The standard US residential voltage is a 120/240V split-phase alternating current (AC) system, delivering 120V for general lighting and receptacles, and 240V for high-wattage appliances. If you are asking what voltage is the US grid, the direct answer is that nominal household power operates at 120V for standard circuits and 240V for heavy loads, derived from a single center-tapped utility transformer.
While older generations and legacy equipment still refer to "110V" or "220V," the modern ANSI C84.1 standard officially designates 120V and 240V as the nominal voltages. This split-phase architecture dictates everything from the double-pole breakers in your panel to the specific NEMA plug shapes on your electric range.
The US Voltage Standard: 120/240V Split-Phase Explained
The US electrical grid delivers high-voltage power (typically 7,200V) to a pole-mounted or pad-mounted transformer near your home. Inside this transformer, the secondary winding is center-tapped. The center tap is bonded to earth ground and becomes your neutral wire. The two outer ends of the winding become your hot legs (Line 1 and Line 2).
- Line 1 to Neutral: 120V (Used for standard NEMA 5-15 outlets and lighting)
- Line 2 to Neutral: 120V (Used for standard outlets on the opposite panel leg)
- Line 1 to Line 2: 240V (Used for HVAC, dryers, ranges, and EV chargers)
Because the two 120V legs are 180 degrees out of phase with each other, their potentials add up when measured across both hot legs, yielding 240V. This allows homes to safely utilize lower voltage for everyday tasks while efficiently delivering high power for heavy appliances without requiring excessively thick wiring.
| Nominal Voltage | Utilization Range A (Min) | Utilization Range A (Max) | Service Voltage (Min) | Typical Application |
|---|---|---|---|---|
| 120V | 114V | 126V | 114V | Standard NEMA 5-15 receptacles, lighting, electronics |
| 240V | 228V | 252V | 228V | Dryers, ranges, HVAC compressors, Level 2 EVSE |
| 208V | 197V | 218V | 197V | Commercial 3-phase wye (Phase-to-Phase, not residential) |
| 277V | 263V | 291V | 263V | Commercial lighting (Phase-to-Neutral on 480Y/277V systems) |
What This Changes in a Real Circuit Installation
The split-phase reality of the US grid fundamentally changes how you wire, protect, and terminate circuits compared to single-voltage systems. A 120V circuit requires a single-pole breaker, one hot wire (black), one neutral (white), and one ground. A 240V circuit requires a double-pole breaker (tying two adjacent bus bars together), two hot wires (black and red), and a ground. If the 240V appliance has 120V control boards (like a modern dryer or range), it also requires a neutral, resulting in a 4-wire setup.
Worked Numeric Example: Sizing a 240V EV Charger Circuit
Let us calculate the wire size and verify the voltage drop for a hardwired Level 2 Electric Vehicle Supply Equipment (EVSE) that draws a continuous 40A load, located 80 feet from the main panel.
- Breaker Sizing (NEC 210.20(A)): EV charging is a continuous load (operating for 3+ hours). The NEC requires the branch circuit to be rated at 125% of the continuous load.
Calculation: 40A × 1.25 = 50A. We must use a 50A double-pole breaker. - Wire Sizing (NEC 310.16): We need a conductor rated for at least 50A in the 75°C column (standard termination rating). 6 AWG THHN copper is rated for 65A at 75°C, which safely exceeds our 50A requirement.
- Voltage Drop Verification: While the NEC recommends keeping branch circuit voltage drop under 3%, we must calculate it to ensure the EVSE receives adequate power. We use the formula: VD = (2 × K × I × L) / CM.
- K (Copper resistivity) = 12.9
- I (Actual continuous current) = 40A
- L (One-way length) = 80 ft
- CM (Circular mils for 6 AWG) = 26,240
Calculation: VD = (2 × 12.9 × 40 × 80) / 26,240 = 3.14V.
Where You Meet This in Practice (and Common Confusions)
You interact with the US split-phase system every time you open a panelboard or plug in an appliance. In a standard residential panel, the main breaker is a 240V double-pole unit. The individual bus bars alternate Line 1 and Line 2 down the chassis. When you install a standard 120V single-pole breaker, it clips onto just one of those legs. When you install a 240V double-pole breaker, it spans across both legs, harvesting the full 240V potential.
Common Confusions: 110V vs. 120V and 240V vs. 208V
The most frequent error DIYers and junior technicians make is conflating historical terminology with modern standards, or confusing residential split-phase with commercial three-phase power.
- 110V / 220V vs. 120V / 240V: In the mid-20th century, utilities delivered slightly lower voltages. Over decades, transformer taps were adjusted to push nominal voltages up to 120V/240V to reduce line losses and handle higher modern appliance loads. If you see "110V" stamped on an old motor, it will run perfectly fine on a modern 120V circuit.
- 240V Split-Phase vs. 208V Three-Phase: This is a critical distinction for commercial work. As detailed in EC&M's breakdown of 208V vs 240V systems, 208V is the phase-to-phase voltage on a commercial 120/208V wye-configured three-phase system. If you plug a purely resistive 240V appliance (like a commercial space heater) into a 208V supply, it will not just run "a little slower." Because power equals voltage squared divided by resistance (P = V²/R), the heat output will drop by exactly 25%. Always verify the nameplate rating and the supply source before terminating heavy loads.
Frequently Asked Questions
Can I plug a 230V European appliance into a US 240V outlet?
Physically, you will need a plug adapter, as European Schuko or BS 1363 plugs do not fit US NEMA receptacles. Electrically, the 230V European nominal voltage falls well within the US 240V Utilization Range A (228V-252V). However, the frequency mismatch is the real danger. The US grid operates at 60Hz, while Europe operates at 50Hz. Plugging a 50Hz AC motor (like a stand mixer or power tool) into a 60Hz US supply will cause it to run 20% faster, draw more current, overheat, and potentially fail. Always check the appliance nameplate for a "50/60Hz" rating before adapting the plug.
Why does the US use 120V for standard outlets when Europe uses 230V?
The US standardized on 120V primarily for historical safety and infrastructure reasons. Early carbon-filament lightbulbs operated best around 110V. Furthermore, 120V presents a lower risk of fatal electrocution and sustained arc flashes compared to 230V. By utilizing the split-phase 240V system, the US achieves the best of both worlds: safer 120V for everyday human contact, and highly efficient 240V for heavy thermal and motor loads.
My multimeter reads 122V at the outlet and 244V at the dryer. Is this dangerous?
No. According to the ANSI C84.1 table above, the maximum allowable utilization voltage for a 120V circuit is 126V, and for a 240V circuit is 252V. Readings in the low 120s and mid 240s are incredibly common, especially if your home is located close to the utility pole transformer where line drop has not yet reduced the voltage.
