Comparing US 120V vs EU 220V for Dishwasher Power

[Bassalisk]

If I have a let's say a dishwasher which needs 1000 Watts to operate. Doesn't that mean that, if you have 120 V you will need to draw more current from the source in order to dishwasher to work properly?
I mean more in comparison to 220 V where you need less current for a dishwasher to operate?

Further more doesn't this mean that the dishwasher at 120 V will get hotter over time as it has more current running through it, comparison to 220 V?

 

[chaoseverlasting]

I don't know about the heating up part because that depends on the system design, even if the same system was run on two different supplies.

It will require more current though, unless there's something I'm missing. I'm only a EE student, though in final year of my engineering. I don't have any practical experience with dishwashers though.

 

[Bassalisk]

I took a dishwasher as an example. It could be any machine which runs on high powered source.

 

[Jiggy-Ninja]

If I have a let's say a dishwasher which needs 1000 Watts to operate. Doesn't that mean that, if you have 120 V you will need to draw more current from the source in order to dishwasher to work properly?
I mean more in comparison to 220 V where you need less current for a dishwasher to operate?

Correct. For the same amount of power (P= IV), the 120V machine will require almost twice as much current.

Further more doesn't this mean that the dishwasher at 120 V will get hotter over time as it has more current running through it, comparison to 220 V?

Incorrect. Heating is determined by power, not current. Since both machines are drawing the same amount of power, and presumably do the same kind of operations, they would both heat up about the same way.

 

[Averagesupernova]

The biggest advantage of double voltage and half the current is the wire size required to carry the load and the distance the load can be from the source carrying the same amount of power when using the same wire size. For instance, a 2400 watt load running at a supply voltage of 240 volts fed with #12 copper wire the max distance from source to load is 140 feet. A 2400 watt load running at a supply voltage of 120 volts fed with the same #12 wire the max distance from the source to load is 35 feet. This is figured with a 2% voltage drop.

 

[MATLABdude]

The biggest advantage of double voltage and half the current is the wire size required to carry the load and the distance the load can be from the source carrying the same amount of power when using the same wire size. For instance, a 2400 watt load running at a supply voltage of 240 volts fed with #12 copper wire the max distance from source to load is 140 feet. A 2400 watt load running at a supply voltage of 120 volts fed with the same #12 wire the max distance from the source to load is 35 feet. This is figured with a 2% voltage drop.

A little bit of extension here for the OP on Averagesupernova's point, but this reason is also why long distance transmission lines are in the tens (or even hundreds) of kilovolts (kV). The conductors required to carry the all the current used at household voltages (without significant loss) would be enormous!

 

[Jiggy-Ninja]

And the biggest disadvantage of double voltage/half current is that it is more dangerous to handle. Increasing the efficiency means decreasing the safety, which is why we step down from transmission line voltages to begin with. Even if it let you use smaller wires with less resistive losses, you wouldn't really want several thousand volts going into your wall plug!

In engineering, there's always a trade-off of some kind involved.

 

[Bassalisk]

Thanks for your help, I understand now the basics of this difference. If you have any more useful statements please do post them.
I am trying to understand this power-amp-volt tradeoff and power in general. Does anybody have any analogies for power? I mean I don't get a lot from I*V, I look at it the way to calculate it but my intuition is not full.

Simplest example that bugs me:

You have a DC brush electric motor(the one you make in class with battery and a coiled up wire) When you turn on the circuit the coil starts to spin and there is a certain fairly low current being registered at ammeter. When you physically stop the coil with your hand let's say, suddenly very large current starts flowing through the coil. See this conversion of current into mechanical energy is really distant to me, if you understand what I'm saying.

 

[Studiot]

A few further pieces of information.

The European standardised voltage is 230 volts +10 -15 volts.

This apparently strange figure arose to harmonise supplies in the member states which varied from 215 to 240 volts.

In the UK our standard was 240 and is often seen at 244 or so. I have seen up to 257, with some damage to equipment resulting.

The EU standard of 230 is a single phase supply.

In fact the power companies supply three phases in the road and the 230 is the voltage between one of these phases and the neutral.

In residential areas the street lighting may be supplied from one phase and the houses from another.

In areas where there are large buildings or industrial premises all htree pahses may be supplied.
Some high powered equipment especially in shops (say a bakers oven) may be run from the voltage between two of the phases. This is 415 volts.

The trend in Europe is for underground cables. So the cable has a metal protective sheath.

This sheath is used for the supply company to povide an Earth connection to the consumer.

In the US practice is very different, although the higher interphase voltage is also used so your dishwasher would use 240 volts.

The supply cables tend to be overhead so no grounding sheath is available. The US consumer is responsible for providing his own earth.

The incoming power line is single phase at high voltage and connected to the primary of a power transformer at the consumer's building.

The consumer receives his supply from the secondary of this transformer, which is cente tapped to supply 120 - 0 - 120 volts. This is a called a split phase supply and the centre is earthed by the consumer so he can have two independent 120 volt supplies. He can also connect across the 240 from the transformer for higher power equipment such as your dishwasher.

Building wiring in the US is different from the UK.

In the the UK ring mains are generally used to supply power circuits at 230 (nominal) volts.

In the US star and branch wiring is generally used.

Unfortunately the colour coding for the wiring is also totally different.

 

[Bassalisk]

I have the ultimate question: WHY did US go for 120 V?

 

[Studiot]

Safety.

You have a significantly (I don't know the %) greater chance of survivng a 120 volt shock than a 240 volts shock.

Also 120 volts is less likely to create a fire hazard through arcing, sparking etc.

In the UK there is an industrial standard for portable equipment used on site to be supplied at 110 volts via a double insulated transformer for this reason. The equipment is then not earthed.

 

[Bassalisk]

So basically we have a trade off between current and voltage through power? And it doesn't matter that much how much you trade as long as you don't go too high?

And another question, why 220V was used in the first place? Because of experimental data said this was most efficient or?

 

[MATLABdude]

Thanks for your help, I understand now the basics of this difference. If you have any more useful statements please do post them.
I am trying to understand this power-amp-volt tradeoff and power in general. Does anybody have any analogies for power? I mean I don't get a lot from I*V, I look at it the way to calculate it but my intuition is not full.

Simplest example that bugs me:

You have a DC brush electric motor(the one you make in class with battery and a coiled up wire) When you turn on the circuit the coil starts to spin and there is a certain fairly low current being registered at ammeter. When you physically stop the coil with your hand let's say, suddenly very large current starts flowing through the coil. See this conversion of current into mechanical energy is really distant to me, if you understand what I'm saying.

Unfortunately, that example isn't purely resistive. If you think about a really simple motor, you've got a long piece of wire (okay, and it's formed into a coil, and there are some magnets). The resistance of that piece of wire is fairly low.

Because the rotor is spinning and cutting across the field lines from the magnet, you actually have a back EMF (counter EMF) generated in the opposite direction of the applied current, reducing the forward current. As you apply increasing load and slow down the motor, you get less back EMF, until you finally stall out the motor and get very high current flowing through it (your applied voltage divided by just the resistance of the wire):
http://en.wikipedia.org/wiki/Counter-electromotive_force

 

[Bassalisk]

Unfortunately, that example isn't purely resistive. If you think about a really simple motor, you've got a long piece of wire (okay, and it's formed into a coil, and there are some magnets). The resistance of that piece of wire is fairly low.

Because the rotor is spinning and cutting across the field lines from the magnet, you actually have a back EMF (counter EMF) generated in the opposite direction of the applied current, reducing the forward current. As you apply increasing load and slow down the motor, you get less back EMF, until you finally stall out the motor and get very high current flowing through it (your applied voltage divided by just the resistance of the wire):
http://en.wikipedia.org/wiki/Counter-electromotive_force

Oh my God XD I totally overruled Lenz Law. Now I get it. Thank you. This forum is best thing I found on internet. You know, these little things you learn help you move forward. I maybe even passed my exams because of these little steps.

 

[Bob S]

If you have a dishwasher designed to operate on 120 V 60 Hz, and you try to run it on 220 V 50 Hz, even with a step-down transformer, you may smoke it, because the induction motor has a volt-second limit before the iron saturates (from Faraday's Law):

Using V = L dI/dt

∫V dt = ∫L(I) dI

Bob S

 

[JaredJames]

Safety.

You have a significantly (I don't know the %) greater chance of survivng a 120 volt shock than a 240 volts shock.

Also 120 volts is less likely to create a fire hazard through arcing, sparking etc.

In the UK there is an industrial standard for portable equipment used on site to be supplied at 110 volts via a double insulated transformer for this reason. The equipment is then not earthed.

Do devices in the US have an earth?

I was under the impression the US had a 2-pin plug setup as opposed to the UK's 3-pin.

I ask because although the safety may be greater using the lower voltage, would not having the Earth setup increase your chances of receiving a shock? If the device isn't earthed and there's a short, you're going to get a belting shock as opposed to it tripping out in with via the Earth circuit.

So more likely to survive, but also more likely to receive a shock?

 

[Evil Bunny]

The consumer receives his supply from the secondary of this transformer, which is cente tapped to supply 120 - 0 - 120 volts. This is a called a split phase supply and the centre is earthed by the consumer so he can have two independent 120 volt supplies. He can also connect across the 240 from the transformer for higher power equipment such as your dishwasher.

It's not earthed so we can have two independent 120V supplies. It is center tapped so we can have two independent 120V supplies. The center tap is what creates the two independent supplies, not the ground rod. Being grounded (or earthed) has nothing to do with it.

 

[Evil Bunny]

Do devices in the US have an earth?

I was under the impression the US had a 2-pin plug setup as opposed to the UK's 3-pin.[/QUOTE]

Yes, we have a third pin as well. This was not always the case, but it is now... The technical term for the third conductor is an "equipment grounding conductor" but it's commonly referred to as the "ground".

 

[turbo]

Do devices in the US have an earth?

I was under the impression the US had a 2-pin plug setup as opposed to the UK's 3-pin.

I ask because although the safety may be greater using the lower voltage, would not having the Earth setup increase your chances of receiving a shock? If the device isn't earthed and there's a short, you're going to get a belting shock as opposed to it tripping out in with via the Earth circuit.

So more likely to survive, but also more likely to receive a shock?

US circuits have an Earth (ground) to which neutral is referenced. That tie-in is made at the entrance panel of your house. If that reference is lost in your house's internal wiring (which recently happened in my house) some very odd things will happen, including possible overheating, fires, etc. All modern internal wiring is not only grounded but polarized.

 

[Skaperen]

I have the ultimate question: WHY did US go for 120 V?

At the time Thomas Edison was making his improvements to the incandescent light bulb (which had been invented much earlier), he apparently figured out that at a lower voltage, the filament would be designed with a lower resistance, which means that it would be thicker and/or shorter. Such a filament at a lower voltage would actually be more reliable than one of equal wattage at a higher voltage. The higher voltage filament would be thinner and evaporate sooner and/or be longer and more vulnerable to vibrations (including startup vibrations).

There was already a migration to using 220 volts as the delivery on various electrical distributions systems of the day (but not very widespread, yet, due to the lack of a practical lightbulb). What Mr. Edison figured out was a way to use a 3 wire scheme to allow him the advantages of distribution at the higher voltage of 220, while also having the advantage of the lower voltage of 110 that made his lightbulbs more reliable. His business model was not in selling the lightbulbs per se, but in selling the electricity that they use.

Although his scheme was based on DC, and thus was a mixed positive/neutral/negative system, it happened to work (most likely to Mr. Edison's displeasure) just as well on AC (as did his light bulbs). So when and where systems were established with AC, they used the same 3-wire scheme. It did, however, come with a 50% cost penalty for the cost of the wire. But that didn't seem to stop them. I presume copper was lower in price back then as it was not in such widespread demand compared to today.

So basically, the AC system adopted Mr. Edison's DC system voltages, which Mr. Edison devised to make the light bulb work more reliably.

You will find that even lower voltages like 12 volts make for even more reliably light bulbs. That reliability can then be traded off to operate the filament at an even higher temperature, gaining some efficiency (but not like what we can do today with FL, HID and LED lighting). Then there is the halogen cycle that can operate at the higher temperature, which can improve the filament reliability.

If Mr. Edison had accepted alternating current, he might have realized that using a transformer to reduce 220 volts down to 10 volts would have produced a much more reliable light bulb. Then he could have delivered electricity at 220 volts and had transformers in each home and office to step that down to 10 or 20 volts. And at that point he could have used even higher distribution voltages and widened the scale of his electricity business.

So, blame the existence of 110, 115, 120 volts on Edison's fascination with DC and his business model of making reliable lightbulbs to sell electricity. BTW, the standard in the USA for common utilization voltage was boosted to 115 volts during WW2 and 120 volts later on.

Anyone know why Japan uses 100 volts?

 

[Skaperen]

Correct. For the same amount of power (P= IV), the 120V machine will require almost twice as much current.


Incorrect. Heating is determined by power, not current. Since both machines are drawing the same amount of power, and presumably do the same kind of operations, they would both heat up about the same way.

Since the conductors are substantially a lower resistance, the heating loss that takes place there is effectively a function of system current (which through the I²R formula is correctly a function of power). When we determine how much current is flowing through a load, we usually only consider the system or applied voltage, and load resistance or impedance, and disregard the distribution resistance in the wires (since in most cases it is insignificant to that calculation).

But that doesn't mean it (power loss in the form of heating in the supply conductors) is not important. It is power waste. Reducing it is good.

One thing to note is that when comparing 120 volt vs. 240 volt utilization, the higher current results in more (double) power loss through heating even though the supply conductors are doubled in cross sectional area. Without that doubling, the loss is actually 4 times as much due to the I²R rule. At double the wire size, just think of it as half the current going through one wire and half going through the other wire (or wire halves of a doubled wire). Each would be dissipating the same power in the 120 volt case as the one wire in the 240 volt case. But that being two wires dissipating power, you still have twice the loss at 120 volts because of the doubling of current.

That said, it isn't really as bad as it may seem with the 3-wire split phase 120/240 volt system. To the extent that loads are balanced between the phases, the system will act like a 240 volt system, between the main breaker panel and the supplying transformer. But the 120 volt branch circuits will still be an area of loss. Where you can run things in 240 volts, you will be reducing the power loss by doing so. And with switch-mode power supplies running more and more electronics these days, lots of things could be (but don't try it unless you know what you are doing).

 

[Bassalisk]

Anyone know why Japan uses 100 volts?

From your writing, and everything else in this thread, i suspect safety. Their nature is like that. Always safety.

 

[Jiggy-Ninja]

From your writing, and everything else in this thread, i suspect safety. Their nature is like that. Always safety.

Or maybe because it's a round number? I don't know.

100V will still zap you pretty good.

 

[Bassalisk]

Or maybe because it's a round number? I don't know.

100V will still zap you pretty good.

Well if you look at it that way only safe voltage would be up to 20-30 V...

 

[Skaperen]

From your writing, and everything else in this thread, i suspect safety. Their nature is like that. Always safety.

It's not that much of a difference. I wonder if it was just some desire for a nice round number, or a different approach to dealing with war time power shortages based on a different pattern of electrical usage where a lower voltage made more sense (e.g. fewer motors, more incandescent lights).