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at7mbe


Aug 24, 2011, 10:23 AM

Post #26 of 29 (5012 views)

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Re: [DavidHF] USA - Mexico Voltage Differences for Wire Size

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Now, which is it?

Both. I was very precise in the construction of my two caveats, although not with the intent to create confusion, which it seems to have done unfortunately.

The first applies to a resistive load, as stated. The wattage of such a load is not constant, but increases as the square of the voltage or current (W=V squared / R, W = I squared x R, or W = V x I). In the interest of verifying that there's no difference between theory and practice, I just hooked up an incandescent flashlight bulb to a lab bench power supply. As I turned up the voltage from 0 to 3 volts, the bulb gets brighter (more power) and the ammeter shows increasing current. So yes, both current and voltage increase in this case.

My second caveat was framed in terms of a constant load (power), which is how YucaLandia phrased his proposition:

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A microwave oven rated at 1500W draws roughly 15% lower amperage at 127V (17 amps) versus 19 amps at 110V.

As his example shows, lower voltage means higher current f ora constant load. Does a 1500 watt microwave really stay at 1500 watts as the voltage varies? I have no idea. But that was the proposition, and that was what I was responding to.

Sorry if my shifting between variable load and constant load in the two caveats caused a misunderstanding.


YucaLandia


Aug 24, 2011, 11:16 AM

Post #27 of 29 (4991 views)

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Re: [at7mbe] USA - Mexico Voltage Differences for Wire Size

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I think the errors are creeping in as typos (like my earlier typo).

I think that at7mb has seemingly interchanged "load" with "power" with "Resistance" in his 2 caveats and subsequent email, which is causing misunderstandings. Load is formally net Resistance in Ohms, which is clearly not watts, but at7mb seems to interchange load, resistance, and power in the text. (as I also mistakenly did above).
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**There is more to this story = a different post.
steve
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Read-on MacDuff
E-visit at http://yucalandia.com

(This post was edited by YucaLandia on Aug 24, 2011, 11:50 AM)


YucaLandia


Aug 24, 2011, 11:28 AM

Post #28 of 29 (4984 views)

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Re: [at7mbe] USA - Mexico Voltage Differences for Wire Size

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 at7mbe's observation about the difference about my text wording "higher", and at7mb's & my earlier interchanging use of "load" and "power", all got me thinking about how my words were contradictory to the theoretical Ohms law RMS example math calculation I included:

"When the voltage is higher than 110V, the current drawn for the same wattage load is proportionally higher. e.g. A microwave oven rated at 1500W draws roughly 15% lower amperage at 127V (17 amps) versus 19 amps at 110V."

Issues arise when convolving calculations based on batteries vs. variable output power supplies, and when mixing "power", vs. "load" (R for pure resistive loads and including phase angles w/inductive loads), and my error in claiming "the same wattage load" - because the household devices wattage does not stay constant with varying voltage.

Here's my attempt to correct my earlier mis-speaks:
Households in Mexico can have variations in AC power ranging from 110V to 137V, mainly depending on the transformer for your block and on neighbor's loads (Air Conditioning etc). Both the wattage used and current used vary with the supply voltage => Power (Watts) = I x V for resistive loads. This means that higher household voltage also give higher wattages uses: (Physics 101 calculations do fit reality).

Fortunately, our home has two "127V" legs coming into the home, with one at 132V and one at 119V, and I have various meters which gives me a chance to measure actual voltages, wattages, and amperages on various real world loads

Table of Measured Wattages and Amperages (RMS values)
132V..........119V.........Item
111W..........93W..........Coffee bean grinder = AC motor
0.85A..........0.81A..........Coffee bean grinder = AC motor

1008W..........827W..........Clothes Iron
7.91A..........7.25A..........Clothes Iron

996W..........810W..........Espresso Water Heater
7.81A..........7.08A..........Espresso Water Heater

42W..........36W..........Floor Fan - Low Speed
50W..........45W..........Floor Fan - Medium Speed
63W..........57W..........Floor Fan - High Speed

130W..........120W..........Plasma Screen TV with constant picture

1610W..........1419W.........."1600W" Microwave Oven


As shown by 16 measurements on a variety of common household inductive and resistive loads, and with at7mb's light bulb, ammeter, and variable VOLTAGE and variable POWER supply issue, when you increase the Voltage using a variable power supply, then I increases too... Clearly, the CFE lines offer variable power, variable voltage, and variable current. So, the power used by devices increases with increasing voltage. (Said another way, neither CFE's power supplied nor the power consumed by household devices are constant with changing voltage, so P is NOT a fixed variable, and P is NOT "load".)

These data also clearly show that higher household voltages also give higher reading of actual watts used, and higher amperages, which is exactly what Robt65 was concerned about.

This also means that at7mb's implied claims (and my faulty math example) of higher household voltages giving lower amperages just does not work in the real world. Fine for teaching theory in an EE or Physics class using a fixed-voltage/fixed-power battery, but not in actually wiring homes in Mexico with voltages that vary from 110V - 137V.

Making measurements: I used a combination of 2 Fluke meters and a Kill-a-Watt meter to cross check the voltage, amperage, and wattage measurements.

at7mb also writes that : "2. if you're plagued by low voltage, the current will increase for a constant load as the voltage falls. ", but as we can see with a variety of household loads (both inductive and pure resistive loads), the opposite is true: lower household voltages give lower current loads (though his argument is true if you have very limited power - as in failing CFE power at 95V). Except for caveat 2, all of this works out, based on: theory, at7mb's experiment, and my home measurements.

Fortunately, all the wire sizes advice given to Robert (above) were correct.

I also measured refrigerators, computers of various kinds, microwave ovens, other fans, incandescent and fluorescent lights etc to find out their actual power usage values when operating and when "turned off" - and everything made sense for those devices.
steve
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Read-on MacDuff
E-visit at http://yucalandia.com

(This post was edited by YucaLandia on Aug 24, 2011, 1:44 PM)


johanson


Aug 24, 2011, 6:01 PM

Post #29 of 29 (4953 views)

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Re: [YucaLandia] USA - Mexico Voltage Differences for Wire Size

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Thanks, guys. Many years ago, before I retired, I knew all of what you posted by heart. You guys brought it back to this old dude and using the terms I learned and still use, W=IE'

Oh and Steve, at one time CFE posted that the allowed voltage in Mexico is 127 +/- 10% which means a line voltage of 114.3 to 139.7 volts is okay. My line voltage at 4 AM in the morning used to be 143 to 145. But during the day when CFE checked it was 134 to 138 V which was OK. It seems that an employee of CFE who was using wire that was too thin, and lived near me would always step up the local transformer to ensure that he would have sufficiently high voltage during periods of high usage

Finally after several visits to not the normal CFE office, but the local CFE HQ, with printouts of the voltage over 24 hour periods for several weeks, and after showing my press card, they took my comments seriously and now after stepping down the transformer about 2 steps, my average voltage during periods of high use are closer to 122 and periods of low use close to 130 V.
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