What is the difference between 120V and 240V?

120V is the standard line-to-neutral voltage at North American wall outlets and feeds lights, electronics and small appliances. 240V is the rest of the world's residential standard and, inside North America, the line-to-line voltage made by combining two 120V legs of a split-phase transformer — used for dryers, ranges, EV chargers and central air. At the same wattage, 240V draws half the current of 120V, which is why heavy loads run on it.

Why is commercial small-equipment voltage 208V instead of 240V in the US?

208V is the line-to-line voltage of a 120/208V three-phase wye system, the standard service for US strip malls, apartments and small offices. Because three-phase voltages are 120° out of step, line-to-line works out to 120 × √3 ≈ 208V, not 240V. Equipment marked 208/230V handles it; equipment marked 240V-only will run hot on a 208V supply.

What is 277V used for?

277V is the line-to-neutral voltage of a 277/480V three-phase wye and is used almost exclusively for commercial and industrial lighting in North America — office towers, big-box retail, warehouses, schools and parking garages. Running fluorescents and LED drivers at 277V instead of 120V more than halves branch-circuit current, saving copper across thousands of fixtures. It is not a wall-outlet voltage and is always fed to fixtures and dedicated switchgear, never general-purpose receptacles.

When do you use 480V instead of 240V?

480V is the standard three-phase voltage for commercial and industrial motors, chillers, pumps, packaged machinery and main switchgear in the US. For any motor over about 5HP, 480V halves the current and roughly quarters the I²R losses compared with 240V, which is why every factory floor and most mid-rise mechanical rooms run on it. The trade-off is arc-flash energy, which is meaningfully higher at 480V and triggers Cat 2+ PPE for any live work.

Why is line-to-line voltage √3 times line-to-neutral on a wye system?

Three-phase voltages are sinusoids 120° out of phase. When you take the vector sum of two of them to get the line-to-line voltage, the geometry works out to √3 (about 1.732) times the line-to-neutral voltage. That is why 120 × √3 = 208, 277 × √3 = 480, and 230 × √3 ≈ 400 (the European industrial standard). The √3 factor is on every three-phase exam question.

Is US residential 240V the same as three-phase 240V?

No. US residential 240V is split-phase: two 120V legs from a single-phase transformer secondary, 180° apart, sharing a neutral. Real three-phase 240V (open delta) exists in some rural and light-commercial services but is uncommon, and it is not interchangeable with split-phase 240V for motor and panel selection. Always confirm the service type from the print before sizing equipment.

120V vs 208V vs 240V vs 277V vs 480V Explained

Plain-English guide to the five voltages every electrician meets — where each is used and the √3 wye maths that ties them together.

Voltage is the first thing an apprentice asks about and the last thing most guides explain clearly. Why is a power point 120V in Detroit and 240V in Brisbane? Why does a rooftop unit on a strip mall pull 208V, the lights in an office tower 277V, and the freight elevator 480V? Same wire, same copper — wildly different numbers. Here's the plain-English breakdown of 120V, 208V, 240V, 277V and 480V, what they're used for, and the maths that ties them together.

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The single-sentence version

Higher voltage moves the same amount of power through smaller wire, so big loads use big voltage. Small loads use small voltage because it's safer to touch. Everything below is a footnote on that.

120V — North American residential

120V is the standard wall-socket voltage across the United States, Canada and Mexico. It feeds lights, toasters, TVs, laptop chargers and anything else you plug in at home. It's a single phase tapped from the centre of a 240V transformer, with the neutral being the centre tap. That's why a US house has two 120V "legs" that combine to make 240V for the dryer and stove.

Why so low? History. Edison's DC system was built around it, and when AC took over the wall sockets stayed at roughly the same level so existing lamps still worked. It's the most touch-tolerant of the common voltages — a shock at 120V can still kill you, but the survival odds are noticeably better than at 240V.

240V — residential outside North America (and big appliances inside it)

240V (often nominally 230V in Europe, 240V in the UK and Australia) is the global residential standard everywhere outside North America. Same job as 120V — running the house — but at double the voltage, which halves the current for the same power and lets you use thinner cable. Australia, the UK, NZ, most of Europe, Asia and Africa run their general-purpose outlets at 230–240V.

In North America 240V still shows up at home, just not in the wall sockets — it powers the electric dryer, range, central air conditioner, EV charger and hot water tank, by combining the two 120V legs. That's the "240V split-phase" arrangement: two hots 180° out of phase, sharing a neutral.

240V is genuinely more dangerous to touch than 120V. The current that flows through your body at the moment of contact is roughly double, and the threshold for muscle lock-on is lower than people expect. Every modern code requires RCDs/GFCIs on socket circuits for this reason.

208V — commercial small loads (US)

208V is what you get when you tap between any two phases of a 120/208V three-phase wye system. Strip malls, small offices, restaurants and apartment buildings in the US are almost always wired this way: 120V single-phase to the wall sockets, 208V single-phase or three-phase to the bigger gear — rooftop AC units, commercial ovens, packaged HVAC.

It's not 240V because the maths of a wye system make the line-to-line voltage √3 times the line-to-neutral voltage (120 × √3 ≈ 208). Equipment rated "208/230V" handles the slight undervoltage fine; equipment marked 240V-only will run hot and die early on a 208V supply. Reading the nameplate is the difference between a 15-year unit and a warranty call.

277V — commercial lighting (US)

277V is the line-to-neutral voltage of a 480/277V three-phase wye, the workhorse system in mid-rise and high-rise commercial buildings in North America. You'll find it feeding fluorescent and LED lighting in office towers, big-box retail, warehouses and schools. Running lights at 277V instead of 120V more than halves the current draw, which means thinner branch-circuit wiring across thousands of fixtures — the savings on copper alone are why every commercial spec written after 1970 defaults to it.

It's not a touch voltage. You will not find a 277V receptacle in a hallway. Maintenance is qualified-personnel-only territory; the arc-flash energy is meaningfully higher than at 120V.

480V — commercial and industrial three-phase

480V (line-to-line in the same 480/277V wye, or 480V delta in older industrial plants) is the standard for motors, pumps, chillers, packaged industrial machinery and the main switchgear in most US commercial buildings. Three-phase 480V is what an electrician working in industry actually sees most days.

Why 480? It's the highest "low-voltage" tier the NEC recognises (anything ≥600V is "medium voltage" and triggers a different rulebook). For a 100HP motor, running it at 480V instead of 240V halves the current, halves the conductor size, and roughly quarters the I²R losses in the feeder. Across a factory that's tens of thousands of dollars a year in saved electricity and copper.

The trade-off is arc-flash. A bolted fault on a 480V bus releases enough energy to kill a person in PPE that would be fine at 240V. Every modern 480V switchgear job comes with arc-flash labels, incident-energy calculations and Cat 2+ PPE for any live work.

The 277V vs 480V relationship — and why the same wye gives both

A 480/277V wye-connected transformer secondary has three "hots" (A, B, C) and a neutral. Measure from any hot to neutral and you get 277V. Measure between any two hots and you get 480V (because 277 × √3 ≈ 480). That single transformer feeds the building's lights at 277V and the mechanical equipment at 480V from the same set of conductors — which is why commercial electricians have to keep both numbers in their head at all times.

The same logic explains 208/120V: a 208/120V wye gives you 120V hot-to-neutral and 208V hot-to-hot. Lights and outlets at 120, packaged HVAC at 208.

The √3 factor in one paragraph

Three-phase voltages are out of step by 120°. When you sum the instantaneous voltages between two hots, the geometry of those out-of-phase sinewaves gives you √3 (about 1.732) times the phase voltage. So 120 × √3 = 208, 277 × √3 = 480, 230 × √3 = 400 (the European industrial standard). It's not a coincidence — it's the same equation in every system.

Power in = power out, current goes down as voltage goes up

This is the whole reason higher voltages exist. For the same kilowatt load:

1.5 kW at 120V = 12.5 A — 2.5 mm² (12 AWG) cable, no problem.
1.5 kW at 240V = 6.25 A — 1.0 mm² (16 AWG) cable would do it.
15 kW at 240V = 62.5 A — chunky single-phase feeder.
15 kW at 480V three-phase = 18 A — small three-phase feeder, much smaller copper.

That's the entire economic case. Power = volts × amps (× √3 × power factor for three-phase). Push volts up, amps come down, conductors shrink, losses shrink, building gets cheaper. The only ceiling is safety — past about 600V the insulation and clearance requirements get expensive enough to undo the savings.

What apprentices keep tripping on

"208V is just bad 240V." No. They're different systems. A 240V-only motor on 208V will overheat. Buy 208/230V-rated gear in 208V buildings.
Mixing line-to-line and line-to-neutral on the nameplate. A 480V motor is line-to-line. A 277V lighting fixture is line-to-neutral. Same wye, different terminals. Read the label.
Assuming "240V split-phase" is three-phase. It isn't. US residential 240V is two 120V legs from the same single-phase transformer secondary, 180° apart. Real three-phase 240V (open delta) exists but is rare and worth flagging on the print.
Forgetting power factor on three-phase calcs. Three-phase power = √3 × V × I × pf. The pf isn't optional — a 0.8 pf motor draws 25% more amps than the kW-only calc suggests.
Treating 277V like a wall outlet voltage. It isn't. Fixtures, ballasts and dedicated switchgear only. Never a general-purpose receptacle.

The cheat sheet

120V — US/Canada wall sockets. Lights, electronics, small appliances. Single phase.
208V — US light commercial three-phase. Strip-mall HVAC, small motors, commercial kitchens. Line-to-line of a 120/208V wye.
240V — Everywhere-except-North-America residential, plus US dryers and stoves. Split-phase in the US, single-phase elsewhere.
277V — US commercial lighting. Office towers, big-box, warehouses. Line-to-neutral of a 277/480V wye.
480V — US commercial and industrial three-phase. Motors, chillers, switchgear. Line-to-line of a 277/480V wye.

What this means for the licence exam

Voltage system questions show up on every electrical licence exam — Australian Cert III capstone, US journeyman, UK 2391, Canadian Red Seal. They look easy and then catch people on the √3, on the difference between line and phase, and on which system is wye versus delta. The fix is reps: 50–100 mixed-voltage calc questions and the patterns become instinct. The Voltly question bank flags voltage-system questions as their own concept and serves more when you're getting them wrong — drill until the √3 conversions are reflexive.

Bottom line

120, 208, 240, 277 and 480 aren't five random numbers — they're three transformer geometries (single-phase split, 120/208 wye, 277/480 wye) and the maths of √3. Learn the geometries and the numbers explain themselves. Then drill the calcs until you don't have to think.

Keep learning

Test yourself on voltage systems — 6 question practice quiz with worked explanations.
The four-year electrical apprentice study guide — when to learn three-phase calcs and how to retain them.
AS/NZS 3000 wiring rules exam guide — Australian/NZ-specific voltage-drop and disconnection-time clauses.
Pass the electrical licence exam first time — the daily rhythm that makes voltage calcs reflexive.
NEC practice exam — US-focused question bank covering 277/480V wye, motor sizing and arc-flash.
Electrical licence practice — mixed three-phase questions across AU, NZ, UK, CA and US standards.