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Old 06-03-2009, 01:16 AM   #57
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Originally Posted by atobols View Post
I shudder to respond here for fear that I will once again incite a string of responses which tell me what a danger to society that I am. Call me a masochist, but here we go...

I plugged my trusty Radio Shack multimeter (a relatively high dollar one, might I mention) into the AC-current outlet just above where my $10 RV-store analog voltage meter was plugged in.

I'll start by saying that the suggestion to check things w/ the multimeter is probably the best, most useful, and most learned-from piece of advice that I've been given on this thread. The results of my "experimentation" are summarized below. Keep in mind that the conditions for the test were as follows: my AS connected via a 100' extension cord to a 15A outlet in my garage.

condition; multimeter V reading; plug-in volt gauge reading
nothing running; 121.5; 122.5
low fan; 118.9; 120
high fan; 117.9; 119.5
low A/C; 104.4 (after 5 min); 104-106 fluctuating

an important point to mention: when first turning the knob to "low A/C" the voltage recovered to about 110. But, it dropped about 1V per minute until it got to 104.4 and I decided to end the test. I immediately inspected the entire length of elec. supply for heat / melting. I felt SLIGHT heat at the end of the 100' cord where it connected to the 15A to 30A plug converter at the AS. By no means could I not hold on to the connector and if my coffee were the same temperature, I would have been microwaving it to warm it up.

So, what do I conclude from this experiment?

1) my analog plug-in gauge is close enough to trust as a reading of voltage in the AS.

2) since the rated V of my A/C unit is 115V, I can run my unit (fan only) on low or high fan without concern for damage to my A/C unit.

3) IF, IF, IF my electrical connection is via a 100' standard-duty extension cord and connected to a 15A service, I should NOT use my A/C. However, if my connection is better than that, a test run can be made and I can trust the reading from my analog gauge as valid and true.
Excellent testing and experiment.

The data clearly demonstrates the effect of wire resistance.

Increasing the wire size, would also demonstrate a reduction in the voltage drop, when the compressor is running.

Andy
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Old 06-03-2009, 09:57 AM   #58
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Quote:
Originally Posted by atobols View Post
I shudder to respond here for fear that I will once again incite a string of responses which tell me what a danger to society that I am. Call me a masochist, but here we go...

I plugged my trusty Radio Shack multimeter (a relatively high dollar one, might I mention) into the AC-current outlet just above where my $10 RV-store analog voltage meter was plugged in.

I'll start by saying that the suggestion to check things w/ the multimeter is probably the best, most useful, and most learned-from piece of advice that I've been given on this thread. The results of my "experimentation" are summarized below. Keep in mind that the conditions for the test were as follows: my AS connected via a 100' extension cord to a 15A outlet in my garage.

condition; multimeter V reading; plug-in volt gauge reading
nothing running; 121.5; 122.5
low fan; 118.9; 120
high fan; 117.9; 119.5
low A/C; 104.4 (after 5 min); 104-106 fluctuating

an important point to mention: when first turning the knob to "low A/C" the voltage recovered to about 110. But, it dropped about 1V per minute until it got to 104.4 and I decided to end the test. I immediately inspected the entire length of elec. supply for heat / melting. I felt SLIGHT heat at the end of the 100' cord where it connected to the 15A to 30A plug converter at the AS. By no means could I not hold on to the connector and if my coffee were the same temperature, I would have been microwaving it to warm it up.
Resistive heating due to the "Small" size of the Extension cord. Too many electrons not enough wire.

I carry two extra 30A RV cords to reduce this problem.

You concluded correctly. The 30A RV cords are available at Wal-Mart for a decent price.
I also have a Kill-A-Watt plugged inside my trailer. I gives me good readings constantly. I could plug it in at the 30-15 junction and find out how much current I am trying to pull.
Fortunately my house is old and I have lots of 20A circuits. One happens to be near the trailers current location in the garage. I spent the night sleeping in the trailer because it has air conditioning and my house currently does not. The voltage still dropped from 120 to about 110 with the AC on.
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Old 06-04-2009, 12:29 AM   #59
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Hi, according to an article in the latest Highways magazine, 105 volts should be the lowest voltage to operate motorized appliances. This is in line with what some of you have said in your posts. Like Michelle, I also keep a Kill-A-Watt plugged into one of my trailer's outlets. [in the kitchen where it is centrally located and easy to see]
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Old 12-18-2009, 03:56 PM   #60
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Regarding Atobols' experiment --

The current draw of an air conditioning unit will fluctuate depending on internal pressures in the sealed system. The initial current draw approaches the "locked rotor" value, typically around 5 times the full load current. It is so named because, prior to modern electronic instruments, it was measured by locking the rotor deliberately and measuring the current with an ammeter while connecting power for a second or so -- long enough to get a reading and short enough to avoid burning up the stator windings.

Otherwise, in an A/C unit that is operating properly, locked rotor current (or something close to it) is drawn briefly, a small fraction of a second. Then current stabilizes at a value below full load, and increases somewhat as the temperature differential between the condensing (outside/hot) and evaporating (inside/cold) coils increases. In general, on a hotter day, an air conditioner will draw slightly more current than on a warm day.

It is this change in current draw that leads to voltage changes. While there is a slight increase in the resistance of the extension cord as it heats up, this effect is negligible. You could confirm this experimentally by using a load that is known to have constant resistance, such as a bunch of incandescent light bulbs, which will draw the same power after the first couple of seconds.

Regarding transfer switches --

The intent of the electrical codes that require transfer switches and disallow the backfeeding of electricity through, say, a welder outlet is to allow a generator system to be operated safely by an average person. The main concern that the codes would have is that someone who doesn't understand the setup might unplug your generator from the wall socket and be exposed to the 240V present on the blades of the plug connector.

Regarding voltages --

The effect that voltage has on various appliances is widely variable. The NEC standard, if I remember correctly, allows a 2% total voltage drop at calculated power required adjusted for demand, no more than 1% of which can be in the branch circuit. That's a pretty tight specification, hard to live up to with the wire sizes and lengths that are common in RV setups, especially with 30A service.

Induction motors are the most demanding regarding voltage. In a house, you have them in your fridge, your sump pump, your washer and dryer, your dishwasher, and your garage door opener. Typically, in an RV, you only have them in your air conditioner(s), because the ammonia refrigeration system in your fridge doesn't use one, and you probably don't have the other appliances. So the main question, regarding voltage, is whether your A/C will start, because lights and kitchen appliances and heating appliances will all work at much lower voltages than the A/C will tolerate. (Most lights and kitchen appliances and heating appliances just degrade in performance by difference in the squares of the voltages, so at 90 volts they have only 60% of the power or brightness or whatever that they have at 115 volts; 115*115=13225, 90*90=8100, 8100/13225=61.2%)

So, how many volts does it take to start the A/C? Well, it's complicated. It varies depending on the specific A/C unit, the ambient temperature (hotter day=higher minimum voltage, because the refrigerant pressure is higher once it's equalized), and whether the A/C has cycled off long enough for the high side/low side pressures to equalize completely. Typically around 90 volts is the minimum. The catch is, that the 90 volts has to be delivered while the A/C is drawing the locked rotor current, usually 40-50 A or so, for the A/C to start reliably. If the voltage is below 108 or 109 volts when the A/C is running, you're probably right on the edge.

Another way to look at it, is that if your voltage drop is calculated to be more than about 20 volts at 50 amps (assuming 115 volts at the outlet in your home or campground, that gives you 95 volts during A/C startup), you're likely to have problems. Solving for the length limitation (ask if you want to see all the math), here's what we get for cord lengths (actual wire lengths are double this):

10 gauge ... 200 feet
8 gauge ... 320 feet
6 gauge ... 500 feet
4 gauge ... 830 feet

Now, those are absolute maximums that will take you right down to 95 volts during A/C startup, which I don't recommend, but it gives you some idea where the edge of the cliff is.

If we stick to the NEC's 1% drop at the branch circuit, that's 1.15 volts, and if we assume that you're running the AC and some other loads for a total of 25 amps, we get these extension cord lengths:

10 gauge ... 23 feet
8 gauge ... 37 feet
6 gauge ... 57 feet
4 gauge ... 95 feet
2 gauge ... 147 feet
1 gauge ... 185 feet

That's pretty conservative but it gives you some idea. If you double the lengths, the voltage drop doubles, if you triple them, it triples, and so on, so you can get a pretty good idea of how much loss you would be getting with various cord configurations.

One of the benefits of 240V setups is that the voltage drop is less of a problem. With a balanced load, there's no current in the neutral, so if everything's set up right (meaning, that the load is perfectly balanced between the two legs), you can go twice as far with the same sized conductors. With RV power, 240V setups are fused or breakered at 50A so you have to use 6 gauge wire or larger.

At higher currents the performance of the terminals and connectors can also become limiting. I see a lot of half-melted extension cord ends, either because the plug wasn't crimped properly at the factory, or because of corrosion or loss of spring pressure in the contacts as they wear. Contact grease helps up to a point but once there starts to be a problem it's time to think in terms of replacing the ends.
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