US residential split-phase power is nominally 240V measured across two hot legs, though older terminology and appliance nameplates often refer to it as 220V or 230V. If you are wiring a new circuit, sizing a breaker, or troubleshooting a tripped thermal overload, you must design for the modern 240V nominal standard while accounting for the acceptable utility tolerance range. What people commonly confuse this with is either 208V three-phase commercial power or the 230V single-phase standard used in Europe and the UK; they are fundamentally different systems with different transformer topologies and cannot be treated as interchangeable.
The Short Answer: Is US Voltage 220 or 240?
The electrical grid in the United States delivers 240V to the residential service panel. The persistence of "220V" in everyday language is a legacy artifact. In the early-to-mid 20th century, utility transformers were indeed tapped to deliver 110V/220V. As grid infrastructure improved and utility companies needed to push more power through existing conductors without exceeding thermal limits, the nominal distribution voltage was incrementally raised to 115V/230V, and eventually to the modern 120V/240V standard.
Appliance manufacturers still print "220V" or "220-240V" on nameplates to ensure backward compatibility with older homes and to account for voltage drop on long branch circuits. However, the National Electrical Code (NEC) and utility standards operate strictly on the 120V/240V baseline. According to the National Fire Protection Association (NFPA), which publishes the NEC, all conductor ampacity and overcurrent protection calculations for residential ranges, dryers, and HVAC equipment must utilize the 240V nominal figure.
The American National Standards Institute (ANSI) dictates that utility delivery voltage at the service entrance must fall within Range A: 228V to 252V (240V ±5%). At the utilization equipment (the receptacle), the acceptable Range A limit is 216V to 252V (240V -10% to +5%) to account for internal voltage drop.
The Math Behind the Split-Phase System
To understand what this voltage actually changes in a real circuit, we have to look at how split-phase power behaves under load. A standard US residential transformer secondary features a center-tapped neutral. You get 120V from either hot leg (L1 or L2) to neutral, and 240V across L1 to L2.
Let us run a worked numeric example using a purely resistive load: a standard 4500-watt electric water heater element. Power ($P$) equals Voltage ($V$) squared divided by Resistance ($R$). First, we find the fixed resistance of the element at its rated 240V:
- Resistance: $R = 240^2 / 4500W = 12.8\Omega$
- Current at 240V: $I = 240V / 12.8\Omega = 18.75A$
Now, what happens if your utility is experiencing heavy summer grid sag and delivers the absolute minimum ANSI C84.1 Range A voltage of 228V to your panel? Because the element's physical resistance remains fixed at $12.8\Omega$, the math shifts:
- Current at 228V: $I = 228V / 12.8\Omega = 17.81A$
- Actual Power Output: $P = 228^2 / 12.8\Omega = 4061W$
The current actually drops, and your water heater loses nearly 10% of its heating capacity. This is why sizing conductors based on the 240V nameplate amperage is critical; if you sized the wire expecting only 17A because you assumed a "220V" system, you would be risking a thermal event when the grid stabilizes and pushes a full 18.75A through the circuit.
Where You Meet This in Practice
You will encounter the 220V vs 240V discrepancy primarily when matching NEMA receptacles to appliance plugs and when reading motor nameplates. Here is how the physical hardware maps to the nominal voltage:
| NEMA Receptacle | Amp Rating | Nominal Voltage | Common Application |
|---|---|---|---|
| 6-15R | 15A | 240V | Window AC units, small shop tools |
| 6-20R | 20A | 240V | Large window AC, small welders |
| 14-30R | 30A | 125/250V | Electric clothes dryers (requires neutral) |
| 14-50R | 50A | 125/250V | Electric ranges, EV Level 2 chargers |
Notice that NEMA designations for straight-blade, non-neutral devices (the 6-series) are strictly rated for 240V. The 14-series includes a neutral pin and carries a dual rating (125/250V) because the appliance uses 120V for control boards and timers, and 240V for the heating elements. When wiring a 14-50R for an EV charger, you must use a 50A double-pole breaker and minimum 6 AWG copper (or 4 AWG aluminum) THHN conductors, referencing the 75°C column in NEC Table 310.16.
Real-World Scenario: The Chattering Well Pump Contactor
Theory is clean; jobsites are not. Here is a worked real-world scenario walkthrough demonstrating why treating 240V and 220V as interchangeable synonyms will destroy your equipment.
- The Setup: A homeowner installs a new 5 HP, 240V single-phase submersible well pump. The pump is located 250 feet from the main panel. The motor nameplate lists a Full Load Amp (FLA) rating of 28A. The installer runs 10 AWG copper wire in PVC conduit, protected by a 35A double-pole breaker. This meets NEC ampacity requirements for the 35A breaker.
- The Numbers: The DC resistance of 10 AWG copper is roughly 1.24 ohms per 1,000 feet. For a 250-foot run, the round-trip conductor length is 500 feet. Total circuit resistance is $0.5 \times 1.24 = 0.62\Omega$. At full load (28A), the voltage drop is $V = I \times R$, which equals $28A \times 0.62\Omega = 17.36V$.
- The Outcome: On a hot July afternoon, the utility grid is strained, and the voltage at the main panel sags to 230V (well within the ANSI C84.1 Range A minimum of 228V). By the time the power travels 250 feet down the well casing, the voltage at the motor terminals is $230V - 17.36V = 212.64V$. The motor struggles to start, the contactor chatters rapidly, arcs across the contacts, and eventually welds shut, burning out the motor start winding.
- What Went Wrong: The installer sized the wire strictly for ampacity (preventing the wire from melting) but ignored voltage drop. NEMA MG-1 standard motors require a voltage tolerance of ±10% (216V to 264V). Because the installer assumed "220V is close enough to 240V," they failed to realize that a 240V system sagging to 230V, combined with a 17V wire drop, pushes the utilization voltage below the 216V absolute minimum. The fix requires upsizing to 6 AWG copper to reduce the voltage drop to under 3%.
FAQ: Clearing Up the Voltage Confusion
Can I plug a 220V European appliance into a US 240V outlet?
Usually, no. While the voltage is close, European appliances operate on 50Hz AC power, whereas the US grid operates at 60Hz. Plugging a 50Hz motor or transformer-based appliance into a 60Hz US 240V supply will cause it to run 20% faster, draw more current, and overheat. Furthermore, the physical plug styles (like the Schuko or BS 1363) require an adapter that does not change the frequency.
Why does my multimeter read 245V at the panel?
Reading 245V is perfectly normal and compliant. Utilities often target the higher end of the ANSI C84.1 Range A (up to 252V) at the distribution transformer to ensure that homes at the very end of the distribution line still receive at least 228V after line losses. As long as your reading is between 228V and 252V, your equipment is operating within design specifications.
Is 208V the same as 220V or 240V?
No. 208V is derived from a 120V three-phase wye system (120V × √3 = 208V), commonly found in commercial buildings and large apartment complexes. A 240V split-phase residential system is entirely different. While many modern HVAC compressors and electric heaters are rated for "208/240V" and will operate on either, a resistive heater designed strictly for 240V will output only 75% of its rated heat if connected to a 208V supply.
When designing circuits, always calculate using the 240V nominal baseline, verify your actual panel voltage with a calibrated True-RMS multimeter, and run voltage drop calculations for any branch circuit exceeding 50 feet. For deeper guidance on power quality and nominal voltage tolerances, reference the Fluke electrical testing guidelines and your local utility's power quality documentation.
