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Old 12-24-2010, 11:35 PM   #1
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volts x amps = watts right?


I just bought a clamp on amp meter and made myself a line splitter, and I'm like a kid with a new toy. Been checking the amp usage of various appliances.

I checked the amp reading of my 550w electric heater, expecting to get at least close to 550w but I came to around 300w. Does this make sense? Or is this an inaccuracy due to not being true RMS. If it's not a true RMS meter (I doubt it, only paid 100 bucks for it) then is there another calculation I should be using?

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Old 12-24-2010, 11:45 PM   #2
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volts x amps = watts right?


Ok, I spoke too soon. Guess the amperage goes up over time. Either a feature of the heater so it does not strain the element too much by giving it full power right away, or maybe just a natural phenomenon of heat elements. Also, is it normal for a bit of current to be read on the ground? I can read 0.02amps.

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Old 12-24-2010, 11:45 PM   #3
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volts x amps = watts right?


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I just bought a clamp on amp meter and made myself a line splitter, and I'm like a kid with a new toy. Been checking the amp usage of various appliances.

I checked the amp reading of my 550w electric heater, expecting to get at least close to 550w but I came to around 300w. Does this make sense? Or is this an inaccuracy due to not being true RMS. If it's not a true RMS meter (I doubt it, only paid 100 bucks for it) then is there another calculation I should be using?
For resistive loads like heaters and incandescent lights, wattage will equal volts*amps. Your meter should be accurate enough for those purposes. With reactive loads like motors, fluorescent lights, and electronics, the non-unity power factor will mean that your readings are artificially high - possibly by quite a bit. There are very few if any circumstances where multiplying volts and amps will give you an artificially low number for power. If you show 300w for that heater, then it's probably running 300w. Did you measure the voltage during operation, or just assume it's 120? Voltage drop may explain the low power operation.
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Old 12-24-2010, 11:52 PM   #4
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volts x amps = watts right?


Yeah measured the voltage before from the same power bar, while the heater was on. It's actually interesting to see the voltage drop a bit when it's turned on. Drops by about 2 volts. When I waited I then got 420watts, so that makes more sense now.

As for reactive loads, what kind of margin am I looking at as far as accuracy? If I'm measuring the load of a computer, should I expect a rather accurate number? The instruction manual also shows one model as being RMS and the other being Average RMS calibrated. Not sure which model mine is since it does not say on it. It's an Ideal.
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Old 12-25-2010, 12:10 AM   #5
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Yeah measured the voltage before from the same power bar, while the heater was on. It's actually interesting to see the voltage drop a bit when it's turned on. Drops by about 2 volts. When I waited I then got 420watts, so that makes more sense now.

As for reactive loads, what kind of margin am I looking at as far as accuracy? If I'm measuring the load of a computer, should I expect a rather accurate number? The instruction manual also shows one model as being RMS and the other being Average RMS calibrated. Not sure which model mine is since it does not say on it. It's an Ideal.
Definitely not normal to read current on the ground. That may not be real, though. If your clamp is near the current carrying conductors, it may read some of their magnetic field even if they are not passing through the clamp's loop. Try plugging it into a GFCI and see what happens. If it doesn't trip, then you're fine and the meter's just picking up the magnetic field from the other wires.

Reactive loads vary greatly in power factor. A motor might have a power factor of 0.8 to 0.9 at full load, all the way down to 0.2 to 0.4 with no load. Electronic devices with switching power supplies are even harder to predict. Some have no power factor correction and might run as low as 0.5. Others have active PFC and run close to 1.0. No way to tell without an RMS wattmeter (not merely an RMS ammeter and voltmeter, since the two must be integrated together to account for phase) or an oscilloscope.
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Old 12-25-2010, 12:21 AM   #6
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Works fine on a GFCI, so guess I was just picking up from the other conductors. I kinda figured it may have been just that.

Now how does the power company meter work, is that true RMS? I'm using this more or less to figure out how much certain things use. Wondering if a killawatt would be better for me. The nice thing with this though is I can test at an open panel as well and is overall more versatile as it's not limited to 120v 15a circuits.
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Old 12-25-2010, 03:37 PM   #7
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Works fine on a GFCI, so guess I was just picking up from the other conductors. I kinda figured it may have been just that.

Now how does the power company meter work, is that true RMS? I'm using this more or less to figure out how much certain things use. Wondering if a killawatt would be better for me. The nice thing with this though is I can test at an open panel as well and is overall more versatile as it's not limited to 120v 15a circuits.
The power company's meter and the kill-a-watt are true wattmeters, not volt-amp meters. They account for power factor. There's no way to measure power factor or true wattage without simultaneously measuring voltage and current with the same device.
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Old 12-25-2010, 05:39 PM   #8
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The power companies meter does NOT account for power factor. At least not the one in your home. Industrial accounts have a PF meter separate from the watt meter and pay a penalty for poor PF.
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Old 12-26-2010, 12:41 AM   #9
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The power companies meter does NOT account for power factor. At least not the one in your home. Industrial accounts have a PF meter separate from the watt meter and pay a penalty for poor PF.
That's a different matter. Your power meter DOES "account" for power factor, in that it reads true wattage, not volt-amps. It measures the instantaneous product of voltage and current, appropriately taking the phase difference into account. You only get charged for watts used, not volt-amps. This means that low power factor devices in your house do not cost you more money. What your electric meter does NOT do is record the power factor or display it. Thus, the power company has no way of knowing what power factor your house is running. The power company's distribution efficiency is lower for low power factor loads. Thus, they want large customers to correct their own power factor and pay a penalty if they don't. So on large commercial installations they have a separate power factor meter that records PF and allows them to charge more for high reactive power.
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Old 01-01-2011, 04:35 PM   #10
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volts x amps = watts right?


Could someone suggest to me how I can make myself a safe to use line splitter?
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Old 01-01-2011, 08:12 PM   #11
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Could someone suggest to me how I can make myself a safe to use line splitter?
Take a short extension cord and strip the outer jacket off a section of the cord exposing the inner insulated wires. Do not strip the inner wires. Then you can clamp around each wire separately.
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Old 01-01-2011, 11:59 PM   #12
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volts x amps = watts right?


That's basically what I did, works great.



For accurate watt calculations I also test the voltage at one of the receptacles connected directly to that circuit, ex: the nearest power bar, or if it's a duplex, the other plug. Then just multiply the values.

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