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Amps to Watts

Old 07-17-2012, 08:36 AM
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Default Amps to Watts

Some of you electrical guys probably know why, but I wish appliances posted the wattage as well as amperage on the tags. Most people are shocked(no pun intended) to hear the genny pull down when they turn on the coffee pot or hair dryer.
I used the conversion formula for each circuit/appliance when installing the transfer switch but why? Had to pull out the fridge etc to get the amps and do the math.
I also heard that after a hurricane or extended power outage that a lot of people had to replace refrigerators and other appliances because they had overloaded their generators and did not supply enough power to them causing damage.
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Old 07-17-2012, 08:42 AM
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There should be a tag on all appliances. If its a 120 volt appliance its the amperage times 120 equaly watts. If its 220 its amps time 220 equals watts.
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Old 07-17-2012, 08:45 AM
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Ohm's Law......http://www.ohmslawcalculator.com/ohm...calculator.php
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Old 07-17-2012, 08:45 AM
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Power= Voltage x Current,

Watts = Volts x Amps

Amps= Watts/Volts
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Old 07-17-2012, 09:35 AM
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Originally Posted by LI Sound Grunt View Post
There should be a tag on all appliances. If its a 120 volt appliance its the amperage times 120 equaly watts. If its 220 its amps time 220 equals watts.
This is not correct.

If the device uses direct current [DC], then Volts x Amps=Watts.
This is not true for alternating current [AC]devices.

For AC, you first must convert the AC volts to Volts RMS] RMS is Root Mean Squared. Since alternating current changes its direction of flow 60 times per second [60 hertz, or 60 cycles] the voltage is not always the same. It varies with the cycle, both in intensity and direction of flow.

For "perfect" AC [a sine wave] the formula is a/ square root of 2. a is the peak amplitude of the wave, which for 120 volts would be 60. So 60 divided by 1.4142136 [the square root of 2] would be 42.4264.

For an A/C device using 120 volts, multiply the amps x 42.4264 to yield the wattage.
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Old 07-17-2012, 09:58 AM
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110/220 IS volts RMS

From many years ago:

I * V = P (current times volts = power) can be re-written as:

P/I = V or P/V = I
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Old 07-17-2012, 10:20 AM
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Originally Posted by Captain Crispy View Post
This is not correct.

If the device uses direct current [DC], then Volts x Amps=Watts.
This is not true for alternating current [AC]devices.

For AC, you first must convert the AC volts to Volts RMS] RMS is Root Mean Squared. Since alternating current changes its direction of flow 60 times per second [60 hertz, or 60 cycles] the voltage is not always the same. It varies with the cycle, both in intensity and direction of flow.

For "perfect" AC [a sine wave] the formula is a/ square root of 2. a is the peak amplitude of the wave, which for 120 volts would be 60. So 60 divided by 1.4142136 [the square root of 2] would be 42.4264.

For an A/C device using 120 volts, multiply the amps x 42.4264 to yield the wattage.
Nope. 120 VAC is 120 volts RMS. You need a refresher course before you overload a generator. Here you go.
http://www.ee.unb.ca/tervo/ee2791/vrms.htm
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Old 07-17-2012, 10:41 AM
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Geek fight. Cool.


What about the forces applied when towing a skier from your T-top?


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Old 07-17-2012, 10:44 AM
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Let's put it in a boating (at least water related) example. Picture a motor driven pump which pumps water in a loop. The water exits the pump, goes through a loop of pvc pipe, and then enters the pump again.

The loop is the circuit.
The speed of the water is like the voltage.
The diameter of the pipe is like the amperage.
The total water passing through (gallons per hour) is a product of the speed of the water x the size of the pipe, so it's like wattage. Increasing the speed (higher voltage) or increasing the pipe diameter (more amperage) will increase the wattage. That's why generators care about watts, since it's like the total gallons per hour being pumped.
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Old 07-17-2012, 10:56 AM
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Originally Posted by jobowker View Post
Picture a motor driven pump ........

I have an engine driven pump that needs motor oil in the crankcase. Don't ask me why.
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Old 07-17-2012, 10:58 AM
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Engine x Motor = $ deficiency.


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Old 07-17-2012, 11:12 AM
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Originally Posted by jobowker View Post
Let's put it in a boating (at least water related) example. Picture a motor driven pump which pumps water in a loop. The water exits the pump, goes through a loop of pvc pipe, and then enters the pump again.

The loop is the circuit.
The speed of the water is like the voltage.
The diameter of the pipe is like the amperage.
The total water passing through (gallons per hour) is a product of the speed of the water x the size of the pipe, so it's like wattage. Increasing the speed (higher voltage) or increasing the pipe diameter (more amperage) will increase the wattage. That's why generators care about watts, since it's like the total gallons per hour being pumped.
I would disagree with a few of these statements (Al wants a geek fight!).

Voltage is the pressure of the water (not speed).
Amperage is the flow of the water
The diameter of the pipe is better thought of as the resisance
Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.
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Old 07-17-2012, 11:13 AM
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For AC

VARS = voltage * current (volt Amp reactive)
Watts = voltage * current * power factor

The power factor is the sine of the phase difference between the voltage and current.
power factor varies between 1.0 for a pure resistive load to 0.0 for a pure inductive or capacitive load.

SO you can have current flow (amps) with out any power. (watts)
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Old 07-17-2012, 11:49 AM
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Originally Posted by ericinmich View Post
I would disagree with a few of these statements (Al wants a geek fight!).

Voltage is the pressure of the water (not speed).
Amperage is the flow of the water
The diameter of the pipe is better thought of as the resisance
Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.

x2 I like your analogy better, and the equations used for both systems are identical.

However, you are wrong on a few accounts. Power by definition is work per period of time.

Force = Mass x Acceleration
Energy(work) = Force/distance
Power = Energy/time

For example 1 hp equals the average amount of work a horse can do in 1 days time. Power = work/time, and 1 HP = 745 Watts

pressure x flow rate is also power in a fluid system. This is how you'd measure the power of a hydraulic motor, such as those in agriculture equipment skid steers etc.

Now let me step off my nerd pedestal, I knew my MS in Mechanical Engineering would pay off on this forum sometime.
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Old 07-17-2012, 11:54 AM
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Originally Posted by ericinmich View Post
I would disagree with a few of these statements (Al wants a geek fight!).

Voltage is the pressure of the water (not speed).
Amperage is the flow of the water
The diameter of the pipe is better thought of as the resisance
Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.
Sure we can make it complicated, but I was keeping it simple. If you have 2 circuits with the same speed but one is a bigger pipe, you'll have more gallons. If you have 2 circuits with the same size pipe but one has a faster flow rate, you'll still get more gallons of water.


That is far easier to visualize, and in my example there would be very little water pressure anyway. And watts are related to time similar to gallons per hour when your meter reads kilowatt hours. Peopel who don't understand the subtleties can easily grasp that passing through a skinnier pipe will make the water go faster (transformer) or two pumps and two circuits next to each other increase the amperage but the voltage stays the same (like batteries in parallel).

This was meant to be a SIMPLE analogy that didn't require math and would be easily visualized.

So quit crapping on my example.
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Old 07-17-2012, 02:00 PM
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Originally Posted by beber View Post
x2 I like your analogy better, and the equations used for both systems are identical.

However, you are wrong on a few accounts. Power by definition is work per period of time.

Force = Mass x Acceleration
Energy(work) = Force/distance
Power = Energy/time

For example 1 hp equals the average amount of work a horse can do in 1 days time. Power = work/time, and 1 HP = 745 Watts

pressure x flow rate is also power in a fluid system. This is how you'd measure the power of a hydraulic motor, such as those in agriculture equipment skid steers etc.

Now let me step off my nerd pedestal, I knew my MS in Mechanical Engineering would pay off on this forum sometime.
Power is an instantanous measurement. (Watts) Energy is power integrated over time (Watt hours or KW hours)
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Old 07-17-2012, 02:27 PM
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So far I've learned that Amps to Watts conversion is an exact estimate.
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Old 07-17-2012, 02:33 PM
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Exactly...............this must be why manufacturers don't tag the appliance with wattage because they don't know what the hell it is....
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Old 07-17-2012, 02:34 PM
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Try this:


ETA: In case you didn't know the unit of power is the watt. So power=watts
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Old 07-17-2012, 02:40 PM
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Originally Posted by Snapper Head View Post
Geek fight. Cool.


What about the forces applied when towing a skier from your T-top?


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