sizes. A 240-volt circuit uses a double-pole breaker and both hot legs of the service.
Is 120 volts dangerous?
Yes. Voltage drives current through the body, and OSHA lists 50 to 150 milliamps as possibly fatal. A 120-volt circuit can easily push that much through wet skin. Never touch live parts, and leave testing and wiring to a licensed electrician.
Why do power lines use such high voltage?
High voltage moves power over long distances more efficiently and at lower cost, the EIA explains. Transformers raise the voltage at the plant and lower it near homes, because lower voltage is safer for people. Higher voltage means less current for the same power, which cuts heat loss in the lines.
Does 240V use less electricity than 120V?
No. Energy use depends on watts and time. A 1,500-watt load uses the same kilowatt-hours at either voltage. At 240 volts it draws half the current, which can mean less heat in the wire, but your bill counts the same energy.
What does 120/240V on a label mean?
It means the appliance uses both voltages from a split-phase supply. A dryer, for example, may run its heater at 240 volts and its motor, timer and light at 120 volts. It needs a circuit with both hot legs and a neutral, sized by an electrician.
What causes low voltage in a house?
Common causes are long or undersized wire runs, too many loads on a circuit, loose connections and utility problems. Large motors starting up can also dip voltage for a moment. Repeated dimming, or one half of the house going weak, needs an electrician or the utility.
What is 208 volts?
It is the voltage between two hot legs of a three-phase supply, common in apartment buildings and businesses. The DOE notes commercial Level 2 chargers often run at 208 volts versus 240 volts at homes. Some appliances work on both; others put out less heat at 208 volts.
Sources
- NIST, SI Units: Electric Current (volt and ohm definitions), retrieved .
- Virginia Cooperative Extension, How to Estimate Appliance Energy Use (pub. 2901-9014), retrieved .
- U.S. DOE Alternative Fuels Data Center, Developing Infrastructure to Charge Electric Vehicles, retrieved .
- U.S. Energy Information Administration, Electricity explained: Delivery to consumers, retrieved .
- OSHA 3075, Controlling Electrical Hazards, retrieved .
- NEC 2023 210.19 Conductors: Minimum Ampacity and Size (code change summary), ElectricalLicenseRenewal.com, retrieved .
- NFPA 70, National Electrical Code, 2026 edition (free access), retrieved .
- Minnesota Dept. of Labor and Industry, NEC 2026 Adoption Review Committee minutes, 25 Sept 2025, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Unit definitions from NIST; appliance voltages from Virginia Cooperative Extension; EV charging voltages from DOE AFDC; code text from a 2023 NEC change summary. Current math is illustrative.
Suggest a correction. We fix errors and say what changed.