how low is too low AC voltage?

[WineStream]

115v came from Carrier's documentation...the rated input by Carrier is 115V @ 60 Hz. Carrier even does the math for you to tell you that it needs 1472W. According to the documentation, the 12.8A is a "Full-Load Amps" rating. The locked motor draw is 60A, but if the motor's locked then I've got other issues beside the electricity going to it.

So, no, I'm not talking myself into anything, just reading what the manufacturer has provided to the world.

[dmac]

You shouldn't connect a generator to your house through a backfeed arrangement. You must use a transfer switch, otherwise you are pumping electricity back through the grid... and could zap a lineman who believes he is working on a cold wire.

I wonder if the 12.8 full load amps is the normal operating draw or the starting amperage? Worth checking with an ammeter. I would want to ensure that things are okay at startup, when the load is highest. The startup condition will occur every time the unit cycles, which is often enough to cause a problem with low voltages.

[Wingeezer]

Originally Posted by dmac

You shouldn't connect a generator to your house through a backfeed arrangement. You must use a transfer switch, otherwise you are pumping electricity back through the grid... and could zap a lineman who believes he is working on a cold wire.

I wonder if the 12.8 full load amps is the normal operating draw or the starting amperage? Worth checking with an ammeter. I would want to ensure that things are okay at startup, when the load is highest. The startup condition will occur every time the unit cycles, which is often enough to cause a problem with low voltages.

************************************************** ***

I'm not arguing with you, I just don't understand this! I haven't done it, but am thinking about it so I want to be sure what I am considering is safe!

If you pull the main fuses coming into the house, how is it possible to backfeed down the incoming line with a small generator that you have connected - lets say to the dryer connection socket?

I have to repeat that I'm not trying to be a smart ass, just trying to get better educated as to whether this is safe or not!

Brian.

[WineStream]

a little off topic but worth clarifying...

any time I connect my generator to the house, the main supply switch (200A) is switched OFF. This is so I'm not powering all of my neighbors nor am I zapping anyone working on the lines.

I claim to be electrically stupid, but I have learned some things along the way...

[markdoane]

I guess the biggest reason not to backfeed is that its ILLEGAL. You need to have an isolator transfer switch like described by dmac.

Gee, I wish John HD was here more often..

[ROBERTSUNRUS]

Hi, so far, unless I missed it, we still haven't got a definite answer on the original question. So my guess-timate is anything below 100 volts is bad. And this must be done for long periods of time to become fatal. Like I said earlier, my in-house portable air conditioner stopped cycling one day so I turned it off. I checked my house voltage and it was down to 88 volts. We were having a brown out. Nothing in my house was damaged due to this and the only thing I did turn off was my portable air conditioner. It has been determined that a poor connection that will melt the plug and or the receptical can and will destroy electrical items regardless of voltage.

[Dwight]

The rule-of-thumb is 120volts +/- 10%, which is 108v to 132v.

[jcanavera]

One other thing to think about. Read the back of your RV voltage gauge. If it's like mine, it may say that the reading is subject to a variance +- 10%. Means if it's showing 105 volts, the actual voltage available may be another 10% lower. Check out the owners manual on your A/C unit. It will give you a voltage range at which the unit can run at.

Buy yourself a digital meter with better tolerances. The analog unit I have is for glancing only. If I see it on the low side, I'll get my digital unit out and get an accurate measurement.

Jack

[markdoane]

I've been informed that it is not illegal to backfeed power from your generator through a welding outlet, without a transfer switch, in Ashland County, Ohio, so I'll avoid making blanket statements about places like Ashland County in the future.

[Airstreamer67]

If the poster is talking about using a plug to backfeed power into his home, this plug is commonly called a "dead man's plug" for very good reason. It has exposed prongs that are hot, and one touch of the prongs can cause death.

I think such a device is absolutely forbidden by any code anywhere in the country. Not only does it present danger to anyone touching the device, unless precautions are taken it also allows electricity to leave the home, be upgraded to thousands of volts by the nearby transformer, and sent for many miles to potentially harm people and property.

The only way to sanely power a home with a generator is by a properly installed transfer switch, or to plug the various appliances directly into the generator and bypass the home's electric system alltogether.

I can safely say that any insurance agent in the country, given the facts, would not condone a deadman's plug backfeeding power into any home he/she insures. Being happy about not paying claims due to thawed meat is one thing. Being unhappy about paying claims on dead clients or their victims is another.

[thecatsandi]

Originally Posted by atobols

So my question is: how low of AC voltage is too low? My plug-in gauge shows that the 105V lies in the "red zone", but is this conservative? If it only dips that low when the A/C is on is it really a concern? I anticipate that the A/C will only be on at night when we're trying to sleep, so shouldn't be much else running to draw current.

106-135 VAC according to my Dometic instructions is the acceptable operating range.

[thecatsandi]

Originally Posted by markdoane

I've been informed that it is not illegal to backfeed power from your generator through a welding outlet, without a transfer switch, in Ashland County, Ohio, so I'll avoid making blanket statements about places like Ashland County in the future.

That would require two exposed plugs. By the NEC (National Electrical code) this is "illegal". Your county code may allow it but I can guarante your insurance company will not.

[Inland RV Center, In]

Electricity, within itself, is very safe.

It's the person who unsafely uses it, that is the crux of the problems, electrical fires and deaths from it.

As they said in class in high school, electricity is a shocking subject, alway has and always will.

But even more shocking, is how a few fight the obvious.

Andy

[WineStream]

I assure everyone that my welder outlet has no exposed or bare connections. You can all sleep easy tonight.

[WineStream]

Originally Posted by jcanavera

Check out the owners manual on your A/C unit. It will give you a voltage range at which the unit can run at.

Buy yourself a digital meter with better tolerances. The analog unit I have is for glancing only. If I see it on the low side, I'll get my digital unit out and get an accurate measurement.

I tried to see what Carrier recommends, but the info available on their website doesn't call out a min limit, it just calls for a supply of 115V and 12.8A. I'm not concerned enough about this to write / call / email them about it.

I will check it w/ my multimeter when we're back home, good idea.

[WineStream]

Originally Posted by jcanavera

Buy yourself a digital meter with better tolerances. The analog unit I have is for glancing only. If I see it on the low side, I'll get my digital unit out and get an accurate measurement.

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.

[Inland RV Center, In]

Originally Posted by atobols

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

[thecatsandi]

Originally Posted by atobols

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.

[ROBERTSUNRUS]

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]

[Jammer]

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.