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Old 13th August 2017, 02:20 AM   #1
psionl0
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120 V or 240 V Mains - which voltage is better?

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/env...rgy/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.
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Old 13th August 2017, 02:28 AM   #2
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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.
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Last edited by Beerina; 13th August 2017 at 02:32 AM.
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Old 13th August 2017, 04:54 AM   #3
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Originally Posted by psionl0 View Post
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.

Last edited by ceptimus; 13th August 2017 at 04:55 AM.
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Old 13th August 2017, 05:14 AM   #4
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Originally Posted by Beerina View Post
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.
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Old 13th August 2017, 05:54 AM   #5
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Originally Posted by psionl0 View Post

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/env...rgy/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.
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Old 13th August 2017, 06:11 AM   #6
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Originally Posted by ceptimus View Post
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.
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Old 13th August 2017, 06:11 AM   #7
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In the US system, the same wires are used in the 120V and 240V parts of the system.

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

Last edited by Speedskater; 13th August 2017 at 06:18 AM. Reason: added content
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Old 13th August 2017, 06:13 AM   #8
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Originally Posted by PhantomWolf View Post
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.

Last edited by Modified; 13th August 2017 at 06:19 AM.
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Old 13th August 2017, 06:16 AM   #9
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Originally Posted by Speedskater View Post
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.
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Old 13th August 2017, 06:20 AM   #10
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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.

Last edited by Speedskater; 13th August 2017 at 06:21 AM.
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Old 13th August 2017, 06:56 AM   #11
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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.
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Old 13th August 2017, 07:01 AM   #12
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Originally Posted by psionl0 View Post
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.

Last edited by ceptimus; 13th August 2017 at 07:20 AM.
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Old 13th August 2017, 07:05 AM   #13
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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.
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Old 13th August 2017, 07:28 AM   #14
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Originally Posted by ceptimus View Post
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).
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Old 13th August 2017, 08:09 AM   #15
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Originally Posted by PhantomWolf View Post
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.
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Old 13th August 2017, 08:11 AM   #16
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Originally Posted by aleCcowaN View Post
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.
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Old 13th August 2017, 09:01 AM   #17
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Originally Posted by Modified
Originally Posted by aleCcowaN View Post
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.
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Old 13th August 2017, 09:08 AM   #18
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Originally Posted by Speedskater View Post
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.
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Old 13th August 2017, 09:25 AM   #19
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Originally Posted by MostlyDead View Post
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...
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Old 13th August 2017, 09:32 AM   #20
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Originally Posted by Dabop View Post
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.
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Old 13th August 2017, 09:34 AM   #21
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I know that it's made 3 phase 120 degrees apart, so if you have 240 two phase power, it's 120 degrees apart, not 180.
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Old 13th August 2017, 09:44 AM   #22
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Originally Posted by bobdroege7 View Post
I know that it's made 3 phase 120 degrees apart, so if you have 240 two phase power, it's 120 degrees apart, not 180.
You should understand what polyphase is in this context before reaching any conclusion.

A 240V two phase power 120 degrees apart is the three-phase system called in Spanish "triángulo abierta" that I mentioned above. When I translate it into "open delta" -which is the right translation of the concept in the name- it seems to refer to a different thing.
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Old 13th August 2017, 09:46 AM   #23
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I think it's rare these days, but 208V three phase with a neutral provides a convenient 120V from each phase to the neutral. Very early in my engineering career I actually got to design a machine for that. We could eliminate the usual transformer to provide 120V for the control circuitry.
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Old 13th August 2017, 09:46 AM   #24
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Originally Posted by MostlyDead View Post
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.
That's not what he's saying. In my shop I have 10 amp and 20 amp 220v circuits that run with 14 and 12 gauge wire running small machines.

The amount of (expected) draw determines the wire size.
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Old 13th August 2017, 09:46 AM   #25
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As we dont work with these 'gauge' wires, I would have to consult google to find out what size they actually are, but I can state with 100% accuracy, and I'll support with links, that if you want the same load (stated in watts) then it will 100% guaranteed halve the current every time you double the voltage

(what is the NEC- I assume from your comment it is some kind of regulatory body in the US???)- The only NEC I have come across here makes cash registers and the like, and altho the power companies certainly have no qualms about taking my cash, I doubt they have much to do with the wiring codes...)

;-)
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Old 13th August 2017, 09:50 AM   #26
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Originally Posted by Dabop View Post
As we dont work with these 'gauge' wires, I would have to consult google to find out what size they actually are, but I can state with 100% accuracy, and I'll support with links, that if you want the same load (stated in watts) then it will 100% guaranteed halve the current every time you double the voltage

(what is the NEC- I assume from your comment it is some kind of regulatory body in the US???)- The only NEC I have come across here makes cash registers and the like, and altho the power companies certainly have no qualms about taking my cash, I doubt they have much to do with the wiring codes...)

;-)
National Electrcal Code, part of National Fire Prevention code
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Old 13th August 2017, 09:51 AM   #27
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Originally Posted by Dabop View Post
(what is the NEC- I assume from your comment it is some kind of regulatory body in the US???)- The only NEC I have come across here makes cash registers and the like, and altho the power companies certainly have no qualms about taking my cash, I doubt they have much to do with the wiring codes...)

;-)
National Electric Code. It's a huge catalog of information put into the most complicated way possible.............
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Old 13th August 2017, 09:54 AM   #28
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Originally Posted by Macgyver1968 View Post
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.
It's different .. voltage and current are 2 sides of the same coin (well, together with resistance) you can't have one without another. Gunpowder and bullet are just two things.

120V is safer for sure, but it's by no means 100% safe.
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Old 13th August 2017, 09:57 AM   #29
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12 0.0808 2.05
13 0.0720 1.83
14 0.0641 1.63
15 0.0571 1.45


OK weird yank wire sizes figured out in real measurements ie mm^2
I have heard of these 'gauge' wires but never had to deal with imperial units, even when we had feet and inches we still used fractions of an inch ^2 for cable sizes

The only 12 gauge I have ever used went boom and was good on rabbits....
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Old 13th August 2017, 09:59 AM   #30
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Originally Posted by DGM View Post
That's not what he's saying. In my shop I have 10 amp and 20 amp 220v circuits that run with 14 and 12 gauge wire running small machines.

The amount of (expected) draw determines the wire size.
Yes, that's what I said. Talking about ovens and water heaters in household 220v, it's all high expected draw. Becomes a pedantic argument to say there is a lighter gauge for household appliance wiring when it basically does not exist.
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Old 13th August 2017, 10:01 AM   #31
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Originally Posted by DGM View Post
National Electric Code. It's a huge catalog of information put into the most complicated way possible.............
Ahhh....


we have our own version of that
less sexy title tho

AS/NZS 3000:2007

doesnt roll off the tongue quite like NEC tho does it
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Old 13th August 2017, 10:07 AM   #32
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Originally Posted by MostlyDead View Post
Yes, that's what I said. Talking about ovens and water heaters in household 220v, it's all high expected draw. Becomes a pedantic argument to say there is a lighter gauge for household appliance wiring when it basically does not exist.
Actually here it does
but thats not what you said, "14 or 12 gauge wiring is used for 120v, but the higher amp draw of 240v"

is a factually incorrect statement
240v doesnt 'draw' more amps, in your country it is used for higher wattage appliances, here it isnt- everything from my led desklamp to the 3600w hot water service is 240v

And to do the same amount of 'work' ie wattage consumed- the voltage when doubled, will halve the current- which is why we use 240vac in the first place
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Old 13th August 2017, 10:12 AM   #33
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Originally Posted by MostlyDead View Post
Yes, that's what I said. Talking about ovens and water heaters in household 220v, it's all high expected draw. Becomes a pedantic argument to say there is a lighter gauge for household appliance wiring when it basically does not exist.
These are not the only examples of 220v appliances used in the US.

Electric baseboard heat and A/C units along with bathroom heaters are good examples that are typically rated as 20 amp or less.

Your blanket statement is incorrect.
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Old 13th August 2017, 10:12 AM   #34
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Originally Posted by Trebuchet View Post
I think it's rare these days, but 208V three phase with a neutral provides a convenient 120V from each phase to the neutral. Very early in my engineering career I actually got to design a machine for that. We could eliminate the usual transformer to provide 120V for the control circuitry.
Exactly: delta 208V is wye 208/sqrt(3)= 120V

but you never had needed a transformer: that's the way it's designed to be. You decided to plug it in one configuration and not the in the other.
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Old 13th August 2017, 10:18 AM   #35
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Originally Posted by aleCcowaN View Post
PS: People who don't know what phase means in power provision shouldn't speak of it.
3 live to neutral voltages each 120 degrees out of phase relative to each other is 3 phase power.

But 2 live to neutral voltages both 180 degrees out of phase relative to each other is NOT 2 phase power. Got it.
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Old 13th August 2017, 10:21 AM   #36
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Originally Posted by ceptimus View Post
No. I'm right and you're wrong.
You are right if you are talking about something completely different to what I am posting.

I am referring to the power lines that run past your house and which you connect to in order to supply your house. (The 7,200 volt reference should have been a clue).
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Old 13th August 2017, 10:22 AM   #37
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Originally Posted by Dabop View Post
Actually here it does
but thats not what you said, "14 or 12 gauge wiring is used for 120v, but the higher amp draw of 240v"

is a factually incorrect statement
240v doesnt 'draw' more amps, in your country it is used for higher wattage appliances, here it isnt- everything from my led desklamp to the 3600w hot water service is 240v

And to do the same amount of 'work' ie wattage consumed- the voltage when doubled, will halve the current- which is why we use 240vac in the first place
Yes, I get that Oz is wired differently than the States, and I of course agree with the wattage calcs. Context. This is the post I responded to:

Originally Posted by Speedskater View Post
In the US system, the same wires are used in the 120V and 240V parts of the system.

================
NEC Article 310
To use your phrasing, this is factually incorrect statement. The 220 supply lines in the States for the high draw appliances listed earlier are not the same gauge as the 110 lines.

ETA: calling it amp draw instead of wattage draw is just construction shorthand around these parts.
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Old 13th August 2017, 10:26 AM   #38
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I add myself to the list of people boggled by that "gauge" thingy of those ethnic standards.

I declined to look for wire section. The "ampacity" of a "14 gauge" (my goodness!) is 15 Ampere in copper, so it must be between 2 and 2.5 square millimetres, if the same rules for wire life and safety are applied. If it's less than that, OMG! such wires in cardboard and match houses!

About some two-person debate: undoubtedly using 240V for the same energy consumption requires half of the current needed under 120V, but that won't make regulations any lighter regarding minimum wire section (or maximum "gauge" if you like that).
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Old 13th August 2017, 10:26 AM   #39
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Originally Posted by Dabop View Post
Ahhh....


we have our own version of that
less sexy title tho

AS/NZS 3000:2007

doesnt roll off the tongue quite like NEC tho does it
It's really called: NFPA 70: National Electrical Code (NEC) (2017).



It changes constantly and is not cheap to keep up with.

http://www.nfpa.org/nec
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Old 13th August 2017, 10:32 AM   #40
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Originally Posted by psionl0 View Post
3 live to neutral voltages each 120 degrees out of phase relative to each other is 3 phase power.

But 2 live to neutral voltages both 180 degrees out of phase relative to each other is NOT 2 phase power. Got it.
Shrugs- well I agree with you psion10, it was referred to as 2 phase power here when it was still used (mostly in country areas in some states)

2 phases 180 deg apart was 2 phase, 3 phases 120 deg apart is 415v/440v 3 phase, uncommon to have a dedicated motor etc using 2 of 3 phases of 240v, but common to split loads over 2 phases in large households where the current drawn was going to exceed the street line supply of 70A (older) or 100A(newer) where you could have 2 of 3 phases connected to the house to provide sufficient power (solid neutral required in the install tho)

(solid neutral is where the neutral is capable of carrying the combined total of all three phases, light or floating neutral has the same sized conductor as any single active or line conductor and a solid neural has the neutral wire is much larger than any single active conductor)
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