120 V or 240 V Mains - which voltage is better?

[psionl0]

Just something I have been wondering about recently.

Most appliances can just as easily be designed for 120V mains as 240V mains so it is six of one and half of the other. More heavy duty appliances will require a higher voltage and sometimes, only three-phase will do (for example, I used to have a three phase motor to pump bore water).

Most places in the world have a 240V mains. The local power lines carry that voltage on 3 lines (plus one for neutral) and it is only necessary to tap into the line to supply the house with either single phase or three phase electricity.

America, Canada, Japan and a few other countries use a 120V mains. There the setup seems a little more complicated. As I understand it, the local power lines carry 7,200V and pole top (or underground) transformers convert it to a "center tapped" 240V supply (http://science.howstuffworks.com/environmental/energy/power7.htm). Each transformer supplies a single house with two phase 120V electricity (although residential complexes may share a transformer). Heavy duty appliances like washing machines can easily be accommodated with 240V but if a household needs a three phase supply then I guess they need an extra pole top transformer.

About the only advantage I can see of a 120V system is that local power lines can be thinner since they carry less current. I know an electrician who has worked in both Australia and the USA. He describes US household wiring as a "mess" and much prefers Australia's 240V system.

I was wondering if anybody involved in power distribution would like to weigh in.

 

[Beerina]

It's to stop you from insta-dying if you get shocked. Or at least reduce your chances.

US houses typically have 240 available for electric stoves, and general home equipment is 120.


My dad, rest his soul, as a child (this would be mid 1940s) fell in the tub, dropping a radio in with him. My grandmother had to run down into the basement and flip the main off.

I'll leave it to you as to whether the 120 volts killed my father when he was 6 or not.

 

[ceptimus]

About the only advantage I can see of a 120V system is that local power lines can be thinner since they carry less current.

That's the wrong way round. For any given appliance of a certain power, the 120V unit will consume twice the current of its 240V equivalent - so a 120V system will usually require thicker wiring and power cords.

There are two causes of thick wiring and cords - one is to provide more conductor cross section to carry higher currents - the other is to have greater insulation thickness to protect against electric shock or shorts. Generally speaking there is a trade off in thickness as the voltage is raised - a higher voltage cable carries less current for the same power, so the conductor can be thinner, but the higher voltage cable also requires thicker insulation to be safe, so the insulation is thicker.

I think with correctly specified cables to "code" the 240V ones will usually be a bit thinner than 120V ones - especially for higher power appliances like kettles.

 

[PhantomWolf]

It's to stop you from insta-dying if you get shocked. Or at least reduce your chances.

US houses typically have 240 available for electric stoves, and general home equipment is 120.


My dad, rest his soul, as a child (this would be mid 1940s) fell in the tub, dropping a radio in with him. My grandmother had to run down into the basement and flip the main off.

I'll leave it to you as to whether the 120 volts killed my father when he was 6 or not.

Volts don't kill you at all.

Consider the Taser, it has a wallop of about 50,000 Volts.

What will kill you is the Amperage, about 100-200 mA across the heart.

 

[surrogate]

America, Canada, Japan and a few other countries use a 120V mains. There the setup seems a little more complicated. As I understand it, the local power lines carry 7,200V and pole top (or underground) transformers convert it to a "center tapped" 240V supply (http://science.howstuffworks.com/environmental/energy/power7.htm). Each transformer supplies a single house with two phase 120V electricity (although residential complexes may share a transformer). Heavy duty appliances like washing machines can easily be accommodated with 240V but if a household needs a three phase supply then I guess they need an extra pole top transformer.

I think it is called split phase, not two phase. US Washing machines (non commercial) are 120V, electric hot water heaters, stoves, dryers, and central heating units are generally 240V.

 

[psionl0]

That's the wrong way round. For any given appliance of a certain power, the 120V unit will consume twice the current of its 240V equivalent - so a 120V system will usually require thicker wiring and power cords.

You are conflating power lines with power cords.

In any case, you didn't read the link I provided. US power poles have 7,200 V at the lines while in the rest of the world it is 240 V.

 

[Speedskater]

In the US system, the same wires are used in the 120V and 240V parts of the system.

================
NEC Article 310

 

[Modified]

Volts don't kill you at all.

Consider the Taser, it has a wallop of about 50,000 Volts.

What will kill you is the Amperage, about 100-200 mA across the heart.

And with double the voltage, all else being equal and where there is no current limitation, you will get twice the current. A Taser is current-limited.

 

[Modified]

In the US system, the same wires are used in the 120V and 240V parts of the system.

They could be, but most houses I've lived in have dedicated lines for all the 240v equipment.

 

[Speedskater]

Yes, of course. But both the 120V and 240V circuits use the same type of wire. With the exact same insulation thickness. Probably from the same reels.

 

[aleCcowaN]

Regarding mains electricity, the most common voltage is 230 V -like Australia's- followed by 220 V and 120V, with several countries using 240V, 115V, 110V and 127V. The most common frequency is 50 Hz, with many countries using 60 Hz. I'm positive Edison's 110V wounds up being statistically safer than 230V when people do what they must avoid or neglect to check and keep their wiring.

Besides that:

There's a continent called America with no unified mains electricity voltage.

120/240 V household electricity is not "two phase" at all. It is one phase, two voltages and 4 wires (that is the "mess" mentioned) In a normal country you have 4 wires to enjoy polyphase and single-phase at the same time in a whole apartment building, and even you can have just 3 wires and have three-phase and single-phase at the same exact voltage with the electric service company just using 2 transformers and not 3 (that is typical in quick developing areas).

I have had at home 3 x 220V without ground, later 3 x 380/220V without ground and now 3x 380/220V with ground. There was a time when I had to warn any electrician coming to do some work at home that he needed to be careful because both wires were "hot". They were very "sceptic" about my knowledge but they kept silent once they had finished their works. One of them was shocked and felt for not using proper footwear while handling the "neutral", without having had the common-knowledge precaution of touching it first with the first knuckle of his index, just below the nail.

I remember that, 35 years ago, each time I had electricity but the lift wasn't working or did strange noises I'd took my first multimeter and check all three phases by measuring the voltage between hot an "neutral" wire, hot and the kitchen tap and "neutral" and the kitchen tap. Public company ages, long gone now.

PS: People who don't know what phase means in power provision shouldn't speak of it.

PS2: There was a kid who said to be shocked in a bathtub just to avoid to be punished by letting a radio drop into it. Maybe we're talking of the same kid.

 

[ceptimus]

You are conflating power lines with power cords.

In any case, you didn't read the link I provided. US power poles have 7,200 V at the lines while in the rest of the world it is 240 V.

No. I'm right and you're wrong.

Power (Watts) is volts times amps. A 1.2 kW appliance, assuming near-unity power factor, will draw ten amps from a 120V supply, but only five amps from a 240V one. If the power factor is bad, it will draw even more amps - the power is actually volts x amps x cos(phi). However, most household appliances, especially the higher power ones, will have good power factors.

We have various transmission line voltages here from 400 kV, 135 kV, down to normal factory 3-phase which is 415V between phases (actually 440V but now called 415 to be Eu-compliant). Phase to neutral (or earth) is then 240V (but now called 230V). For houses the three-phase runs down the street (usually underground) and only a single phase, plus neutral and earth is bought into each house. Different houses along the street run on different phases in an attempt to balance the load across the three phases. This means that if you run an extension cord from your house, and your neighbour does the same, then you can potentially get an electric shock from phase-to-phase of 415V (or 440V) in the unlikely event that both extension leads or appliances plugged into them are faulty and you touch both appliances at the same time.

Everything in the UK is 50Hz which is slightly unusual as the majority of countries now operate at 60Hz, regardless of the voltage.

 

[ceptimus]

When the UK changed from 240V to 230V (to be compliant with Europe), the voltage didn't change at all. They just started putting 230V stickers on things - there was already a sufficient tolerance that meant that our 240V supply was compliant with the new regulations and labels.

 

[aleCcowaN]

We have various transmission line voltages here from 400 kV, 135 kV, down to normal factory 3-phase which is 415V between phases (actually 440V but now called 415 to be Eu-compliant).

It was 415V when it was 240V. For 230V it corresponds 400V. It's just basic trigonometry, so to speak: 230V x 2 x sin 60°. Of course they're allowed to provide up to 6% more than that, and in some countries even 10% just to force consumption when the overall load is low and technical minimals from nuclear power plants and fossil fuel plants are to be kept. I once measured 247V at home (normal is 223V) at 3:30 am (I was curious why the light seemed to be so bright).

 

[Macgyver1968]

Volts don't kill you at all.

Consider the Taser, it has a wallop of about 50,000 Volts.

What will kill you is the Amperage, about 100-200 mA across the heart.

I never liked that phrase. To me, it's like saying "It's not the gunpowder that kills you, it's the lead projectile." When in fact, it's the right combination of both that kills you.

 

[Modified]

120/240 V household electricity is not "two phase" at all. It is one phase, two voltages and 4 wires (that is the "mess" mentioned)

It's two 120 volt lines relative to ground, 180 degrees out of phase. There is no 240 volt line.

 

[aleCcowaN]

120/240 V household electricity is not "two phase" at all. It is one phase, two voltages and 4 wires (that is the "mess" mentioned)

It's two 120 volt lines relative to ground, 180 degrees out of phase. There is no 240 volt line.

What doesn't contradict what I said. That "180 degrees out of phase" is what you see, not two phases. It's called a split-phase electric power. This figure in it

shows the only transformer for the only phase in it. An autotransformer isn't neither two-or-more phased.

 

[Thermal]

In the US system, the same wires are used in the 120V and 240V parts of the system.

================
NEC Article 310

Per NEC, it depends on the amperage. 14 or 12 gauge wiring is used for 120v, but the higher amp draw of 240v normally requires 10 or 8 gauge wiring, in the instance of the household appliances listed earlier.

 

[Dabop]

Per NEC, it depends on the amperage. 14 or 12 gauge wiring is used for 120v, but the higher amp draw of 240v normally requires 10 or 8 gauge wiring, in the instance of the household appliances listed earlier.

Um hate to be contrary, but this is 100% completely, totally and utterly wrong...
W=V x A (commonly known as watts law)
shows conclusively that doubling the voltage will HALVE the current for the same wattage drawn

thus for the same circuit loading (ie watts consumed by the load) the current will HALVE when using 240VAC compared to 120VAC

either that or I have been doing it wrong for the last 35 years.....along with every other electrician in the world

as an example, being the middle of winter, I have my 2400W fan heater going in the bedroom ATM
this draws 10A from my 240VAC (nominal) outlet (which is now classified as 230VAC, despite the normal voltage as read by my Fluke meter usually being 247V...)
If I was in the states, using their 110/120VAC system, to get the same 2400W heating, I would have to halve the resistance of the heating element, which would result in a 20A draw to get the same 2400W heating effect

Thus by halving the voltage I have doubled the current- as was foretold by Watts Law...

 

[Thermal]

Um hate to be contrary, but this is 100% completely, totally and utterly wrong...
W=V x A (commonly known as watts law)
shows conclusively that doubling the voltage will HALVE the current for the same wattage drawn

thus for the same circuit loading (ie watts consumed by the load) the current will HALVE when using 240VAC compared to 120VAC

either that or I have been doing it wrong for the last 35 years.....along with every other electrician in the world

Ok. So you are wiring up a 50 amp 220v circuit for, say, an electric oven with I would assume 14 gauge, and passing inspection per NEC? I hope you'll forgive me for not continuing this argument.

ETA: we were talking about wiring for 240v household appliances (stoves, water heaters, dryers, etc.). All obviously high wattage draw. While you are right on the watt calc, it was irrelevant to the subject of 240v household appliances. They are all high wattage, and in construction terms automatically high amperage.