Why does the US run at 120V when some of the world runs at 240V?

When a load is applied from either 120v conductor to the neutral (as is the case for typical receptacles, lights, and so forth, it is using 120 volts.

However, when a load is applied from one 120v conductor to the other, without using the neutral, the voltage being used is the sum of both 120v conductors (240 volts). This is the case for many HVAC systems, electric furnaces, clothes dryers, and so forth.

So the answer is that both 'some of the world' and the U.S. distribute 240v to the residence.

It seems like the difference you are talking about is that on the non-U.S. systems, their receptacles are 240v, while ours are 120v. Why that is, I'm not even sure. My guess is that lower voltages tend to be safer, which is why you are receiving 240v at the home instead of the thousands of volts generated by the power plant.

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In terms of power production - all power is the same. It is then transmitted over High Voltage cables - usually above 10K Volts. The power is then stepped down before it reaches our homes.

U.S, Japanese and some other countries receive 110V in the form of 2 wires - 1 Live and 1 Neutral

Some may argue that the US is behind or has just managed to stay afloat with this old system longer.

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The US is at 120 volts, not 110 volts. It was increased sometime around the 1950s.

The historic reason for 110 volts was due to the DC power systems created by Thomas Edison. I think he chose 110 volts because that is what his light bulb worked on. Later on these systems were converted to AC so you didn't need a power plant on every corner but the voltage wasn't changed so existing lighting didn't need to be replaced (they didn't care if they got AC or DC)

An interesting question is why the rest of the world did not wind up using 110 volts. How did 220/230/240V get started over there?

The US system theoretically could be made as good as (slightly better, actually) than the European system with no infrastructure change, except to houses themselves. US houses get 240 volts at the panel. If wall outlets all were fed with 240V you'd have the lower current and higher power advantage of the European system and it would be safer too, since each "hot" would still be only 120V from ground (not 240V) which keeps the reduced shock hazard advantage. Of course it is still possible to touch the two hots.

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As one has stated above, it was Thomas Edison who promoted the use of (then) 100 volts as some tragic experiences in the early days of power distribution showed that 100 volts was not usually lethal for a shock. Remember that in the early days, bare wires were strung though ceramic insulators, both exterior and interior, and so there were many more shock hazards present. As technology advanced, good, long life insulation was wrapped on conductors. Speaking from personal experiences, one as a child, I am glad that the few shocks I've experienced were with 120v., not 220v. power.

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Just remember it's not the volts that kill, it's the current (the amps).

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Yes, but at 220v your body's resistance will draw twice the current and that may well be over the threshold to kill you. In countries in Europe and elsewhere, where 230 volts is the general standard mains supply voltage for domestic houses, offices, factories, etc., they have to make sure that their wiring systems are very safe by using high quality insulation and wiring methods for all wiring upgrades and new work.

For additional safety the most recent wiring regulations insist that an RCD (Residual Current Device) must be included in the main Consumer Distribution Unit (Breaker Box in US parlance) to cut the supply very quickly if any significant difference is detected between the currents flowing in the live (hot) and neutral wires. A good description of how an RCD works is given at this manufacturer's web site:

http://www.greenbrook.co.uk/pdf/pbcatpg21-22.pdf

An RCD works in a different way to a simple Ground Fault Current Interruptor (GFCI). An RCD will trip if there is any significant difference between the currents flowing in the live (hot) and neutral wires. A simple GFCI will trip if any significant current is detected flowing in the main Earth (ground) wire to the actual Earth (or Ground) spike.

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In up-to-date domestic installations in the UK no actual Earth "spike" is used. Instead the protective safety wiring (or casing) of the incoming mains supply cable is used because that is most likely to be reliably "grounded" to the Earth.

That is different to US and Canadian standards and other countries' which use the same. In those countries the incoming mains supply is two "hot" wires supplying 240 volts balanced around "Ground Potential" which is always 0V. (Zero Volts). An Earth Spike is used at each property (house, apartment, office, factory, works site or whatever) to provide a common Neutral (i.e. the "White" wire) for the two resulting 120 volt "hots". One "hot" is coloured "Black", the other one is coloured "Red".

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First answer by Lucyfanclub. Last edit by Martinel. Contributor trust: 247 [recommend contributor]. Question popularity: 178 [recommend question]