What is the household AC electrical power service in the US?

Answer 1

In short: 240/120 V @ 60 Hz frequency.

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In the US, lighting and low-power appliances run at 120 volts plus or minus 10%; meaning 108 volts to 132 volts at 60 cycles per second called hertz (Hz). To get the actual figures for your locality it is best to ask a local licensed electrician or your local electricity supply company.

However, US houses get 240 volts at 60 Hz at the panel. If all the wall outlets were fed with that 240 volts you'd have the lower current and higher power advantage of the European 50 Hz system and it would be safer too, since each "hot" would still be only 120 volts from ground, which reduces the shock hazard compared to some European 230 volt receptacles or socket outlets.

However the most important difference is how the Neutral wire is connected:

240 volt 50 Hz service

Please note: there is no 50 Hz service in countries which use a 60 Hz, such as the US and Canada. The following information is given just for comparison purposes.

A 240 volt 50 Hz appliance can have up to 3 wires altogether:

• one single 240 volt "live" or "hot" wire;
• a neutral wire;
• a safety "earth" or "ground" wire but this is not required if the appliance is of the type known as "double insulated".

The hot wire feeds alternating voltage from the power station to the load and, because the voltage is alternating, the load draws an alternating current. Then the neutral wire returns the current to the power station to complete the circuit.

240 volts 60 Hz service

An appliance that is designed to be connected to strictly 240 volts 60 Hz is connected with only a two-wire cable plus a safety ground wire. (For example 240 volt 60 Hz base board heaters use that.)

The only time a cable with three wires plus safety ground is used is if 120/240 volts is needed in the equipment. (For example kitchen ranges or washing machines which have time clocks or programmers that require only a 120 volt feed.)

Thus an appliance designed to run on a 240/120 volt 60 Hz supply can have up to 4 wires altogether:

• two 120 volt 60 Hz live "hot legs" which run in opposing phase to one another: when one hot leg goes "+" (120 volts positive) the other leg goes "-" (120 volts negative);
• a neutral wired as a "central" common current return conductor:
• a safety "ground" wire but this is not required if the appliance is of the type known as "double insulated".

One of the two hot wires feeds a 120 volt alternating voltage from the power station to the 120 volt load - the clock or programmer - and, because the voltage is alternating, that load draws an alternating current. Then the neutral wire returns the current to the power station to complete the circuit.

Important conclusion

There is a 240 volt voltage difference between the neutral and the hot conductor in the 50 Hz system and only a 120 volt voltage difference between the neutral and each of the hot conductors in the 60 Hz system.

That is why an appliance designed to be connected onto the 50 Hz system cannot be used safely on the 60 Hz system without first having a proper technical inspection done, followed by any necessary modification work done to ensure that the appliance can be operated safely because, in the main breaker box, at the point where the 60 Hz "neutral" gets connected to the ground, this difference will cause serious problems!

A licensed electrician or electrical engineer would be able to consider whether or not a particular large appliance, that was manufactured to work on 50 Hz-only, could be modified to run safely at the higher 60 Hz frequency. However it won't usually be worth the expense of doing the work because it would be more cost-effective to buy (new or secondhand) an equivalent large appliance designed to work on 60 Hz.

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In the U.S. it's 120 volts AC running at a frequency of 60 Hz. It was increased from 110 volts in the 1950s.

The historic reason for 110 volts is the Direct Current (DC) power systems created by Thomas Edison. It is believed he chose 110 volts because that is what his light bulb worked on. Later, these systems were converted to Alternating Current (AC), so you didn't need a power plant on every street corner, but the voltage level wasn't changed, so existing lighting didn't need to be replaced. (At that time 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 the European system (slightly better than, actually) with no infrastructure change except to houses themselves. US houses get 240 volts at the panel through the Black and the Red hots. If wall outlets were all 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. If the wiring has been done correctly it should be impossible for someone to touch both Black hot and Red hot at the the same time by accident.

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, single un-insulated wires were strung though ceramic insulators, both exterior and interior, and so there were many more shock hazards present. Each splice was made by wrapping one wire around the other one and soldered. The use of junction boxes was non existent. As technology advanced good, long life, insulation was wrapped around the conductors.

Scroll down to related links and look at "Household AC electrical power in different countries around the world - voltages, frequencies and plug types - Worldmap for AC Voltage and Frequency - Wikipedia".