Almost everywhere outside North America, homes run on a single 220 to 240V supply. Here, a typical house gets both 120V and 240V out of the same service, from a single transformer, on a single set of wires. Neither arrangement is an accident. Both trace back to specific engineering decisions made over a century apart.
Quick orientation before the mechanics below: a transformer is the device, usually the gray can on a utility pole or a green box on a pad near the street, that changes voltage between the level the grid uses for transmission and the level a home actually uses. Read the full explanation of how transformers work → for the details; here, what matters is that one transformer per neighborhood is doing double duty, supplying both voltages a home needs at once.
Where 120 volts came from
The number starts with Thomas Edison's earliest DC power stations in the 1880s, which delivered roughly 100 to 110 volts to nearby customers. That figure wasn't arbitrary. The carbon-filament light bulbs of that era, the only real electrical load most customers had, performed best and lasted longest at close to 100V. But the copper wire running from the generating station to a customer's lamp had real resistance, and voltage drops as current travels down a wire. Supplying a bit above the bulb's ideal voltage, around 110V at the source, compensated for that drop so the bulb still received close to its ideal 100V by the time the current reached it. Higher voltage than that was also considered a meaningfully greater fire and shock risk, given how primitive wire insulation still was at the time.
When AC and the Westinghouse-Tesla system won out over Edison's DC in the following decade, the AC systems that replaced those DC networks largely inherited the same voltage neighborhood, gradually standardizing at 120V rather than switching to something else, since it kept working for the same lamps and appliances people already owned.
How one transformer delivers two voltages
One winding, one neutral, two usable voltages
The mechanism that lets a single home get both 120V and 240V is called split-phase service, and despite the name, it isn't two separate phases. It's a single AC phase, delivered through a transformer with a center-tapped secondary winding. That center tap is grounded and becomes the system's neutral wire. From that neutral, one end of the winding sits at 120V in one direction; the other end sits at 120V in the opposite direction, at the same instant. Measure from either end to the neutral, and a lighting circuit or a standard outlet sees 120V. Measure straight across, from one end of the winding to the other, skipping the neutral entirely, and a dryer or range sees the full 240V, since the two 120V halves add together instead of canceling out.
That's the whole trick: one transformer, one winding, one neutral, and a home gets a gentler voltage for everyday lighting and outlets plus a stronger voltage for heavy loads, without the utility needing to run two separate services or install two separate transformers.
Why the neutral has to be grounded
There's a piece of this that's easy to take for granted: none of those 120V or 240V figures are fixed numbers on their own. They only become fixed, trustworthy values because the neutral, that center tap, is deliberately connected to earth ground.
An isolated transformer winding, left ungrounded, has no defined relationship to the earth at all. Its two ends still sit at a set voltage relative to each other, that's fixed by the transformer's own physics, but relative to the ground under your feet, the whole winding is free to float. In practice, parasitic capacitance between the windings and nearby grounded objects, metal conduit, building steel, plumbing, tends to pull an ungrounded system toward some ambiguous mid-point voltage, but that voltage isn't stable, and under fault conditions it can shift dramatically and without warning.
Grounding the center tap fixes this by tying that one point of the circuit directly to the earth. The earth itself is, for practical purposes, close to an infinitely large conductor: connecting a small residential system to it doesn't meaningfully change the earth's own potential, no matter how much current briefly flows into or out of it. That makes earth an ideal reference, a sink large enough to treat as a fixed 0V no matter what else is happening on the circuit. Once the neutral is pinned to that reference, each hot leg's voltage becomes just as fixed: 120V away from a stable zero, not 120V away from some unknown, drifting number.
One terminology note for accuracy: in this standard split-phase setup, the grounded conductor and the neutral are the same wire, but that's not universally true of every electrical system. Some other configurations intentionally ground one of the actual hot legs instead of a true center-tapped neutral, so "grounded conductor" and "neutral" are related but not strictly interchangeable terms in every context, just in this one.
This matters for safety beyond just predictable numbers, too. If a hot wire ever faults against a grounded metal surface, an appliance's metal case, for instance, that solid earth connection gives the fault current an intended, low-resistance path back to the source, which is exactly what allows a breaker to detect the fault and trip. Without that ground reference, the same fault could quietly energize a metal surface at some undefined voltage with no path to trip anything, leaving it live and dangerous with no warning at all.
There's a second, more common version of a "floating neutral" worth knowing about, distinct from the case above, since it can happen on systems that were properly grounded to begin with: the neutral conductor itself breaking somewhere between the panel and the utility transformer while both hot legs stay energized. That fault turns the two 120V legs into a voltage divider instead of two independent circuits, and it's dangerous precisely because nothing looks obviously wrong at first glance. Read the full breakdown of the open neutral fault →
Why 240V for the big stuff
For a given amount of power, doubling the voltage cuts the current in half, and current, not voltage, is what determines how thick a wire needs to be and how much energy gets lost as heat along the way. An electric range, a dryer, a water heater, or an EV charger all draw enough power that running them at 120V would require thick, expensive wiring and would waste more energy as heat in the circuit. Running them at 240V instead lets the same job get done with thinner wire and less waste, which is exactly why those circuits use different outlets entirely, built so a 120V device physically can't be plugged into a 240V circuit by mistake.
One labeling quirk worth knowing, since it trips people up: those outlets and plugs are usually stamped 125V and 250V, not 120V and 240V. That's not a different voltage standard, it's a design-rating convention NEMA settled on back in the 1930s, deliberately set a bit above the actual nominal supply voltage as built-in safety headroom. If you've ever looked at a range or window AC plug and seen "250V" stamped on it, that's genuinely what it says, it's just an equipment rating, not the voltage itself.
Why it never changed
Europe's electrification happened somewhat later than America's, largely in the early 1900s, at a point when the efficiency case for higher voltage was already well understood, and it happened through more centralized, coordinated national utilities and eventual continent-wide standardization efforts. The result is the 230V standard most of the world uses today.
America's electrification happened earlier, faster, and through a much more fragmented landscape of competing local utilities, each building out infrastructure around whatever equipment and standards they already had. By the time anyone might have seriously proposed switching the whole country to a higher voltage standard, there were already millions of 120V light sockets, appliances, and homes wired around that number. Replacing all of it would have meant rewiring virtually every building in the country and replacing enormous amounts of installed equipment, a cost nobody was willing to take on just to gain some transmission efficiency. The split-phase 120/240V compromise, quietly, without a public decision or a single "we're keeping 120V" declaration, became permanent simply because it was already everywhere.
North America isn't even the most extreme case of a country stuck with its earliest electrical decisions. Japan runs on 100V, even closer to Edison's original number, and famously has two different grid frequencies in the same country, 50Hz in the east and 60Hz in the west, because the two regions bought their first generators from different European and American manufacturers in the 1890s and never fully reconciled the difference. Once millions of homes and businesses are wired around a standard, changing it stops being a technical question and becomes a cost question, and the cost usually wins.
Sources
- Wikipedia, "Split-phase electric power," on the center-tapped transformer mechanism and North American residential wiring conventions.
- Sense, "What is Split-Phase Power?" on the distinction between true two-phase power and North America's single-phase, three-wire system.
- Mike Holt's Electrical Forum, discussion thread on the historical origins of 120V standardization in early U.S. electrification, and separately on NEMA's 125V/250V equipment rating convention versus actual nominal supply voltage, professional trade community discussion, presented here as such rather than a formal study.
- International Electrotechnical Commission, IEC 60038, the international standard defining 230V as the harmonized nominal voltage for 50Hz residential systems.
- National Electrical Code, Article 100 and Article 250, definitions and requirements for system grounding and bonding, on why an ungrounded secondary produces floating, unreliable voltages relative to earth.
- Mayfield Renewables, "Transformer Grounding: Navigating NEC Article 250 and Separately Derived Systems," on floating voltages from capacitive coupling in ungrounded systems.
This is general educational information intended to build foundational understanding, not a substitute for hands-on training or a licensed electrician's evaluation of specific equipment.
You're welcome to quote this article elsewhere with attribution and a link back to the original at hancoelectric.com.