well I guess if you're all the same level of wrong it doesn't matter then does it

 

Im sure you have ur MASTERS dont you?

 

..says the 'electrician' who doesn't know ohm's law. And doesn't know what a phase is.. And doesn't know what a kilowatt-hour meter measures.

 

Beer 4U2Ok im done.........

 

No it is NOT!!! You're proving my point. The resistance of a given load does not change. When you feed the same load higher voltage, the current drops proportionately, allowing for the use of smaller conductors, hence the advantage of using 240 vs 120. 10Ax240V or 20Ax120V is still 2400W. You were already told that at least twice. Go back to school please. I hope your boss' liability insurance is paid up. He'll need it at some point because of you.

 

What point? What are you smoking? This has already been established. You can work an equation.... wow im impressed... 2400 watts is 2.4 kwh so what is your point? You like to quote things that have already been stated is all your about. When you have a valid point to make then speak. Until then fade away and be quite.

 

Yes, it is technically one phase, but each leg is 180 degrees out-of-phase, which is correct/accepted terminology. It shouldn't be confusing to someone that knows what they're doing (unfortunately, that's not always the case). Best to heir on the side of caution I suppose.

Also, some clarification is needed from a previous post regarding power:

I must be missing something here, because if you have a constant load (fixed resistance), the current will increase when the voltage is increased according to ohms law. Your example rather seems to demonstrate the current that would be measured for a 120V/1000W, 240V/1000W, and 480V/1000W ballast. For example, if a ballast is rated for 1000W at 120V, then the current draw will be 8.33A, and if the ballast is rated for 1000W at 240V, the current draw will be 4.17A as in your example. There's a difference. Increasing the voltage isn't going to magically decrease the current, but if you apply the correct voltage to a device base on the nameplate, then you can calculate the current using ohms law. Same as my light bulb example. If you have a 60W light bulb, it is rated for 60W at 120V. If you increase the voltage to 240V, the current isn't going to decrease, it is going to increase by a factor of two and you're going to burn out the element. However, if you have light bulb rated for 60W at 240V, then the current would be less, which would allow for using a smaller gauge wire. I understand what you're trying to say, but you needed to be more clear. In summary:

If you increase the voltage to a fixed load, the current will increase.
If you have a 120V/1000W load and a 240V/1000W load, more current will be flowing in the 120V circuit, but increasing the voltage in that same circuit isn't going to magically decrease the current to keep the power constant. Take a dimmer switch for example, by adjusting the switch you are increasing/decreasing the voltage to the light, which makes the bulb burn brighter/dimmer. If the power remained the same, then the bulb would stay the same brightness, which obviously isn't the case.

 

This thread needs to be closed.

 

...but we're having such fun here, minus the hightened tempers. Everyone just chill. People have different ways of thinking of electricity, but the underlying principles are the same. I'd be happy to address any questions regarding my post in a private message if necessary. Cheers.

 

I suspect the original poster has long fled the scene of the accident.

 

The point that you DO NOT know anything about electrical theory. Since I'm so wrong, please explain to the class how you come to the conclusion that the motor that draws 20A @ 120v (2400W) will be so much cheaper to run with 10A @ 240V (2400W), when the POCO uses a WATTHOUR meter. Please. Do tell. Or tell us what POCO uses AMPHOUR meters so we can all move there and run our appliances for half price.

Oh and that was real cute deleting the post (#44) that I called you out on since it proved you don't actually know what you're talking about. Coward. I hope the moderators can undo your edit.

 

That is sort of true. The legs are 180 degrees of the same sine wave around 0. That is why it is called 'Split phase'. When one refers to 'phases', the term 'phase' is supposed to refer to each separate sine wave, 90 (2 phase) or 120 (3 phase) degrees apart. One should never refer to the legs of split phase as 'phases', because they are not phases.

We are discussing devices that can be used as dual-voltage. Like motors, HID or fluorescent ballasts, etc. Not simply increasing or decreasing the voltage to an arbitrary device. The dual voltage ballast or motor has a specific resistance which normally does not change with regard to the voltage applied. Therefore, doubling the voltage WOULD halve the current that it draws.

The dimmer switch is a little different. It is inserting a variable amount of resistance into the circuit between the supply and the bulb. This resistance wastes some of the power as heat before it gets to the bulb. The total power dissipated by the dimmer and bulb together (therefore the total current), regardless of the dimmer setting, would be slightly higher than the bulb by itself at full brightness.

You also have to remember that incandescent light bulbs do not behave according to Ohm's law. The resistance of the filament increases proportionately in relation to its temperature, which increases exponentially in relation to the voltage supplied to it. It is not a direct linear relationship between voltage and current. That also throws off the dimmer switch/bulb analogy when compared to a fixed resistance load like a ballast or motor.

 

I agree, they are not phases, but they are out of phase, and yes, if you are talking about inductive loads such as light ballast, then I completely agree about the current decreasing with increased voltage, but this is characteristic to this type of circuit, whereas other circuits the opposite would be true (e.g., increasing the voltage to an incandescent bulb would increase the current).

 

That isn't the explanation I have read. What I have read is it interrupts the voltage before the filament can reach full temperature then turns backs on. It does it very rapidly keeping the filament at a lower temperature and therefore less light output. I have read rheostats are generally used only on DC circuits. Varistats are used but a dimer isn't a varistat though. It doesn't change the voltage. Of course I could be totally wrong.

 

There are different types of dimmers on the market. The classical version is basically a variable resistor that divides power between the switch and light bulb. E.g., when the dimmer is at half setting, 30W is being emitted as heat within the switch, and 30W is emitted by the bulb. (60W bulb of course). Other type would be a switched-mode type where the duty cycle of the applied signal is varied to either increase or decrease the power to the light. These are much more efficient (and more expensive).

 

The newer pushbutton electronic and EnergyStar dimmers work this way (this dimming method also creates 'singing' filaments due to the 'chopping' action of the circuit). These actually save electricity because it is modulating the voltage rather than wasting it as heat. But the plain jane $6 Lutron slider or knob is a rheostat.

 

Probably worth explaining what the 120/240 switch is actually doing. When switching to 240, you are adding inductance to the circuit, which decreases current while maintaining the same power delivered to the load.

 

I sure hope this discussion isn't closed.

There is a lot of good information being disseminated and also some serious misconceptions with a few outright errors. I am a retired engineer who spent more than thirty years in the field. Unlike some engineers I did not just sit at a desk and push theoretical concepts but actually worked in the installation and operation of what I designed. My experience is both technical (theoretical/educational) and practice in actually doing the physical work.

Nobody has correctly answered the exercise I posted back in post #19. I'll come back to that shortly but first I want to clear up a common misconception. Dimmers, as are commonly used in residential lighting are NOT simple rheostats (variable resistors) but are electronic circuits that vary the effective applied voltage to the lamps. Before the advent of inexpensive semiconductor devices, including triacs, silicon controlled rectifiers and the like rheostats were indeed used for lamp dimming. The problem with a rheostat, as has been pointed out, is that it converts a portion of the power of the circuit to waste heat. This was extremely uneconomical and is the primary reason that residential dimmers did not come into common usage until the semiconductor revolution. Further, the rheostats were of a size that could not have been installed in any common electrical box used in residential work.

There had been some usage of variable autotransformers in high-end homes but high cost along with relatively large physical dimensions kept them out of the middle class homes.

The use of the term "phase" when describing the secondary windings of a single-phase transformers is incorrect but also in common usage, especially by electricians (and non-electricians) who have never been properly educated on the technical aspects of polyphase systems. As a practical matter it really doesn't make any difference if a person refers to the conductors from a single-phase transformer as "legs" or "phases" but to an engineer it DOES make a difference. It is the engineer in me that is responsible for this nit-picking. My first real job out of college was working for an electric utility that had two ancient, turn-of-the-century steam-electric generating stations along with several "customers" that required 2400 volt (nominal) two-phase power. If I had referred to the secondary wiring of a single-phase transformer as "two-phase" I am quite sure that at least one of my supervisors or co-workers would have cheerfully made me grab those energized 2400 volt connections with both hands. The generating stations had several pieces of equipment that utilized the 2400 volt two-phase power.

Here's the bottom line: Silver, and others, that refer to the conductors from the center-tapped secondary of a common single phase distribution transformers are technically incorrect with that usage but in the common usage of the term it is usually understood what they mean. Being an engineer it really irks me when incorrect, but common, terms are used in a technical nature. It causes me to think that the person has a lesser education in the area where they are supposed to be proficient and usually when I explain the differences the person understands and stops using the term incorrectly.

Since I don't know how much room I have left iin this post I am going to open a new post to discuss the exercise.

 

My exercise of post #19 was of an installation of a common 4800 watt, 240 volt single phase water heater. I asked four questions that are easily answered by the application of Ohm's law. Any practical electrician should be able to answer the questions with a little thought and actually doing the math. At first sight many electricians (and interested lay people) will jump to a very common misconception rather than do the math. The questions are:
1. If both 120 volts and 240 volts are available, which voltage will result in lower operating costs?

2. What would be the effect of using the other voltage?

3. What is the calculated amperage if using 240 volts?

4. What is the calculated amperage if using 120 volts?

The answers are:
1. The operating costs will be the same regardless of what voltage is used.

2. If 120 volts is supplied to the heater it will draw one-half the amperage it would if 240 volts is applied. (Many people incorrectly answer that the amperage at 120 volts will be double the amperage at 240 volts.) It will take four times as long to heat the same quantity of water to the same temperature as when using 240 volts. The cost of operation will be identical.

3. and 4. The amperage of a 4800 watt heater rated at 240 volts is 20 amperes. If the same heater is connected to 120 volts the amperage will be 10 amperes.

If anyone disputes these answers I will show the math that proves the answers.

 

You have it backwards. The amperage IS double at 120. The time it takes to bring the water up to temperature is also the same regardless of the supply voltage. There is 4800W being dissipated into the water whether it is 20A@240v or 40A@120v. The cost of operation will be the same regardless of the supply voltage.

The 4800W heater at 120V draws 40A.

I=P/E. 4800/240=20. 4800/120=40.

 

Sorry Matt but you are incorrect. You are failing to remember that the heating element has a fixed resistance and calculating the current flow based upon the different voltages working with the same resistance.

E=IR, R=E/I and I=E/R. Power (in watts) is P=EI

A 4800 watt 240 volt heating element has a resistance of 12 ohms. (4800 watts divided by 240 volts equals a current flow of 20 amperes. 240 volts divided by 20 amperes equals 12 ohms.)

When this 12 ohm resistance is powered by a 120 volt source it draws 10 amperes. (120 volts divided by 12 ohms equals 10 amperes.

The result is that a heating element nominally rated at 4800 watts at 240 volts becomes a heating element of only 1200 watts when powered by a 120 volt circuit. 1200 watts is one-fourth of the original rating of 4800 watts. Using a heat source of 1200 watts would take four times longer to raise the temperature of a specific quantity of water a specific number of degrees than would using a heater of 4800 watts.

 

From the real world. Please don't try this at home but I once rewired an electric dryer to work on straight 120v. Took over 2 hours to dry a small load of clothes.

(Very old garage apartment with a 30a fuse box and 120v 2 wire drop.)

 

Touché! You're absolutely right, most people would make the exact same mistake.

And the operating costs would be the same because it doesn't matter if you use 4800W over 1 hour or 1200W per hour for 4 hours, you still get charged for 4800W/h to heat that water up to the same temp (although one could get into the semantics of the radiation loss of the tank over the 4 hours would make it take more time to reach the same temp as a 4800w unit, therefore the 240v setup would cost less).

 

Yep, it is a VERY common and completely understandable mistake. It goes with the adage of "a little knowledge is a dangerous thing".

Ray, with a full load my dryer takes close to 2 hours to complete a cycle. In all fairness it does have only only a 2500 watt heating element vs the more common 4800 or 5000 watt elements of most American-built clothes dryers.

 

I am glad you guys keep it civil here and for any members if it get out of the hand in here one of the moderators in here will close it.

Just keep it civil thanks.


Now with Ohm laws I have work on 208Y120 volt system pretty often and when I run into electric heating system like baseboard or water heater or stove etc they will loose 25% of wattage and btu rating over standard 240 volts.

So that why a quite few electric water heaters will listed both wattage at both voltage level ( Both 240 and 208 volts )

Now to run any heating elements even regular light bulbs indentscent bulbs if you drop the voltage to half you cut the wattage back to 25% of oringal wattage at rated voltage that always work with Ohm laws.

If you want talk about more on this one I think I will or other members in here will start the new thread and go from there.

Merci.
Marc

 

Thanks, Marc and I apologize for moving the topic away from the original post. I do think that it has been educational for both those doing electrical work professionally and for the interested amateur.

I will have to "beg off" if the discussion turns to power calculations in three-phase circuits. My background and expertise is with communication and controls, not three-phase power.

 

Furd.,

It is not a issue at all and thanks for being straight with us.

I deal alot of three phase system in both USA and France so I am no stranger with it.

Merci.
Marc

 

And this was the only electric dryer I ever used. Grew up with a gas dryer and switched to gas when I moved from there. Gas has always taken about 40 minutes. I assumed a correctly connected gas dryer did the same.

The wiring in that apartment was something to behold.... or scare the Bjeezus out of you. Single fuse fuse box with a lamp holder jerry rigged on the line side, yes line side, of the disconnect to act as a second fuse holder. No, it was there when I moved in. Not that dumb.

 

I had it wrong too.


Furd
Ok im not sure about the codes on this one being 10 ft from the panel first of all.
1. 240 V is my answer
2. more energy efficient
3. 18.8 amps
4. 37.5 amps

You told me 2500 watts though I was still wrong. Neat little exercise with the elements i never thought of that.
pS. For the record I was not referring to Phase as to a transformer. I use that term like when in a panel or pull in in wire etc. I know leg is the "proper" word but around here(TN) we just say phase and I usually work with 3 phase so it rubs off. AB and C phase. I dont work with pole transformers or anything on the pole for that matter i am not a wireman or lineman. From the meterbase back is where I work.

 

There is no code prohibition for having a water heater ten feet from the panel. It can be adjacent to the panel or 1,000 feet away as long as it meets the other criteria such as not encroaching upon the 30 inch (minimum) width "working space" in front of the panel and the necessity of having a local disconnect if it is located more than fifty feet or not in a straight line of vision to the panel. I used ten feet to eliminate any correction due to voltage drop in the branch circuit.

I think I originally used the figure of 4500 (not 2500) watts and then later changed it to 4800 watts. I think that 4800 watt elements are more common than are 4500 watt elements. The exact numbers are not as important as is understanding the relationship of ohms, volts, amperes and watts in a given circuit.

I hope that I have made it easier to understand that the old "rule of thumb" that states "twice the voltage means half the amperage and half the voltage means twice the amperage" does not always apply. Truth is, that most utilization equipment is made for a specific voltage and significant deviation from that voltage will either cause poor (or no) operation (low voltage) or will destroy the equipment (high voltage). Motors are the notable exception with the majority of motors rated in excess of 1/2 horsepower having the ability to use either of two common voltages. If you connect a "standard" 120 volt incandescent lamp to 240 volts it will shine very brightly for maybe a second or two before burning out. On the other hand if you connected a 240 volt incandescent lamp (these were common in traffic signals before LEDs) to a 120 volt supply the lamp would light dimly and probably last for years before burning out.

I also hope I have shown that when a piece of equipment can be used on either of two voltages that with the exception of a tiny fraction of a percent due to heat losses the efficiency and cost of operation is identical regardless of the voltage used. The cost of installation MAY be significantly different but the cost of operation is the same.

And Silver, you don't have to be working directly with utility distribution transformers for the terminology to apply. It doesn't matter if the transformer is two feet from the panel or two thousand feet away the conductors you are working with are merely extensions of the transformer's windings. Since you state that you work commercial sites you will sooner or later be involved with a "separately derived system" consisting of a transformer connected to a higher voltage supply and having a lower voltage secondary running directly to a panelboard for distribution. This could be a three-phase delta primary connected to a 480 volt three-phase system and being transformed to a wye-connected secondary outputting 208/120 volts three-phase to the secondary panelboard where all the branch circuits are single-phase 120 volt loads. In this case the four busses in the panelboard would properly be referred to as phases, A, B, C and Neutral. If this panelboard supplied any three-phase loads they would be 208 volts and connect via a three-pole circuit breaker to the A, B, and C phases with no neutral connection. The single phase loads (generally 120 volt convenience receptacles and lighting) would connect via a single pole circuit breaker and the neutral bus.

Or, you might have a single-phase transformer that connects to two of the three phases of the higher voltage three-phase system and the transformer secondary will be ONLY a single phase output. This output could be at a single voltage or by using "taps" on the transformer secondary have multiple secondary voltages. Most common is a center-tapped secondary where the voltage from either end of the secondary winding to the center tap is one-half the voltage from one end of the secondary winding to the other end. In this case the secondary conductors AND THEIR EXTENSIONS to a panelboard are NOT properly referred to as "phases" but as "legs". As long as you understand the difference between "legs" and "phases" you can use whichever term is most comfortable in your own work environment but please understand that when discussing the issue on an international forum you WILL be called to account when you use a term contrary to its proper definition.


Probably the most wonderful thing about life is that there is always more to learn. Even five years into my retirement I learn new things almost every day.

 

Furd .,

Again thanks for your wonderfull time to explain the details which I don't always have time to do it due my work shedule and the hours conflect.

Silver.,

I am glad you came foward and get the past to be gones.

I know you will learn a bit as well just take time you will catch on pretty fast.

Just like I learn the European system it only took a short while but get used to Americian system it take a hair longer but figured out myself pretty fast.

Once I learn both system it become a second nature for me.

If you have more question or other items you want to talk you are more than welcome to start the new thread and we will go from there.

{ yeah I know you will ask me if I am on other fourms and Oui I am in couple electrician fourms and I am pretty well knowen over there. }

Merci.
Marc

 

Cool thanks again for all the helpful knowledge. I do appreciate it and I will prob have many more questions. I dont want to be a electrician that just knows how to do things but one that can do it and know why he is doing it and what makes it works. There is a big difference in my opinion. Thanks again.Beer 4U2

 

You're welcome, Silver. Wanting to know the why in addition to the how already puts you ahead of the majority of people in the construction trades. I commend you.

I joined this forum several years ago to give back some of the information that I have learned over my lifetime. While some was learned all on my own there were also a countless number of people that helped me along the way. I thank every one of them for what they shared.