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02-11-2008, 07:53 AM   #1
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## Amp / Watt analysis

So I'm on an energy kick now determining how much power different applicances are pulling and I've got a question or two that I hope you can clarify. Here's a chart I pulled off the internet, not universally exact, but helpful I thought. http://www.generatoroutlet.com/wattage_chart.htm First, what is that actual voltage of a plug? I hear 110, then I hear 120, is there a right or wrong to that? (I'm in the US, I believe that matters too) Not a huge difference, but I like knowing the correct answer.

Second, my wife's hair dryer says right on it 1875 watts. So I go by 110volts and get roughly 17amps. The plug she uses is on a circuit with other fixtures being used at the same time, all on a 15amp breaker.... but the breaker never trips. Reasoning?

Lastly, the electric dryer, we'll say uses 5000watts = 23amps because its wired 220; the hair dryer pulls 17amps. I know it has everything to do with the voltage but I can't get my head around how the amperage is so similar. Yes, I'm analytical, but does anybody have a brief way to ease my confusion? Thanks all!

02-11-2008, 08:38 AM   #2
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The wattage of a plug is zero! The voltage at the plug will be between 110-120 volts. The standard used is 120 volts.

If you could use your dryer on 120 volts, the amps would be 41.66
BesT way I can try to explain it.

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 02-11-2008, 10:58 AM #3 You talking to me?     Join Date: Dec 2007 Location: sw mi Posts: 7,551 Rewards Points: 6,290 try these two pages. The first has a pie chart using Ohm's law and shows the relationship each part has to the other and the second is a calculator so you can play with the numbers. Notice the relationship especially between volts, amps (current), and watts (power) http://www.the12volt.com/ohm/ohmslaw.asp http://ourworld.compuserve.com/homep...en/ohmslaw.htm

 02-11-2008, 11:26 AM #4 Member   Join Date: Apr 2007 Location: Fenton, MI Posts: 474 Rewards Points: 264 I get the relationship, thanks for the links. My second question still stands, how can a 1875 watt hair dryer not trip a 15amp breaker on a circuit with other fixtures? The idea that the little hand held device pulls more than 15 amps on its own makes my mind wonder.... I guess the reason behind my curiosity is that every system has to have a bottleneck, or restriction of some type. For my electrical its the 100amp panel. That panel dictates what can and cannot get a certain amount of power. In my head its confusing how the panel directs nearly the same amount of amperage to the electric dryer as it does to the hair dryer. That link to the12volt.com compares amps to water running down a river. How does the hair dryer require a similar flow of water to that of the clothes dryer? Believe me, I know I'm analytical, it bugs me too; sorry if I'm frustrating you! I have an insatiable need to comprehend the way things work....
02-11-2008, 11:32 AM   #5
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Quote:
 Originally Posted by moneymgmt First, what is that actual voltage of a plug? I hear 110, then I hear 120, is there a right or wrong to that? (I'm in the US, I believe that matters too) Not a huge difference, but I like knowing the correct answer.
The utility generally provides +/- 5%, based on 240 volts. So, you may be as low as 228 V or as high as 252 V. This means between 114 V to 126 V based on 120 volts.

110 was what Edison first provided distribution at. And it went up slightly to compensate for greater demand on the system.

Quote:
 Second, my wife's hair dryer says right on it 1875 watts. So I go by 110volts and get roughly 17amps. The plug she uses is on a circuit with other fixtures being used at the same time, all on a 15amp breaker.... but the breaker never trips. Reasoning?
The trip function of a breaker during overload is based on the heating effect of the current. 17 amps on a 15 A breaker may never trip if the conditions are right. If your panel is in a cold garage, or outside, say, in the winter, then you could get quite a bit more out of it than 15 amps. On the other hand, if it is really hot where your breakers are, then a 15 A breaker might trip at only 10 amps.

Quote:
 Lastly, the electric dryer, we'll say uses 5000watts = 23amps because its wired 220; the hair dryer pulls 17amps. I know it has everything to do with the voltage but I can't get my head around how the amperage is so similar. Yes, I'm analytical, but does anybody have a brief way to ease my confusion? Thanks all!
Amps vs. volts is a linear relationship (in most cases anyway). All things being equal, if you double the voltage, the amount of amps will double. Now, the wattage is proportional to the SQUARE of the current. So if the voltage doubles, and the amperage doubles, then the wattage QUADRUPLES.

Consider what would happen if you changed the voltage on the hair dryer to 220. Of course it would burn out, but just toy with the numbers hypothetically. If it operates on 110 V and draws 17 amps, then at 220 V it would draw 34 amps and heat up to 7480 watts! But the formula holds! The wattage quadrupled!

If you ran the clothes dryer on 120 V, it would draw half the amps or 11.5 A. But the watts are 1/4 of what they were, or 1250 W.

Does that help?

InPhase277

Last edited by InPhase277; 02-11-2008 at 11:34 AM. Reason: Bad format

 02-11-2008, 11:45 AM #6 Member   Join Date: Apr 2007 Location: Fenton, MI Posts: 474 Rewards Points: 264 InPhase.... good explaination, I wish I was cured, but... I was always of the understanding that electricity was drawn, not pushed. For example, a 60 watt lightbulb might be wired on a 15 amp circuit but that doesn't mean the bulb gets 15amps x 120volts = 1800watts; it would explode. In your example of putting the hair dryer on a 220 plug, how would it draw more than 1875watts? I really want to appologize a thousand times bc I feel like an idiot going around in circles, but I really appreciate you taking the time to try explaining it to me.
02-11-2008, 12:01 PM   #7
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Quote:
 Originally Posted by moneymgmt InPhase.... good explaination, I wish I was cured, but... I was always of the understanding that electricity was drawn, not pushed. For example, a 60 watt lightbulb might be wired on a 15 amp circuit but that doesn't mean the bulb gets 15amps x 120volts = 1800watts; it would explode. In your example of putting the hair dryer on a 220 plug, how would it draw more than 1875watts? I really want to appologize a thousand times bc I feel like an idiot going around in circles, but I really appreciate you taking the time to try explaining it to me.
These are great questions that even some electricians don't understand. Keep it up.

The amount of current that flows (amps) in a circuit is dependent on two things: the voltage across the circuit, and the resistance of the circuit. The relation is known as Ohm's Law, and is as follows:

Amps = Volts / Resistance (ohms)

Now, you can rearrange that algebraically to get anything you want.

If the resistance stays constant in a circuit, then a change in voltage changes the amount of current linearly. A hair dryer that uses 1875 watts at 110 volts would have an internal resistance of about 6.5 ohms.

So, do the math with me:

Amps = 110 / 6.5 = ~17 A

Now do 220 V:

Amps = 220 / 6.5 = ~34 A

You see?

InPhase277

 02-11-2008, 12:09 PM #8 Member   Join Date: Feb 2008 Location: Atlanta, Ga/Hamilton, Al Posts: 2,487 Rewards Points: 2,350 I should add to that that electricity is pushed, that's what volts are. It is the pressure on the circuit. Volts are pushing, and if the resistance of the circuit lets it happen, more amps will be pushed through. When we say current is drawn, it is really an allusion to the resistance of a circuit. InPhase277
 02-11-2008, 12:15 PM #9 Member   Join Date: Apr 2007 Location: Fenton, MI Posts: 474 Rewards Points: 264 Ok, I'm getting there. Ohms as "internal resistance" is a good way to explain it. Now the only way I know to change voltage is with the plug (110 vs 220). So what about with lights? Where does the voltage come into play? The switch?
 02-11-2008, 12:57 PM #10 electrician   Join Date: Feb 2008 Posts: 326 Rewards Points: 250 [quote=moneymgmt;96896] In my head its confusing how the panel directs nearly the same amount of amperage to the electric dryer as it does to the hair dryer. That link to the12volt.com compares amps to water running down a river. How does the hair dryer require a similar flow of water to that of the clothes dryer? [quote] Try to think of voltage as head pressure. The analogy that was taught to me in trade school was a dam. A dam will back up a certain amount of water. The more water backed up, the more head pressure (voltage) will be built up. If you were to have a pipe (resistance) at the bottom of the dam a certain amount of water (current) would flow out the pipe. If you dam up more water, the water level will rise creating more pressure (voltage). If the pipe is the same (resistance), more water (current) will flow out the pipe faster because of the increased head pressure (voltage). As to the light switch, I have no idea what you are trying to say.
 02-11-2008, 01:11 PM #11 Member   Join Date: Apr 2007 Location: Fenton, MI Posts: 474 Rewards Points: 264 That's a perfect analogy, thanks. Now that I have all the variables down, I'm trying to equate those to actual parts of the circuit. As I'm seeing it the receptacle creates the voltage, is that correct or not? So in the hair dryer example, the outlet creates the voltage on the current. If that is true, then what creates voltage on a light circuit where there is no receptacle? I hope I'm not way off bc I'm starting to feel more confident in the terminology!
 02-11-2008, 01:34 PM #12 electrician   Join Date: Feb 2008 Posts: 326 Rewards Points: 250 Receptacles don't create voltage, they just "tap into" voltages at different voltage levels. Look at this an see if it makes any sense to you Attached Thumbnails
02-11-2008, 01:57 PM   #13
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Quote:
 Originally Posted by moneymgmt That's a perfect analogy, thanks. Now that I have all the variables down, I'm trying to equate those to actual parts of the circuit. As I'm seeing it the receptacle creates the voltage, is that correct or not? So in the hair dryer example, the outlet creates the voltage on the current. If that is true, then what creates voltage on a light circuit where there is no receptacle? I hope I'm not way off bc I'm starting to feel more confident in the terminology!

OK. Here is the best way to equate it. The water analogy is good, but done to death.

The pressure in a pipe is created by the pump. Water flows because of the difference in pressure between the pump and the open end of the pipe. In other words, the pump creates a high pressure system that pushes water out to the low pressure of the air outside the pipe. A difference in pressure is necessary to keep the flow up. When the pump stops, the pressure inside the pipe equalizes with the pressure outside, and water stops flowing. The amount of water that flows is proportional to the friction in the pipe and the difference in pressure between the pump and the open end of the pipe.

In an electrical circuit, the same principle applies. A generator (charge pump, if you will) is creating the difference in pressure at the power plant. Physicists call it a potential difference. Look at a battery. It has a positive pole and a negative pole. These two poles are at a different potential (pressure). Nature hates an imbalance and prefers things to be at the lowest energy level possible. But the poles of a battery are separated and the charges can't equalize, but they "want" to. If you connect them with a wire, a low resistance path has opened for the charge to flow (current), and it will flow until the two poles are at equal potential. The battery is dead.

With a generator, as long as it keeps turning, the difference of potential is kept up, so current will flow as long as a path is present between it's poles.

Voltage is pressure equivalent to PSI in a pipe. Amps are the rate of charge flow, equivalent to gallons per minute. Resistance (ohms), is the friction in the pipe.

In a receptacle, there is a potential between the two poles. The air has too much resistance for the current to flow, until something is plugged in. Whether or not you plug something in, the pressure is always there, waiting for a path to push current through.

In a switch, the potential is still there when the switch is off. The gap between the contacts inside the switch has too much resistance for current to flow. When you throw the switch, the gap disappears and the current now flows through the resistance of the circuit, which is much lower than the air.

Voltage is always present, just waiting for a path.

InPhase277

 06-17-2008, 05:22 PM #14 Newbie   Join Date: Jun 2008 Posts: 2 Rewards Points: 10 Hope it’s not too late to post but... The answer to the Q about the 1875 watt dryer pulling 17 amps is: 1875 / 110 would be about 17 amps but here is the trick: 1875 / 125 would give 15 amps. I'm sure that if you look closely at the dryer you will see it is rated at 1875 watts at 125 volts. So its a B.S. marketing trick to have "more power" Hope that makes you feel better but I am shocked that no one else could answer this... Honestly I think there may be good reason to be concerned, at the least make sure the outlet has a snug or tight hold on the plugs prongs, other wise there could be arching and the production of a lot of heat and potentially fire. Lots more could be said here... P.S. I feel these items should be banned as I have seen extensive damage caused by appliances properly used that draw less power. A space heater at 1500 watts can melt an outlet out of a wall.
06-17-2008, 06:19 PM   #15
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Wouldn't it be the same amps on each leg.
Amps = 110 / 6.5 = ~17 A

Now do 220 V:

Amps = 220 / 6.5 = ~34 A

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